Renamed lang_ext -> bindings, README updates.

7 files changed, 2073 insertions, 0 deletions

diff --git a/bindings/cpp/upb/handlers.hpp b/bindings/cpp/upb/handlers.hpp
new file mode 100644
index 0000000..b083f15
--- /dev/null
+++ b/bindings/cpp/upb/handlers.hpp
@@ -0,0 +1,142 @@
+/*
+ * upb - a minimalist implementation of protocol buffers.
+ *
+ * Copyright (c) 2011 Google Inc.  See LICENSE for details.
+ * Author: Josh Haberman <jhaberman@gmail.com>
+ *
+ * Note!  This file is a proof-of-concept for C++ wrappers and does not
+ * yet build.
+ *
+ * upb::Handlers is a generic visitor-like interface for iterating over a
+ * stream of protobuf data.  You can register function pointers that will be
+ * called for each message and/or field as the data is being parsed or iterated
+ * over, without having to know the source format that we are parsing from.
+ * This decouples the parsing logic from the processing logic.
+ */
+
+#ifndef UPB_HANDLERS_HPP
+#define UPB_HANDLERS_HPP
+
+#include "upb_handlers.h"
+
+namespace upb {
+
+typedef upb_flow_t Flow;
+
+class FieldHandlers : public upb_fhandlers {
+ public:
+  typedef upb_value_handler ValueHandler;
+  typedef upb_startfield_handler StartFieldHandler;
+  typedef upb_endfield_handler EndFieldHandler;
+
+  // The FieldHandlers will live at least as long as the upb::Handlers to
+  // which it belongs, but can be Ref'd/Unref'd to make it live longer (which
+  // will prolong the life of the underlying upb::Handlers also).
+  void Ref()   { upb_fhandlers_ref(this); }
+  void Unref() { upb_fhandlers_unref(this); }
+
+  // Functions to set this field's handlers.
+  // These return "this" so they can be conveniently chained, eg.
+  //   message_handlers->NewField(...)
+  //       ->SetStartSequenceHandler(&StartSequence),
+  //       ->SetEndSequenceHandler(&EndSequence),
+  //       ->SetValueHandler(&Value);
+  FieldHandlers* SetValueHandler(ValueHandler* h) {
+    upb_fhandlers_setvalue(this, h); return this;
+  }
+  FieldHandlers* SetStartSequenceHandler(StartFieldHandler* h) {
+    upb_fhandlers_setstartseq(this, h); return this;
+  }
+  FieldHandlers* SetEndSequenceHandler(EndFieldHandler* h) {
+    upb_fhandlers_endseq(this, h); return this;
+  }
+  FieldHandlers* SetStartSubmessageHandler(StartFieldHandler* h) {
+    upb_fhandlers_setstartsubmsg(this, h); return this;
+  }
+  FieldHandlers* SetEndSubmessageHandler(EndFieldHandler* h) {
+    upb_fhandlers_endsubmsg(this, h); return this;
+  }
+
+  // Get/Set the field's bound value, which will be passed to its handlers.
+  Value GetBoundValue() { return upb_fhandlers_getfval(this); }
+  FieldHandlers* SetBoundValue(Value val) {
+    upb_fhandlers_setfval(this, val); return this;
+  }
+
+ private:
+  FieldHandlers();  // Only created by upb::Handlers.
+  ~FieldHandlers(); // Only destroyed by refcounting.
+};
+
+
+class MessageHandlers : public upb_mhandlers {
+ public:
+  typedef upb_startmsg_handler StartMessageHandler;
+  typedef upb_endmsg_handler EndMessageHandler;
+
+  // The MessageHandlers will live at least as long as the upb::Handlers to
+  // which it belongs, but can be Ref'd/Unref'd to make it live longer (which
+  // will prolong the life of the underlying upb::Handlers also).
+  void Ref()   { upb_mhandlers_ref(this); }
+  void Unref() { upb_mhandlers_unref(this); }
+
+  // Functions to set this message's handlers.
+  // These return "this" so they can be conveniently chained, eg.
+  //   handlers->NewMessage()
+  //       ->SetStartMessageHandler(&StartMessage)
+  //       ->SetEndMessageHandler(&EndMessage);
+  MessageHandlers* SetStartMessageHandler(StartMessageHandler* h) {
+    upb_mhandlers_setstartmsg(this, h); return this;
+  }
+  MessageHandlers* SetEndMessageHandler(EndMessageHandler* h) {
+    upb_mhandlers_setendmsg(this, h); return this;
+  }
+
+  // Functions to create new FieldHandlers for this message.
+  FieldHandlers* NewFieldHandlers(uint32_t fieldnum, upb_fieldtype_t type,
+                                  bool repeated) {
+    return upb_mhandlers_newfhandlers(this, fieldnum, type, repeated);
+  }
+  FieldHandlers* NewFieldHandlers(FieldDef* f) {
+    return upb_mhandlers_newfhandlers_fordef(f);
+  }
+
+  // Like the previous but for MESSAGE or GROUP fields.  For GROUP fields, the
+  // given submessage must not have any fields with this field number.
+  FieldHandlers* NewFieldHandlersForSubmessage(uint32_t n, FieldType type,
+                                               bool repeated,
+                                               MessageHandlers* subm) {
+    return upb_mhandlers_newsubmsgfhandlers(this, n, type, repeated, subm);
+  }
+
+  FieldHandlers* NewFieldHandlersForSubmessage(FieldDef* f,
+                                               MessageHandlers* subm) {
+    return upb_mhandlers_newsubmsgfhandlers_fordef(f);
+  }
+
+
+ private:
+  MessageHandlers();  // Only created by upb::Handlers.
+  ~MessageHandlers(); // Only destroyed by refcounting.
+};
+
+class Handlers : public upb_handlers {
+ public:
+  // Creates a new Handlers instance.
+  Handlers* New() { return static_cast<Handlers*>(upb_handlers_new()); }
+
+  void Ref()   { upb_handlers_ref(this); }
+  void Unref() { upb_handlers_unref(this); }
+
+  // Returns a new MessageHandlers object.  The first such message that is
+  // obtained will be the top-level message for this Handlers object.
+  MessageHandlers* NewMessageHandlers() { return upb_handlers_newmhandlers(); }
+
+ private:
+  FieldHandlers();  // Only created by Handlers::New().
+  ~FieldHandlers(); // Only destroyed by refcounting.
+};
+
+}  // namespace upb
+
+#endif
diff --git a/bindings/lua/test.lua b/bindings/lua/test.lua
new file mode 100644
index 0000000..42bce25
--- /dev/null
+++ b/bindings/lua/test.lua
@@ -0,0 +1,109 @@
+
+require "upb"
+
+symtab = upb.SymbolTable{
+  upb.MessageDef{fqname="A", fields={
+    upb.FieldDef{name="a", type=upb.TYPE_INT32, number=1},
+    upb.FieldDef{name="b", type=upb.TYPE_DOUBLE, number=2}}
+  }
+}
+
+symtab = upb.SymbolTable{
+  upb.MessageDef{fqname="A", fields={
+    upb.FieldDef{name="a", type=upb.TYPE_INT32, number=1},
+    upb.FieldDef{name="b", type=upb.TYPE_DOUBLE, number=2}}
+  },
+  upb.MessageDef{fqname="B"}
+}
+A, B, C = symtab:lookup("A", "B")
+print(A)
+print(B)
+print(C)
+
+a = A()
+a2 = upb.Message(A)
+print("YO!  a.a=" .. tostring(a.a) .. ", a2.a=" .. tostring(a2.a))
+a.a = 2
+a2.a = 3
+print("YO!  a.a=" .. tostring(a.a) .. ", a2.a=" .. tostring(a2.a))
+
+A = symtab:lookup("A")
+if not A then
+  error("Could not find A")
+end
+
+f = io.open("../../upb/descriptor.pb")
+if not f then
+  error("Couldn't open descriptor.pb, try running 'make descriptorgen'")
+end
+symtab:parsedesc(f:read("*all"))
+symtab:load_descriptor()
+symtab:load_descriptor_file()
+
+upb.pb.load_descriptor(f:read("*all"))
+
+upb.pb.load_descriptor_file("../../src/descriptor.pb", symtab)
+
+f = io.open("../../benchmarks/google_messages.proto.pb")
+if not f then
+  error("Couldn't open google_messages.proto.pb, try running 'make benchmarks'")
+end
+symtab:parsedesc(f:read("*all"))
+
+for _, def in ipairs(symtab:getdefs(-1)) do
+  print(def:name())
+end
+
+SpeedMessage1 = symtab:lookup("benchmarks.SpeedMessage1")
+SpeedMessage2 = symtab:lookup("benchmarks.SpeedMessage2")
+print(SpeedMessage1:name())
+
+msg = MyType()
+msg:Decode(str)
+
+msg:DecodeJSON(str)
+
+msg = upb.pb.decode(str, MyType)
+str = upb.pb.encode(msg)
+
+msg = upb.pb.decode_text(str, MyType)
+str = upb.pb.encode_text(msg)
+
+upb.clear(msg)
+upb.msgdef(msg)
+upb.has(msg, "foo_bar")
+
+msg = upb.json.decode(str, MyType)
+
+msg = upb.pb.DecodeText(str)
+msg = upb.pb.EncodeText(msg)
+upb.
+
+upb.pb.decode_into(msg, str)
+
+str = upb.json.Encode(msg)
+upb.json.DecodeInto(msg, str)
+f = assert(io.open("../../benchmarks/google_message1.dat"))
+msg:Parse(f:read("*all"))
+print(msg:ToText())
+print(upb.json.encode(msg))
+
+msg = SpeedMessage2()
+f = assert(io.open("../../benchmarks/google_message2.dat"))
+msg:Parse(f:read("*all"))
+print(msg:ToText())
+--msg:Serialize()
+--msg:FromText(str)
+-- print(msg.field129)
+-- print(msg.field271)
+--print(msg.field15.field15)
+--msg.field15.field15 = "my override"
+--print(msg.field15.field15)
+-- print(msg.field1)
+-- print(msg.field1)
+-- msg.field1 = "YEAH BABY!"
+-- print(msg.field1)
+-- print(msg.field129)
+-- msg.field129 = 5
+-- print(msg.field129)
+--]]
diff --git a/bindings/lua/upb.c b/bindings/lua/upb.c
new file mode 100644
index 0000000..fe523e7
--- /dev/null
+++ b/bindings/lua/upb.c
@@ -0,0 +1,1012 @@
+/*
+ * upb - a minimalist implementation of protocol buffers.
+ *
+ * Copyright (c) 2009 Google Inc.  See LICENSE for details.
+ * Author: Josh Haberman <jhaberman@gmail.com>
+ *
+ * A Lua extension for upb.
+ */
+
+#include <stdlib.h>
+#include <math.h>
+#include <float.h>
+#include "lauxlib.h"
+#include "upb/def.h"
+#include "upb/msg.h"
+#include "upb/pb/glue.h"
+
+#if LUA_VERSION_NUM == 501
+#define lua_rawlen lua_objlen
+#endif
+
+static bool streql(const char *a, const char *b) { return strcmp(a, b) == 0; }
+
+static bool lupb_isint(double n) { return (double)(int)n == n; }
+
+static uint8_t lupb_touint8(lua_State *L, int narg, const char *name) {
+  lua_Number n = lua_tonumber(L, narg);
+  if (n > UINT8_MAX || n < 0 || rint(n) != n)
+    luaL_error(L, "Invalid %s", name);
+  return n;
+}
+
+static uint32_t lupb_touint32(lua_State *L, int narg, const char *name) {
+  lua_Number n = lua_tonumber(L, narg);
+  if (n > UINT32_MAX || n < 0 || rint(n) != n)
+    luaL_error(L, "Invalid %s", name);
+  return n;
+}
+
+static void lupb_pushstring(lua_State *L, upb_strref *ref) {
+  if (ref->ptr) {
+    lua_pushlstring(L, ref->ptr, ref->len);
+  } else {
+    // Lua requires a continguous string; must copy+allocate.
+    char *str = upb_strref_dup(ref);
+    lua_pushlstring(L, str, ref->len);
+    free(str);
+  }
+}
+
+static void lupb_pushvalue(lua_State *L, upb_value val, upb_fielddef *f) {
+  switch (f->type) {
+    case UPB_TYPE(INT32):
+    case UPB_TYPE(SINT32):
+    case UPB_TYPE(SFIXED32):
+    case UPB_TYPE(ENUM):
+      lua_pushnumber(L, upb_value_getint32(val)); break;
+    case UPB_TYPE(INT64):
+    case UPB_TYPE(SINT64):
+    case UPB_TYPE(SFIXED64):
+      lua_pushnumber(L, upb_value_getint64(val)); break;
+    case UPB_TYPE(UINT32):
+    case UPB_TYPE(FIXED32):
+      lua_pushnumber(L, upb_value_getuint32(val)); break;
+    case UPB_TYPE(UINT64):
+    case UPB_TYPE(FIXED64):
+      lua_pushnumber(L, upb_value_getuint64(val)); break;
+    case UPB_TYPE(DOUBLE):
+      lua_pushnumber(L, upb_value_getdouble(val)); break;
+    case UPB_TYPE(FLOAT):
+      lua_pushnumber(L, upb_value_getfloat(val)); break;
+    case UPB_TYPE(BOOL):
+      lua_pushboolean(L, upb_value_getbool(val)); break;
+    default: luaL_error(L, "internal error");
+  }
+}
+
+// Returns a scalar value (ie. not a submessage) as a upb_value.
+static upb_value lupb_getvalue(lua_State *L, int narg, upb_fielddef *f,
+                               upb_strref *ref) {
+  assert(!upb_issubmsg(f));
+  upb_value val;
+  if (upb_fielddef_type(f) == UPB_TYPE(BOOL)) {
+    if (!lua_isboolean(L, narg))
+      luaL_error(L, "Must explicitly pass true or false for boolean fields");
+    upb_value_setbool(&val, lua_toboolean(L, narg));
+  } else if (upb_fielddef_type(f) == UPB_TYPE(STRING)) {
+    size_t len;
+    ref->ptr = luaL_checklstring(L, narg, &len);
+    ref->len = len;
+    upb_value_setstrref(&val, ref);
+  } else {
+    // Numeric type.
+    lua_Number num = 0;
+    num = luaL_checknumber(L, narg);
+    switch (upb_fielddef_type(f)) {
+      case UPB_TYPE(INT32):
+      case UPB_TYPE(SINT32):
+      case UPB_TYPE(SFIXED32):
+      case UPB_TYPE(ENUM):
+        if (num > INT32_MAX || num < INT32_MIN || num != rint(num))
+          luaL_error(L, "Cannot convert %f to 32-bit integer", num);
+        upb_value_setint32(&val, num);
+        break;
+      case UPB_TYPE(INT64):
+      case UPB_TYPE(SINT64):
+      case UPB_TYPE(SFIXED64):
+        if (num > INT64_MAX || num < INT64_MIN || num != rint(num))
+          luaL_error(L, "Cannot convert %f to 64-bit integer", num);
+        upb_value_setint64(&val, num);
+        break;
+      case UPB_TYPE(UINT32):
+      case UPB_TYPE(FIXED32):
+        if (num > UINT32_MAX || num < 0 || num != rint(num))
+          luaL_error(L, "Cannot convert %f to unsigned 32-bit integer", num);
+        upb_value_setuint32(&val, num);
+        break;
+      case UPB_TYPE(UINT64):
+      case UPB_TYPE(FIXED64):
+        if (num > UINT64_MAX || num < 0 || num != rint(num))
+          luaL_error(L, "Cannot convert %f to unsigned 64-bit integer", num);
+        upb_value_setuint64(&val, num);
+        break;
+      case UPB_TYPE(DOUBLE):
+        if (num > DBL_MAX || num < -DBL_MAX) {
+          // This could happen if lua_Number was long double.
+          luaL_error(L, "Cannot convert %f to double", num);
+        }
+        upb_value_setdouble(&val, num);
+        break;
+      case UPB_TYPE(FLOAT):
+        if (num > FLT_MAX || num < -FLT_MAX)
+          luaL_error(L, "Cannot convert %f to float", num);
+        upb_value_setfloat(&val, num);
+        break;
+    }
+  }
+  return val;
+}
+
+static void lupb_typecheck(lua_State *L, int narg, upb_fielddef *f) {
+  upb_strref ref;
+  lupb_getvalue(L, narg, f, &ref);
+}
+
+//static void lupb_msg_getorcreate(lua_State *L, upb_msg *msg, upb_msgdef *md);
+static void lupb_fielddef_getorcreate(lua_State *L, upb_fielddef *f);
+static upb_msgdef *lupb_msgdef_check(lua_State *L, int narg);
+static void lupb_msg_pushnew(lua_State *L, void *md);
+
+void lupb_checkstatus(lua_State *L, upb_status *s) {
+  if (!upb_ok(s)) {
+    upb_status_print(s, stderr);
+    // Need to copy the string to the stack, so we can free it and not leak
+    // it (since luaL_error() does not return).
+    char buf[strlen(s->str)+1];
+    strcpy(buf, s->str);
+    upb_status_uninit(s);
+    luaL_error(L, "%s", buf);
+  }
+  upb_status_uninit(s);
+}
+
+
+/* object cache ***************************************************************/
+
+// We cache all the lua objects (userdata) we vend in a weak table, indexed by
+// the C pointer of the object they are caching.
+
+static void *lupb_cache_getorcreate_size(
+    lua_State *L, void *cobj, const char *type, size_t size) {
+  // Lookup our cache in the registry (we don't put our objects in the registry
+  // directly because we need our cache to be a weak table).
+  void **obj = NULL;
+  lua_getfield(L, LUA_REGISTRYINDEX, "upb.objcache");
+  assert(!lua_isnil(L, -1));  // Should have been created by luaopen_upb.
+  lua_pushlightuserdata(L, cobj);
+  lua_rawget(L, -2);
+  // Stack: objcache, cached value.
+  if (lua_isnil(L, -1)) {
+    // Remove bad cached value and push new value.
+    lua_pop(L, 1);
+    // We take advantage of the fact that all of our objects are currently a
+    // single pointer, and thus have the same layout.
+    obj = lua_newuserdata(L, size);
+    *obj = cobj;
+    luaL_getmetatable(L, type);
+    assert(!lua_isnil(L, -1));  // Should have been created by luaopen_upb.
+    lua_setmetatable(L, -2);
+
+    // Set it in the cache.
+    lua_pushlightuserdata(L, cobj);
+    lua_pushvalue(L, -2);
+    lua_rawset(L, -4);
+  }
+  lua_insert(L, -2);
+  lua_pop(L, 1);
+  return obj;
+}
+
+// Most types are just 1 pointer and can use this helper.
+static bool lupb_cache_getorcreate(lua_State *L, void *cobj, const char *type) {
+  return lupb_cache_getorcreate_size(L, cobj, type, sizeof(void*)) != NULL;
+}
+
+static void lupb_cache_create(lua_State *L, void *cobj, const char *type) {
+  bool created =
+      lupb_cache_getorcreate_size(L, cobj, type, sizeof(void*)) != NULL;
+  (void)created;  // For NDEBUG
+  assert(created);
+}
+
+
+/* lupb_def *******************************************************************/
+
+// All the def types share the same C layout, even though they are different Lua
+// types with different metatables.
+typedef struct {
+  upb_def *def;
+} lupb_def;
+
+static lupb_def *lupb_def_check(lua_State *L, int narg) {
+  void *ldef = luaL_checkudata(L, narg, "upb.msgdef");
+  if (!ldef) ldef = luaL_checkudata(L, narg, "upb.enumdef");
+  if (!ldef) luaL_typerror(L, narg, "upb def");
+  return ldef;
+}
+
+static void lupb_def_getorcreate(lua_State *L, upb_def *def, int owned) {
+  bool created = false;
+  switch(def->type) {
+    case UPB_DEF_MSG:
+      created = lupb_cache_getorcreate(L, def, "upb.msgdef");
+      break;
+    case UPB_DEF_ENUM:
+      created = lupb_cache_getorcreate(L, def, "upb.enumdef");
+      break;
+    default:
+      luaL_error(L, "unknown deftype %d", def->type);
+  }
+  if (!owned && created) {
+    upb_def_ref(def);
+  } else if (owned && !created) {
+    upb_def_unref(def);
+  }
+}
+
+
+/* lupb_fielddef **************************************************************/
+
+typedef struct {
+  upb_fielddef *field;
+} lupb_fielddef;
+
+static lupb_fielddef *lupb_fielddef_check(lua_State *L, int narg) {
+  lupb_fielddef *f = luaL_checkudata(L, narg, "upb.fielddef");
+  if (!f) luaL_typerror(L, narg, "upb fielddef");
+  return f;
+}
+
+static int lupb_fielddef_index(lua_State *L) {
+  lupb_fielddef *f = lupb_fielddef_check(L, 1);
+  const char *str = luaL_checkstring(L, 2);
+  if (streql(str, "name")) {
+    lua_pushstring(L, upb_fielddef_name(f->field));
+  } else if (streql(str, "number")) {
+    lua_pushinteger(L, upb_fielddef_number(f->field));
+  } else if (streql(str, "type")) {
+    lua_pushinteger(L, upb_fielddef_type(f->field));
+  } else if (streql(str, "label")) {
+    lua_pushinteger(L, upb_fielddef_label(f->field));
+  } else if (streql(str, "subdef")) {
+    lupb_def_getorcreate(L, upb_fielddef_subdef(f->field), false);
+  } else if (streql(str, "msgdef")) {
+    lupb_def_getorcreate(L, UPB_UPCAST(upb_fielddef_msgdef(f->field)), false);
+  } else {
+    luaL_error(L, "Invalid fielddef member '%s'", str);
+  }
+  return 1;
+}
+
+static void lupb_fielddef_set(lua_State *L, upb_fielddef *f,
+                              const char *field, int narg) {
+  if (!upb_fielddef_ismutable(f)) luaL_error(L, "fielddef is not mutable.");
+  if (streql(field, "name")) {
+    const char *name = lua_tostring(L, narg);
+    if (!name || !upb_fielddef_setname(f, name))
+      luaL_error(L, "Invalid name");
+  } else if (streql(field, "number")) {
+    if (!upb_fielddef_setnumber(f, lupb_touint32(L, narg, "number")))
+      luaL_error(L, "Invalid number");
+  } else if (streql(field, "type")) {
+    if (!upb_fielddef_settype(f, lupb_touint8(L, narg, "type")))
+      luaL_error(L, "Invalid type");
+  } else if (streql(field, "label")) {
+    if (!upb_fielddef_setlabel(f, lupb_touint8(L, narg, "label")))
+      luaL_error(L, "Invalid label");
+  } else if (streql(field, "type_name")) {
+    const char *name = lua_tostring(L, narg);
+    if (!name || !upb_fielddef_settypename(f, name))
+      luaL_error(L, "Invalid type_name");
+  } else if (streql(field, "default_value")) {
+    if (!upb_fielddef_type(f))
+      luaL_error(L, "Must set type before setting default_value");
+    upb_strref ref;
+    upb_fielddef_setdefault(f, lupb_getvalue(L, narg, f, &ref));
+  } else {
+    luaL_error(L, "Cannot set fielddef member '%s'", field);
+  }
+}
+
+static int lupb_fielddef_new(lua_State *L) {
+  upb_fielddef *f = upb_fielddef_new();
+  lupb_cache_create(L, f, "upb.fielddef");
+
+  if (lua_gettop(L) == 0) return 1;
+
+  // User can specify initialization values like so:
+  //   upb.FieldDef{label=upb.LABEL_REQUIRED, name="my_field", number=5,
+  //                type=upb.TYPE_INT32, default_value=12, type_name="Foo"}
+  luaL_checktype(L, 1, LUA_TTABLE);
+  // Iterate over table.
+  lua_pushnil(L);  // first key
+  while (lua_next(L, 1)) {
+    luaL_checktype(L, -2, LUA_TSTRING);
+    const char *key = lua_tostring(L, -2);
+    lupb_fielddef_set(L, f, key, -1);
+    lua_pop(L, 1);
+  }
+  return 1;
+}
+
+static void lupb_fielddef_getorcreate(lua_State *L, upb_fielddef *f) {
+  bool created = lupb_cache_getorcreate(L, f, "upb.fielddef");
+  if (created) upb_fielddef_ref(f);
+}
+
+static int lupb_fielddef_newindex(lua_State *L) {
+  lupb_fielddef *f = lupb_fielddef_check(L, 1);
+  lupb_fielddef_set(L, f->field, luaL_checkstring(L, 2), 3);
+  return 0;
+}
+
+static int lupb_fielddef_gc(lua_State *L) {
+  lupb_fielddef *lfielddef = lupb_fielddef_check(L, 1);
+  upb_fielddef_unref(lfielddef->field);
+  return 0;
+}
+
+static const struct luaL_Reg lupb_fielddef_mm[] = {
+  {"__gc", lupb_fielddef_gc},
+  {"__index", lupb_fielddef_index},
+  {"__newindex", lupb_fielddef_newindex},
+  {NULL, NULL}
+};
+
+
+/* lupb_msgdef ****************************************************************/
+
+static upb_msgdef *lupb_msgdef_check(lua_State *L, int narg) {
+  lupb_def *ldef = luaL_checkudata(L, narg, "upb.msgdef");
+  luaL_argcheck(L, ldef != NULL, narg, "upb msgdef expected");
+  return upb_downcast_msgdef(ldef->def);
+}
+
+static int lupb_msgdef_gc(lua_State *L) {
+  lupb_def *ldef = luaL_checkudata(L, 1, "upb.msgdef");
+  upb_def_unref(ldef->def);
+  return 0;
+}
+
+static int lupb_msgdef_call(lua_State *L) {
+  upb_msgdef *md = lupb_msgdef_check(L, 1);
+  lupb_msg_pushnew(L, md);
+  return 1;
+}
+
+static int lupb_msgdef_new(lua_State *L) {
+  upb_msgdef *md = upb_msgdef_new();
+  lupb_cache_create(L, md, "upb.msgdef");
+
+  if (lua_gettop(L) == 0) return 1;
+
+  // User can specify initialization values like so:
+  //   upb.MessageDef{fqname="MyMessage", extstart=8000, fields={...}}
+  luaL_checktype(L, 1, LUA_TTABLE);
+  // Iterate over table.
+  lua_pushnil(L);  // first key
+  while (lua_next(L, 1)) {
+    luaL_checktype(L, -2, LUA_TSTRING);
+    const char *key = lua_tostring(L, -2);
+
+    if (streql(key, "fqname")) {  // fqname="MyMessage"
+      const char *fqname = lua_tostring(L, -1);
+      if (!fqname || !upb_def_setfqname(UPB_UPCAST(md), fqname))
+        luaL_error(L, "Invalid fqname");
+    } else if (streql(key, "fields")) {  // fields={...}
+      // Iterate over the list of fields.
+      lua_pushnil(L);
+      luaL_checktype(L, -2, LUA_TTABLE);
+      while (lua_next(L, -2)) {
+        lupb_fielddef *f = lupb_fielddef_check(L, -1);
+        if (!upb_msgdef_addfield(md, f->field)) {
+          // TODO: more specific error.
+          luaL_error(L, "Could not add field.");
+        }
+        lua_pop(L, 1);
+      }
+    } else {
+      // TODO: extrange=
+      luaL_error(L, "Unknown initializer key '%s'", key);
+    }
+    lua_pop(L, 1);
+  }
+  return 1;
+}
+
+static int lupb_msgdef_fqname(lua_State *L) {
+  upb_msgdef *m = lupb_msgdef_check(L, 1);
+  lua_pushstring(L, m->base.fqname);
+  return 1;
+}
+
+static int lupb_msgdef_fieldbyname(lua_State *L) {
+  upb_msgdef *m = lupb_msgdef_check(L, 1);
+  upb_fielddef *f = upb_msgdef_ntof(m, luaL_checkstring(L, 2));
+  if (f) {
+    lupb_fielddef_getorcreate(L, f);
+  } else {
+    lua_pushnil(L);
+  }
+  return 1;
+}
+
+static int lupb_msgdef_fieldbynum(lua_State *L) {
+  upb_msgdef *m = lupb_msgdef_check(L, 1);
+  int num = luaL_checkint(L, 2);
+  upb_fielddef *f = upb_msgdef_itof(m, num);
+  if (f) {
+    lupb_fielddef_getorcreate(L, f);
+  } else {
+    lua_pushnil(L);
+  }
+  return 1;
+}
+
+static const struct luaL_Reg lupb_msgdef_mm[] = {
+  {"__call", lupb_msgdef_call},
+  {"__gc", lupb_msgdef_gc},
+  {NULL, NULL}
+};
+
+static const struct luaL_Reg lupb_msgdef_m[] = {
+  {"fieldbyname", lupb_msgdef_fieldbyname},
+  {"fieldbynum", lupb_msgdef_fieldbynum},
+  {"fqname", lupb_msgdef_fqname},
+  {NULL, NULL}
+};
+
+
+/* lupb_enumdef ***************************************************************/
+
+static upb_enumdef *lupb_enumdef_check(lua_State *L, int narg) {
+  lupb_def *ldef = luaL_checkudata(L, narg, "upb.enumdef");
+  return upb_downcast_enumdef(ldef->def);
+}
+
+static int lupb_enumdef_gc(lua_State *L) {
+  upb_enumdef *e = lupb_enumdef_check(L, 1);
+  upb_def_unref(UPB_UPCAST(e));
+  return 0;
+}
+
+static int lupb_enumdef_name(lua_State *L) {
+  upb_enumdef *e = lupb_enumdef_check(L, 1);
+  lua_pushstring(L, e->base.fqname);
+  return 1;
+}
+
+static const struct luaL_Reg lupb_enumdef_mm[] = {
+  {"__gc", lupb_enumdef_gc},
+  {NULL, NULL}
+};
+
+static const struct luaL_Reg lupb_enumdef_m[] = {
+  {"name", lupb_enumdef_name},
+  {NULL, NULL}
+};
+
+
+/* lupb_symtab ****************************************************************/
+
+typedef struct {
+  upb_symtab *symtab;
+} lupb_symtab;
+
+static upb_accessor_vtbl *lupb_accessor(upb_fielddef *f);
+
+// Inherits a ref on the symtab.
+// Checks that narg is a proper lupb_symtab object.  If it is, leaves its
+// metatable on the stack for cache lookups/updates.
+lupb_symtab *lupb_symtab_check(lua_State *L, int narg) {
+  return luaL_checkudata(L, narg, "upb.symtab");
+}
+
+// narg is a lua table containing a list of defs to add.
+void lupb_symtab_doadd(lua_State *L, upb_symtab *s, int narg) {
+  luaL_checktype(L, narg, LUA_TTABLE);
+  // Iterate over table twice.  First iteration to count entries and
+  // check constraints.
+  int n = 0;
+  lua_pushnil(L);  // first key
+  while (lua_next(L, narg)) {
+    lupb_def_check(L, -1);
+    ++n;
+    lua_pop(L, 1);
+  }
+
+  // Second iteration to build deflist and layout.
+  upb_def **defs = malloc(n * sizeof(*defs));
+  n = 0;
+  lua_pushnil(L);  // first key
+  while (lua_next(L, 1)) {
+    upb_def *def = lupb_def_check(L, -1)->def;
+    defs[n++] = def;
+    upb_msgdef *md = upb_dyncast_msgdef(def);
+    if (md) {
+      upb_msg_iter i;
+      for(i = upb_msg_begin(md); !upb_msg_done(i); i = upb_msg_next(md, i)) {
+        upb_fielddef *f = upb_msg_iter_field(i);
+        upb_fielddef_setaccessor(f, lupb_accessor(f));
+      }
+      upb_msgdef_layout(md);
+    }
+    lua_pop(L, 1);
+  }
+
+  upb_status status = UPB_STATUS_INIT;
+  upb_symtab_add(s, defs, n, &status);
+  free(defs);
+  lupb_checkstatus(L, &status);
+}
+
+static int lupb_symtab_new(lua_State *L) {
+  upb_symtab *s = upb_symtab_new();
+  lupb_cache_create(L, s, "upb.symtab");
+  if (lua_gettop(L) == 0) return 1;
+  lupb_symtab_doadd(L, s, 1);
+  return 1;
+}
+
+static int lupb_symtab_add(lua_State *L) {
+  lupb_symtab *s = lupb_symtab_check(L, 1);
+  lupb_symtab_doadd(L, s->symtab, 2);
+  return 0;
+}
+
+static int lupb_symtab_gc(lua_State *L) {
+  lupb_symtab *s = lupb_symtab_check(L, 1);
+  upb_symtab_unref(s->symtab);
+  return 0;
+}
+
+static int lupb_symtab_lookup(lua_State *L) {
+  lupb_symtab *s = lupb_symtab_check(L, 1);
+  for (int i = 2; i <= lua_gettop(L); i++) {
+    upb_def *def = upb_symtab_lookup(s->symtab, luaL_checkstring(L, i));
+    if (def) {
+      lupb_def_getorcreate(L, def, true);
+    } else {
+      lua_pushnil(L);
+    }
+    lua_replace(L, i);
+  }
+  return lua_gettop(L) - 1;
+}
+
+static int lupb_symtab_getdefs(lua_State *L) {
+  lupb_symtab *s = lupb_symtab_check(L, 1);
+  upb_deftype_t type = luaL_checkint(L, 2);
+  int count;
+  upb_def **defs = upb_symtab_getdefs(s->symtab, &count, type);
+
+  // Create the table in which we will return the defs.
+  lua_createtable(L, count, 0);
+  for (int i = 0; i < count; i++) {
+    upb_def *def = defs[i];
+    lupb_def_getorcreate(L, def, true);
+    lua_rawseti(L, -2, i + 1);
+  }
+  free(defs);
+  return 1;
+}
+
+static const struct luaL_Reg lupb_symtab_m[] = {
+  {"add", lupb_symtab_add},
+  {"getdefs", lupb_symtab_getdefs},
+  {"lookup", lupb_symtab_lookup},
+  {NULL, NULL}
+};
+
+static const struct luaL_Reg lupb_symtab_mm[] = {
+  {"__gc", lupb_symtab_gc},
+  {NULL, NULL}
+};
+
+
+/* lupb_msg********************************************************************/
+
+// Messages are userdata.  Primitive values (numbers and bools, and their
+// hasbits) are stored right in the userdata.  Other values are stored using
+// integer entries in the environment table and no hasbits are used (since
+// "nil" in the environment table can indicate "not present").
+//
+// The environment table looks like:
+//   {msgdef, <string, submessage, and array fields>}
+
+// Must pass a upb_fielddef as the pointer.
+static void lupb_array_pushnew(lua_State *L, void *f);
+
+static void *lupb_msg_check(lua_State *L, int narg, upb_msgdef **md) {
+  void *msg = luaL_checkudata(L, narg, "upb.msg");
+  luaL_argcheck(L, msg != NULL, narg, "msg expected");
+  // If going all the way to the environment table for the msgdef is an
+  // efficiency issue, we could put the pointer right in the userdata.
+  lua_getfenv(L, narg);
+  lua_rawgeti(L, -1, 1);
+  // Shouldn't have to check msgdef userdata validity, environment table can't
+  // be accessed from Lua.
+  lupb_def *lmd = lua_touserdata(L, -1);
+  *md = upb_downcast_msgdef(lmd->def);
+  return msg;
+}
+
+static void lupb_msg_pushnew(lua_State *L, void *md) {
+  void *msg = lua_newuserdata(L, upb_msgdef_size(md));
+  luaL_getmetatable(L, "upb.msg");
+  assert(!lua_isnil(L, -1));  // Should have been created by luaopen_upb.
+  lua_setmetatable(L, -2);
+  upb_msg_clear(msg, md);
+  lua_getfenv(L, -1);
+  lupb_cache_getorcreate(L, md, "upb.msgdef");
+  lua_rawseti(L, -2, 1);
+  lua_pop(L, 1);  // Pop the fenv.
+}
+
+static int lupb_msg_new(lua_State *L) {
+  upb_msgdef *md = lupb_msgdef_check(L, 1);
+  lupb_msg_pushnew(L, md);
+  return 1;
+}
+
+static int lupb_msg_index(lua_State *L) {
+  upb_msgdef *md;
+  void *m = lupb_msg_check(L, 1, &md);
+  upb_fielddef *f = upb_msgdef_ntof(md, luaL_checkstring(L, 2));
+  if (!f) luaL_argerror(L, 2, "not a field name");
+  if (upb_isprimitivetype(upb_fielddef_type(f))) {
+    upb_value v = upb_msg_has(m, f) ? upb_msg_get(m, f) : upb_fielddef_default(f);
+    lupb_pushvalue(L, v, f);
+  } else {
+    lua_getfenv(L, 1);
+    lua_rawgeti(L, -1, f->offset);
+    if (lua_isnil(L, -1)) {
+      // Need to lazily create array, string, or submessage.
+      if (upb_isseq(f)) {
+        lupb_array_pushnew(L, f);
+      } else if (upb_isstring(f)) {
+        // TODO: (need to figure out default string ownership).
+      } else if (upb_issubmsg(f)) {
+        lupb_msg_pushnew(L, upb_downcast_msgdef(upb_fielddef_subdef(f)));
+      } else {
+        luaL_error(L, "internal error");
+      }
+      lua_rawseti(L, -2, f->offset);
+    }
+  }
+  return 1;
+}
+
+static int lupb_msg_newindex(lua_State *L) {
+  upb_msgdef *md;
+  void *m = lupb_msg_check(L, 1, &md);
+  upb_fielddef *f = upb_msgdef_ntof(md, luaL_checkstring(L, 2));
+  if (!f) luaL_error(L, "not a field name");
+  if (upb_isprimitivetype(upb_fielddef_type(f))) {
+    if (lua_isnil(L, 3))
+      upb_msg_clearbit(m, f);
+    else
+      upb_msg_set(m, f, lupb_getvalue(L, 3, f, NULL));
+  } else {
+    if (!lua_isnil(L, 3)) lupb_typecheck(L, 3, f);
+    lua_getfenv(L, 1);
+    lua_pushvalue(L, 3);
+    lua_rawseti(L, -1, f->offset);
+  }
+  return 0;
+}
+
+static const struct luaL_Reg lupb_msg_mm[] = {
+  {"__index", lupb_msg_index},
+  {"__newindex", lupb_msg_newindex},
+  {NULL, NULL}
+};
+
+// Functions that operate on msgdefs but do not live in the msgdef namespace.
+static int lupb_clear(lua_State *L) {
+  upb_msgdef *md;
+  void *m = lupb_msg_check(L, 1, &md);
+  upb_msg_clear(m, md);
+  return 0;
+}
+
+static int lupb_has(lua_State *L) {
+  upb_msgdef *md;
+  void *m = lupb_msg_check(L, 1, &md);
+  upb_fielddef *f = upb_msgdef_ntof(md, luaL_checkstring(L, 2));
+  if (!f) luaL_argerror(L, 2, "not a field name");
+  lua_pushboolean(L, upb_msg_has(m, f));
+  return 1;
+}
+
+static int lupb_msgdef(lua_State *L) {
+  upb_msgdef *md;
+  lupb_msg_check(L, 1, &md);
+  lupb_def_getorcreate(L, UPB_UPCAST(md), false);
+  return 1;
+}
+
+// Accessors for arrays, strings, and submessages need access to the current
+// userdata's environment table, which can only be stored in Lua space.
+// Options for storing it are:
+//
+// - put the env tables for all messages and arrays in the registry, keyed by
+//   userdata pointer (light userdata), or by a reference using luaL_ref().
+//   Then we can just let upb's parse stack track the stack of env tables.
+//   Easy but requires all messages and arrays to be in the registry, which
+//   seems too heavyweight.
+//
+// - store the stack of env tables in the Lua stack.  Convenient, but requires
+//   special code to handle resumable decoders.
+//
+// There is also the question of how to obtain the lua_State* pointer.
+// The main options for this are:
+//
+// - make our closure point to a struct:
+//     struct { void *msg; lua_State *L; }
+//   But then we can't use standard accessors, which expect the closure
+//   to point to the data itself.  Using the standard accessors for
+//   primitive values is both a simplicity and a performance win.
+//
+// - store a lua_State* pointer inside each userdata.  Convenient and
+//   efficient, but makes every message sizeof(void*) larger.
+//   Currently we take this route.
+//
+// - use thread-local storage.  Convenient and efficient, but not portable.
+
+typedef void createfunc_t(lua_State *L, void *param);
+
+static upb_sflow_t lupb_msg_start(void *m, upb_fielddef *f, bool array,
+                                  createfunc_t *pushnew, void *param) {
+  lua_State *L = *(lua_State**)m;
+  int offset = array ? lua_rawlen(L, -1) : f->offset;
+  if (!lua_checkstack(L, 3)) luaL_error(L, "stack full");
+  lua_rawgeti(L, -1, offset);
+  if (lua_isnil(L, -1)) {
+    lua_pop(L, 1);
+    pushnew(L, param);
+    lua_pushvalue(L, -1);
+    lua_rawseti(L, -3, offset);
+  }
+  void *subval = lua_touserdata(L, -1);
+  lua_getfenv(L, -1);
+  lua_replace(L, -2);  // Replace subval userdata with fenv.
+  return UPB_CONTINUE_WITH(subval);
+}
+
+static upb_flow_t lupb_msg_string(void *m, upb_value fval, upb_value val,
+                                  bool array) {
+  // Could add lazy materialization of strings here.
+  upb_fielddef *f = upb_value_getfielddef(fval);
+  lua_State *L = *(lua_State**)m;
+  int offset = array ? lua_rawlen(L, -1) : f->offset;
+  if (!lua_checkstack(L, 1)) luaL_error(L, "stack full");
+  lupb_pushstring(L, upb_value_getstrref(val));
+  lua_rawseti(L, -2, offset);
+  return UPB_CONTINUE;
+}
+
+static upb_sflow_t lupb_msg_startseq(void *m, upb_value fval) {
+  upb_fielddef *f = upb_value_getfielddef(fval);
+  return lupb_msg_start(m, f, false, lupb_array_pushnew, f);
+}
+
+static upb_sflow_t lupb_msg_startsubmsg(void *m, upb_value fval) {
+  upb_fielddef *f = upb_value_getfielddef(fval);
+  return lupb_msg_start(m, f, false, lupb_msg_pushnew, upb_fielddef_subdef(f));
+}
+
+static upb_sflow_t lupb_msg_startsubmsg_r(void *a, upb_value fval) {
+  upb_fielddef *f = upb_value_getfielddef(fval);
+  return lupb_msg_start(a, f, true, lupb_msg_pushnew, upb_fielddef_subdef(f));
+}
+
+static upb_flow_t lupb_msg_stringval(void *m, upb_value fval, upb_value val) {
+  return lupb_msg_string(m, fval, val, false);
+}
+
+static upb_flow_t lupb_msg_stringval_r(void *a, upb_value fval, upb_value val) {
+  return lupb_msg_string(a, fval, val, true);
+}
+
+#define STDMSG(type, size) static upb_accessor_vtbl vtbl = { \
+    &lupb_msg_startsubmsg, \
+    &upb_stdmsg_set ## type, \
+    &lupb_msg_startseq, \
+    &lupb_msg_startsubmsg_r, \
+    &upb_stdmsg_set ## type ## _r, \
+    &upb_stdmsg_has, \
+    &upb_stdmsg_getptr, \
+    &upb_stdmsg_get ## type, \
+    &upb_stdmsg_seqbegin, \
+    &upb_stdmsg_ ## size ## byte_seqnext, \
+    &upb_stdmsg_seqget ## type};
+
+#define RETURN_STDMSG(type, size) { STDMSG(type, size); return &vtbl; }
+
+static upb_accessor_vtbl *lupb_accessor(upb_fielddef *f) {
+  switch (f->type) {
+    case UPB_TYPE(DOUBLE): RETURN_STDMSG(double, 8)
+    case UPB_TYPE(FLOAT): RETURN_STDMSG(float, 4)
+    case UPB_TYPE(UINT64):
+    case UPB_TYPE(FIXED64): RETURN_STDMSG(uint64, 8)
+    case UPB_TYPE(INT64):
+    case UPB_TYPE(SFIXED64):
+    case UPB_TYPE(SINT64): RETURN_STDMSG(int64, 8)
+    case UPB_TYPE(INT32):
+    case UPB_TYPE(SINT32):
+    case UPB_TYPE(ENUM):
+    case UPB_TYPE(SFIXED32): RETURN_STDMSG(int32, 4)
+    case UPB_TYPE(UINT32):
+    case UPB_TYPE(FIXED32): RETURN_STDMSG(uint32, 4)
+    case UPB_TYPE(BOOL): { STDMSG(bool, 1); return &vtbl; }
+    case UPB_TYPE(GROUP):
+    case UPB_TYPE(MESSAGE): RETURN_STDMSG(ptr, 8)  // TODO: 32-bit
+    case UPB_TYPE(STRING):
+    case UPB_TYPE(BYTES): {
+      STDMSG(ptr, 8);
+      vtbl.set = &lupb_msg_stringval;
+      vtbl.append = &lupb_msg_stringval_r;
+      return &vtbl;
+    }
+  }
+  return NULL;
+}
+
+
+/* lupb_array ****************************************************************/
+
+// Array: we store all elements in the environment table.  Could optimize by
+// storing primitive arrays in our own memory; this would be significantly more
+// space efficient.  Lua array elements are 16 bytes each; our own array would
+// be 1/4 the space for 32-bit integers, or 1/16 the space for booleans.
+//
+// The first element of the environment table stores our type (which will be
+// either an integer from upb.TYPE_* or a upb.msgdef), the remaining elements
+// store the elements.  We always keep all elements contiguous so we can use
+// lua_objlen()/lua_rawlen() (for Lua 5.1/5.2 respectively) to report its len).
+
+// narg is offset of environment table.
+static size_t lupb_array_getlen(lua_State *L, int narg) {
+  return lua_rawlen(L, narg) - 1;
+}
+
+static void lupb_array_check(lua_State *L, int narg) {
+  if (!luaL_checkudata(L, narg, "upb.array"))
+    luaL_typerror(L, narg, "upb array");
+}
+
+static void lupb_array_pushnew(lua_State *L, void *f) {
+  (void)L;
+  (void)f;
+}
+
+static int lupb_array_new(lua_State *L) {
+  (void)L;
+  return 0;
+}
+
+static int lupb_array_len(lua_State *L) {
+  lupb_array_check(L, 1);
+  lua_getfenv(L, 1);
+  lua_pushnumber(L, lupb_array_getlen(L, -1));
+  return 1;
+}
+
+static int lupb_array_index(lua_State *L) {
+  assert(lua_gettop(L) == 2);  // __index should always be called with 2 args.
+  lupb_array_check(L, 1);
+  lua_Number num = luaL_checknumber(L, 2);
+  if (!lupb_isint(num)) luaL_typerror(L, 2, "integer");
+
+  lua_getfenv(L, 1);
+  size_t len = lupb_array_getlen(L, -1);
+  if (num < 1 || num > len) luaL_error(L, "array bounds check failed");
+  lua_rawgeti(L, -1, num + 1);
+  return 1;
+}
+
+static int lupb_array_newindex(lua_State *L) {
+  assert(lua_gettop(L) == 3);  // __newindex should always be called with 3 args.
+  lupb_array_check(L, 1);
+  lua_Number num = luaL_checknumber(L, 2);
+  if (rint(num) != num) luaL_typerror(L, 2, "integer");
+
+  lua_getfenv(L, 1);
+  size_t len = lupb_array_getlen(L, -1);
+  // We only allow extending the index one beyond the end.
+  if (num < 1 || num > len + 1) luaL_error(L, "array bounds check failed");
+  lua_pushvalue(L, 3);
+  lua_rawseti(L, -2, num);
+  return 0;
+}
+
+static const struct luaL_Reg lupb_array_mm[] = {
+  {"__len", lupb_array_len},
+  {"__index", lupb_array_index},
+  {"__newindex", lupb_array_newindex},
+  {NULL, NULL}
+};
+
+/* lupb toplevel **************************************************************/
+
+static const struct luaL_Reg lupb_toplevel_m[] = {
+  {"SymbolTable", lupb_symtab_new},
+  {"MessageDef", lupb_msgdef_new},
+  {"FieldDef", lupb_fielddef_new},
+
+  {"Message", lupb_msg_new},
+  {"Array", lupb_array_new},
+
+  {"clear", lupb_clear},
+  {"msgdef", lupb_msgdef},
+  {"has", lupb_has},
+
+  {NULL, NULL}
+};
+
+// Register the given type with the given methods and metamethods.
+static void lupb_register_type(lua_State *L, const char *name,
+                               const luaL_Reg *m, const luaL_Reg *mm) {
+  luaL_newmetatable(L, name);
+  luaL_register(L, NULL, mm);  // Register all mm in the metatable.
+  lua_createtable(L, 0, 0);
+  if (m) {
+    // Methods go in the mt's __index method.  This implies that you can't
+    // implement __index and also set methods yourself.
+    luaL_register(L, NULL, m);
+    lua_setfield(L, -2, "__index");
+  }
+  lua_pop(L, 1);  // The mt.
+}
+
+static void lupb_setfieldi(lua_State *L, const char *field, int i) {
+  lua_pushnumber(L, i);
+  lua_setfield(L, -2, field);
+}
+
+int luaopen_upb(lua_State *L) {
+  lupb_register_type(L, "upb.msgdef", lupb_msgdef_m, lupb_msgdef_mm);
+  lupb_register_type(L, "upb.enumdef", lupb_enumdef_m, lupb_enumdef_mm);
+  lupb_register_type(L, "upb.fielddef", NULL, lupb_fielddef_mm);
+  lupb_register_type(L, "upb.symtab", lupb_symtab_m, lupb_symtab_mm);
+
+  lupb_register_type(L, "upb.msg", NULL, lupb_msg_mm);
+  lupb_register_type(L, "upb.array", NULL, lupb_msg_mm);
+
+  // Create our object cache.
+  lua_createtable(L, 0, 0);
+  lua_createtable(L, 0, 1);  // Cache metatable.
+  lua_pushstring(L, "v");    // Values are weak.
+  lua_setfield(L, -2, "__mode");
+  lua_setfield(L, LUA_REGISTRYINDEX, "upb.objcache");
+
+  luaL_register(L, "upb", lupb_toplevel_m);
+
+  // Register constants.
+  lupb_setfieldi(L, "LABEL_OPTIONAL", UPB_LABEL(OPTIONAL));
+  lupb_setfieldi(L, "LABEL_REQUIRED", UPB_LABEL(REQUIRED));
+  lupb_setfieldi(L, "LABEL_REPEATED", UPB_LABEL(REPEATED));
+
+  lupb_setfieldi(L, "TYPE_DOUBLE", UPB_TYPE(DOUBLE));
+  lupb_setfieldi(L, "TYPE_FLOAT", UPB_TYPE(FLOAT));
+  lupb_setfieldi(L, "TYPE_INT64", UPB_TYPE(INT64));
+  lupb_setfieldi(L, "TYPE_UINT64", UPB_TYPE(UINT64));
+  lupb_setfieldi(L, "TYPE_INT32", UPB_TYPE(INT32));
+  lupb_setfieldi(L, "TYPE_FIXED64", UPB_TYPE(FIXED64));
+  lupb_setfieldi(L, "TYPE_FIXED32", UPB_TYPE(FIXED32));
+  lupb_setfieldi(L, "TYPE_BOOL", UPB_TYPE(BOOL));
+  lupb_setfieldi(L, "TYPE_STRING", UPB_TYPE(STRING));
+  lupb_setfieldi(L, "TYPE_GROUP", UPB_TYPE(GROUP));
+  lupb_setfieldi(L, "TYPE_MESSAGE", UPB_TYPE(MESSAGE));
+  lupb_setfieldi(L, "TYPE_BYTES", UPB_TYPE(BYTES));
+  lupb_setfieldi(L, "TYPE_UINT32", UPB_TYPE(UINT32));
+  lupb_setfieldi(L, "TYPE_ENUM", UPB_TYPE(ENUM));
+  lupb_setfieldi(L, "TYPE_SFIXED32", UPB_TYPE(SFIXED32));
+  lupb_setfieldi(L, "TYPE_SFIXED64", UPB_TYPE(SFIXED64));
+  lupb_setfieldi(L, "TYPE_SINT32", UPB_TYPE(SINT32));
+  lupb_setfieldi(L, "TYPE_SINT64", UPB_TYPE(SINT64));
+
+  return 1;  // Return package table.
+}
diff --git a/bindings/python/setup.py b/bindings/python/setup.py
new file mode 100644
index 0000000..8abaff8
--- /dev/null
+++ b/bindings/python/setup.py
@@ -0,0 +1,14 @@
+from distutils.core import setup, Extension
+
+setup(name='upb',
+      version='0.1',
+      ext_modules=[
+          Extension('upb.__init__', ['upb.c'],
+              include_dirs=['../../'],
+              define_macros=[("UPB_UNALIGNED_READS_OK", 1)],
+              library_dirs=['../../upb'],
+              libraries=['upb_pic'],
+          ),
+      ],
+      packages=['upb']
+      )
diff --git a/bindings/python/test.py b/bindings/python/test.py
new file mode 100644
index 0000000..29a6c45
--- /dev/null
+++ b/bindings/python/test.py
@@ -0,0 +1,72 @@
+
+import upb
+import unittest
+
+class TestFieldDef(unittest.TestCase):
+  def test_construction(self):
+    fielddef1 = upb.FieldDef()
+    self.assertTrue(fielddef1.number is None)
+    self.assertTrue(fielddef1.name is None)
+    self.assertTrue(fielddef1.type is None)
+    self.assertEqual(fielddef1.label, upb.LABEL_OPTIONAL)
+
+    fielddef2 = upb.FieldDef(number=5, name="field2",
+                             label=upb.LABEL_REQUIRED, type=upb.TYPE_INT32,
+                             type_name="MyType")
+
+    self.assertTrue(id(fielddef1) != id(fielddef2))
+    self.assertEqual(fielddef2.number, 5)
+    self.assertEqual(fielddef2.name, "field2")
+    self.assertEqual(fielddef2.label, upb.LABEL_REQUIRED)
+    self.assertEqual(fielddef2.type, upb.TYPE_INT32)
+    self.assertEqual(fielddef2.type_name, "MyType")
+
+    fielddef2.number = 8
+    self.assertEqual(fielddef2.number, 8)
+
+    fielddef2.name = "xxx"
+    self.assertEqual(fielddef2.name, "xxx")
+
+    fielddef2.label = upb.LABEL_REPEATED
+    self.assertEqual(fielddef2.label, upb.LABEL_REPEATED)
+
+    fielddef2.type = upb.TYPE_FLOAT
+    self.assertEqual(fielddef2.type, upb.TYPE_FLOAT)
+
+  def test_nosubclasses(self):
+    def create_subclass():
+      class MyClass(upb.FieldDef):
+        pass
+
+    self.assertRaises(TypeError, create_subclass)
+
+  # TODO: test that assigning invalid values is properly prevented.
+
+class TestMessageDef(unittest.TestCase):
+  def test_construction(self):
+    msgdef1 = upb.MessageDef()
+    self.assertTrue(msgdef1.fqname is None)
+    self.assertEqual(msgdef1.fields(), [])
+
+    fields = [upb.FieldDef(number=1, name="field1", type=upb.TYPE_INT32)]
+    msgdef2 = upb.MessageDef(fqname="Message2", fields=fields)
+
+    self.assertEqual(set(msgdef2.fields()), set(fields))
+
+    f2 = upb.FieldDef(number=2, name="field2", type=upb.TYPE_INT64)
+    msgdef2.add_field(f2)
+
+    fields.append(f2)
+    self.assertEqual(set(msgdef2.fields()), set(fields))
+
+class TestSymbolTable(unittest.TestCase):
+  def test_construction(self):
+    s = upb.SymbolTable()
+    self.assertEqual(s.defs(), []);
+
+    s.add_def(upb.MessageDef(fqname="A"))
+    self.assertTrue(s.lookup("A") is not None)
+    self.assertTrue(s.lookup("A") is s.lookup("A"))
+
+if __name__ == '__main__':
+  unittest.main()
diff --git a/bindings/python/upb.c b/bindings/python/upb.c
new file mode 100644
index 0000000..ffb3d52
--- /dev/null
+++ b/bindings/python/upb.c
@@ -0,0 +1,724 @@
+/*
+ * upb - a minimalist implementation of protocol buffers.
+ *
+ * Copyright (c) 2009 Google Inc.  See LICENSE for details.
+ * Author: Josh Haberman <jhaberman@gmail.com>
+ *
+ * Python extension exposing the core of upb: definitions, handlers,
+ * and a message type.
+ */
+
+#include <stddef.h>
+#include <Python.h>
+#include "upb/def.h"
+#include "upb/msg.h"
+
+static bool streql(const char *a, const char *b) { return strcmp(a, b) == 0; }
+
+PyObject *PyUpb_Error(const char *str) {
+  PyErr_SetString(PyExc_TypeError, str);
+  return NULL;
+}
+
+int PyUpb_ErrorInt(const char *str) {
+  PyErr_SetString(PyExc_TypeError, str);
+  return -1;
+}
+
+#define PyUpb_CheckStatus(status) \
+  if (!upb_ok(status)) return PyUpb_Error((status)->str);
+
+static upb_accessor_vtbl *PyUpb_AccessorForField(upb_fielddef *f);
+
+
+/* Object cache ***************************************************************/
+
+// For objects that are just wrappers around a C object pointer, we keep a
+// cache mapping C pointer -> wrapper object.  This allows us to consistently
+// vend the same Python object given the same C object.  This prevents us from
+// creating too many Python objects unnecessarily.  More importantly, it provides
+// the expected semantics:
+//
+//   if field.subdef is field.subdef:
+//     print "Sanity prevails."
+//
+// If we conjured up a new wrapper object every time, the above would not be
+// true.
+//
+// The cost is having to put all such objects in a table, but since this only
+// applies to schema-level objects (defs, handlers, etc) this seems acceptable.
+// We do *not* have to put all message objects in this table.
+//
+// We use weak refs so that the cache does not prevent the wrapper objects from
+// being collected.  The table is stored as a static variable; to use
+// sub-interpreters this would need to change, but I believe that using
+// sub-interpreters is exceedingly rare in practice.
+
+typedef struct {
+  PyObject_HEAD;
+  void *obj;
+  PyObject *weakreflist;
+} PyUpb_ObjWrapper;
+
+static PyObject *obj_cache = NULL;
+static PyObject *reverse_cache = NULL;
+static PyObject *weakref_callback = NULL;
+
+// Utility functions for manipulating Python dictionaries keyed by pointer.
+
+static PyObject *PyUpb_StringForPointer(void *ptr) {
+  PyObject *o = PyString_FromStringAndSize((const char *)&ptr, sizeof(void*));
+  assert(o);
+  return o;
+}
+
+static PyObject *PyUpb_ObjCacheDeleteCallback(PyObject *self, PyObject *ref) {
+  // Python very unfortunately clears the weakref before running our callback.
+  // This prevents us from using the weakref to find the C pointer we need to
+  // remove from the cache.  As a result we are forced to keep a second map
+  // mapping weakref->C pointer.
+  PyObject *ptr_str = PyDict_GetItem(reverse_cache, ref);
+  assert(ptr_str);
+  int err = PyDict_DelItem(obj_cache, ptr_str);
+  assert(!err);
+  err = PyDict_DelItem(reverse_cache, ref);
+  assert(!err);
+  return Py_None;
+}
+
+static PyObject *PyUpb_ObjCacheGet(void *obj, PyTypeObject *type) {
+  PyObject *kv = PyUpb_StringForPointer(obj);
+  PyObject *ref = PyDict_GetItem(obj_cache, kv);
+  PyObject *ret;
+  if (ref) {
+    ret = PyWeakref_GetObject(ref);
+    assert(ret != Py_None);
+    Py_INCREF(ret);
+  } else {
+    PyUpb_ObjWrapper *wrapper = (PyUpb_ObjWrapper*)type->tp_alloc(type, 0);
+    wrapper->obj = obj;
+    wrapper->weakreflist = NULL;
+    ret = (PyObject*)wrapper;
+    ref = PyWeakref_NewRef(ret, weakref_callback);
+    assert(PyWeakref_GetObject(ref) == ret);
+    assert(ref);
+    PyDict_SetItem(obj_cache, kv, ref);
+    PyDict_SetItem(reverse_cache, ref, kv);
+  }
+  assert(ret);
+  Py_DECREF(kv);
+  return ret;
+}
+
+
+/* PyUpb_Def ******************************************************************/
+
+static PyTypeObject *PyUpb_TypeForDef(upb_def *def);
+
+static void PyUpb_Def_dealloc(PyObject *obj) {
+  PyUpb_ObjWrapper *wrapper = (void*)obj;
+  upb_def_unref((upb_def*)wrapper->obj);
+  obj->ob_type->tp_free(obj);
+}
+
+PyObject *PyUpb_Def_GetOrCreate(upb_def *def) {
+  return def ? PyUpb_ObjCacheGet(def, PyUpb_TypeForDef(def)) : Py_None;
+}
+
+// Will need to expand once other kinds of defs are supported.
+#define Check_Def(o, badret) Check_MessageDef(o, badret)
+
+
+/* PyUpb_FieldDef *************************************************************/
+
+static PyTypeObject PyUpb_FieldDefType;
+static int PyUpb_FieldDef_setattro(PyObject *o, PyObject *key, PyObject *val);
+
+#define Check_FieldDef(o, badret) \
+  (void*)(((PyUpb_ObjWrapper*)o)->obj); do { \
+    if(!PyObject_TypeCheck(o, &PyUpb_FieldDefType)) { \
+      PyErr_SetString(PyExc_TypeError, "must be a upb.FieldDef"); \
+      return badret; \
+    } \
+  } while(0)
+
+static PyObject *PyUpb_FieldDef_GetOrCreate(upb_fielddef *f) {
+  return PyUpb_ObjCacheGet(f, &PyUpb_FieldDefType);
+}
+
+static PyObject *PyUpb_FieldDef_new(PyTypeObject *subtype,
+                                    PyObject *args, PyObject *kwds) {
+  return PyUpb_ObjCacheGet(upb_fielddef_new(), subtype);
+}
+
+static int PyUpb_FieldDef_init(PyObject *self, PyObject *args, PyObject *kwds) {
+  if (!kwds) return 0;
+  PyObject *key, *value;
+  Py_ssize_t pos = 0;
+  while (PyDict_Next(kwds, &pos, &key, &value))
+    PyUpb_FieldDef_setattro(self, key, value);
+  return 0;
+}
+
+static void PyUpb_FieldDef_dealloc(PyObject *obj) {
+  PyUpb_ObjWrapper *wrapper = (void*)obj;
+  if (wrapper->weakreflist) PyObject_ClearWeakRefs(obj);
+  upb_fielddef_unref((upb_fielddef*)wrapper->obj);
+  obj->ob_type->tp_free(obj);
+}
+
+static PyObject *PyUpb_FieldDef_getattro(PyObject *obj, PyObject *attr_name) {
+  upb_fielddef *f = Check_FieldDef(obj, NULL);
+  if (!upb_fielddef_ismutable(f)) {
+    PyErr_SetString(PyExc_TypeError, "fielddef is not mutable.");
+    return NULL;
+  }
+  const char *name = PyString_AsString(attr_name);
+  if (streql(name, "name")) {
+    const char *name = upb_fielddef_name(f);
+    return name == NULL ? Py_None : PyString_FromString(name);
+  } else if (streql(name, "number")) {
+    uint32_t num = upb_fielddef_number(f);
+    return num == 0 ? Py_None : PyInt_FromLong(num);
+  } else if (streql(name, "type")) {
+    uint8_t type = upb_fielddef_type(f);
+    return type == 0 ? Py_None : PyInt_FromLong(type);
+  } else if (streql(name, "label")) {
+    return PyInt_FromLong(upb_fielddef_label(f));
+  } else if (streql(name, "type_name")) {
+    const char *name = upb_fielddef_typename(f);
+    return name == NULL ? Py_None : PyString_FromString(name);
+  } else if (streql(name, "subdef")) {
+    // NYI;
+    return NULL;
+  } else if (streql(name, "msgdef")) {
+    // NYI;
+    return NULL;
+  } else {
+    return PyUpb_Error("Invalid fielddef member.");
+  }
+}
+
+static int PyUpb_FieldDef_setattro(PyObject *o, PyObject *key, PyObject *val) {
+  upb_fielddef *f = Check_FieldDef(o, -1);
+  const char *field = PyString_AsString(key);
+  if (!upb_fielddef_ismutable(f))
+    return PyUpb_ErrorInt("fielddef is not mutable.");
+  if (streql(field, "name")) {
+    const char *name = PyString_AsString(val);
+    if (!name || !upb_fielddef_setname(f, name))
+      return PyUpb_ErrorInt("Invalid name");
+  } else if (streql(field, "number")) {
+    // TODO: should check truncation.  Non-security issue.
+    // Non-int will return -1, which is already invalid as a field number.
+    if (!upb_fielddef_setnumber(f, PyInt_AsLong(val)))
+      return PyUpb_ErrorInt("Invalid number");
+  } else if (streql(field, "type")) {
+    // TODO: should check truncation.  Non-security issue.
+    if (!upb_fielddef_settype(f, PyInt_AsLong(val)))
+      return PyUpb_ErrorInt("Invalid type");
+  } else if (streql(field, "label")) {
+    // TODO: should check truncation.  Non-security issue.
+    if (!upb_fielddef_setlabel(f, PyInt_AsLong(val)))
+      return PyUpb_ErrorInt("Invalid label");
+  } else if (streql(field, "type_name")) {
+    const char *name = PyString_AsString(val);
+    if (!name || !upb_fielddef_settypename(f, name))
+      return PyUpb_ErrorInt("Invalid type_name");
+  } else if (streql(field, "default_value")) {
+    // NYI
+    return -1;
+  } else {
+    return PyUpb_ErrorInt("Invalid fielddef member.");
+  }
+  return 0;
+}
+
+static PyTypeObject PyUpb_FieldDefType = {
+  PyObject_HEAD_INIT(NULL)
+  0,                                      /* ob_size */
+  "upb.FieldDef",                         /* tp_name */
+  sizeof(PyUpb_ObjWrapper),               /* tp_basicsize */
+  0,                                      /* tp_itemsize */
+  &PyUpb_FieldDef_dealloc,                /* tp_dealloc */
+  0,                                      /* tp_print */
+  0,                                      /* tp_getattr */
+  0,                                      /* tp_setattr */
+  0,                                      /* tp_compare */
+  0, /* TODO */                           /* tp_repr */
+  0,                                      /* tp_as_number */
+  0,                                      /* tp_as_sequence */
+  0,                                      /* tp_as_mapping */
+  0,                                      /* tp_hash */
+  0,                                      /* tp_call */
+  0,                                      /* tp_str */
+  &PyUpb_FieldDef_getattro,               /* tp_getattro */
+  &PyUpb_FieldDef_setattro,               /* tp_setattro */
+  0,                                      /* tp_as_buffer */
+  Py_TPFLAGS_DEFAULT,                     /* tp_flags */
+  0,                                      /* tp_doc */
+  0,                                      /* tp_traverse */
+  0,                                      /* tp_clear */
+  0,                                      /* tp_richcompare */
+  offsetof(PyUpb_ObjWrapper, weakreflist),/* tp_weaklistoffset */
+  0,                                      /* tp_iter */
+  0,                                      /* tp_iternext */
+  0,                                      /* tp_methods */
+  0,                                      /* tp_members */
+  0,                                      /* tp_getset */
+  0,                                      /* tp_base */
+  0,                                      /* tp_dict */
+  0,                                      /* tp_descr_get */
+  0,                                      /* tp_descr_set */
+  0,                                      /* tp_dictoffset */
+  &PyUpb_FieldDef_init,                   /* tp_init */
+  0,                                      /* tp_alloc */
+  &PyUpb_FieldDef_new,                    /* tp_new */
+  0,                                      /* tp_free */
+};
+
+
+/* PyUpb_MessageDef ***********************************************************/
+
+static PyTypeObject PyUpb_MessageDefType;
+static int PyUpb_MessageDef_setattro(PyObject *o, PyObject *key, PyObject *val);
+
+#define Check_MessageDef(o, badret) \
+  (void*)(((PyUpb_ObjWrapper*)o)->obj); do { \
+    if(!PyObject_TypeCheck(o, &PyUpb_MessageDefType)) { \
+      PyErr_SetString(PyExc_TypeError, "must be a upb.MessageDef"); \
+      return badret; \
+    } \
+  } while(0)
+
+static PyObject *PyUpb_MessageDef_new(PyTypeObject *subtype,
+                                      PyObject *args, PyObject *kwds) {
+  return PyUpb_ObjCacheGet(upb_msgdef_new(), subtype);
+}
+
+static PyObject *PyUpb_MessageDef_add_fields(PyObject *o, PyObject *args);
+
+static int PyUpb_MessageDef_init(PyObject *self, PyObject *args, PyObject *kwds) {
+  if (!kwds) return 0;
+  PyObject *key, *value;
+  Py_ssize_t pos = 0;
+  while (PyDict_Next(kwds, &pos, &key, &value)) {
+    const char *field = PyString_AsString(key);
+    if (streql(field, "fields")) {
+      PyUpb_MessageDef_add_fields(self, value);
+    } else {
+      PyUpb_MessageDef_setattro(self, key, value);
+    }
+  }
+  return 0;
+}
+
+static PyObject *PyUpb_MessageDef_getattro(PyObject *obj, PyObject *attr_name) {
+  upb_msgdef *m = Check_MessageDef(obj, NULL);
+  const char *name = PyString_AsString(attr_name);
+  if (streql(name, "fqname")) {
+    const char *fqname = upb_def_fqname(UPB_UPCAST(m));
+    return fqname == NULL ? Py_None : PyString_FromString(fqname);
+  }
+  return PyObject_GenericGetAttr(obj, attr_name);
+}
+
+static int PyUpb_MessageDef_setattro(PyObject *o, PyObject *key, PyObject *val) {
+  upb_msgdef *m = Check_MessageDef(o, -1);
+  if (!upb_def_ismutable(UPB_UPCAST(m))) {
+    PyErr_SetString(PyExc_TypeError, "MessageDef is not mutable.");
+    return -1;
+  }
+  const char *name = PyString_AsString(key);
+  if (streql(name, "fqname")) {
+    const char *fqname = PyString_AsString(val);
+    if (!fqname || !upb_def_setfqname(UPB_UPCAST(m), fqname))
+      return PyUpb_ErrorInt("Invalid fqname");
+  } else {
+    return PyUpb_ErrorInt("Invalid MessageDef member.");
+  }
+  return 0;
+}
+
+static PyObject *PyUpb_MessageDef_fields(PyObject *obj, PyObject *args) {
+  upb_msgdef *m = Check_MessageDef(obj, NULL);
+  PyObject *ret = PyList_New(0);
+  upb_msg_iter i;
+  for(i = upb_msg_begin(m); !upb_msg_done(i); i = upb_msg_next(m, i)) {
+    upb_fielddef *f = upb_msg_iter_field(i);
+    PyList_Append(ret, PyUpb_FieldDef_GetOrCreate(f));
+  }
+  return ret;
+}
+
+static PyObject *PyUpb_MessageDef_add_fields(PyObject *o, PyObject *fields) {
+  upb_msgdef *m = Check_MessageDef(o, NULL);
+  if (!PySequence_Check(fields)) return PyUpb_Error("Must be a sequence");
+  Py_ssize_t len = PySequence_Length(fields);
+  if (len > UPB_MAX_FIELDS) return PyUpb_Error("Too many fields.");
+  upb_fielddef *f[len];
+  int i;
+  for (i = 0; i < len; i++) {
+    PyObject *field = PySequence_GetItem(fields, i);
+    f[i] = Check_FieldDef(field, NULL);
+  }
+  upb_msgdef_addfields(m, f, len);
+  return Py_None;
+}
+
+static PyObject *PyUpb_MessageDef_add_field(PyObject *o, PyObject *field) {
+  upb_msgdef *m = Check_MessageDef(o, NULL);
+  upb_fielddef *f = Check_FieldDef(field, NULL);
+  upb_msgdef_addfield(m, f);
+  return Py_None;
+}
+
+static PyMethodDef PyUpb_MessageDef_methods[] = {
+  {"add_field", &PyUpb_MessageDef_add_field, METH_O, "Adds a list of fields."},
+  {"add_fields", &PyUpb_MessageDef_add_fields, METH_O, "Adds a list of fields."},
+  {"fields", &PyUpb_MessageDef_fields, METH_NOARGS, "Returns list of fields."},
+  {NULL, NULL}
+};
+
+static PyTypeObject PyUpb_MessageDefType = {
+  PyObject_HEAD_INIT(NULL)
+  0,                                      /* ob_size */
+  "upb.MessageDef",                       /* tp_name */
+  sizeof(PyUpb_ObjWrapper),               /* tp_basicsize */
+  0,                                      /* tp_itemsize */
+  &PyUpb_Def_dealloc,                     /* tp_dealloc */
+  0,                                      /* tp_print */
+  0,                                      /* tp_getattr */
+  0,                                      /* tp_setattr */
+  0,                                      /* tp_compare */
+  0, /* TODO */                           /* tp_repr */
+  0,                                      /* tp_as_number */
+  0,                                      /* tp_as_sequence */
+  0,                                      /* tp_as_mapping */
+  0,                                      /* tp_hash */
+  0,                                      /* tp_call */
+  0,                                      /* tp_str */
+  &PyUpb_MessageDef_getattro,             /* tp_getattro */
+  &PyUpb_MessageDef_setattro,             /* tp_setattro */
+  0,                                      /* tp_as_buffer */
+  Py_TPFLAGS_DEFAULT,                     /* tp_flags */
+  0,                                      /* tp_doc */
+  0,                                      /* tp_traverse */
+  0,                                      /* tp_clear */
+  0,                                      /* tp_richcompare */
+  offsetof(PyUpb_ObjWrapper, weakreflist),/* tp_weaklistoffset */
+  0,                                      /* tp_iter */
+  0,                                      /* tp_iternext */
+  PyUpb_MessageDef_methods,               /* tp_methods */
+  0,                                      /* tp_members */
+  0,                                      /* tp_getset */
+  0,                                      /* tp_base */
+  0,                                      /* tp_dict */
+  0,                                      /* tp_descr_get */
+  0,                                      /* tp_descr_set */
+  0,                                      /* tp_dictoffset */
+  &PyUpb_MessageDef_init,                 /* tp_init */
+  0,                                      /* tp_alloc */
+  &PyUpb_MessageDef_new,                  /* tp_new */
+  0,                                      /* tp_free */
+};
+
+
+static PyTypeObject *PyUpb_TypeForDef(upb_def *def) {
+  switch(def->type) {
+    case UPB_DEF_MSG: return &PyUpb_MessageDefType;
+    default: return NULL;
+  }
+}
+
+/* PyUpb_SymbolTable **********************************************************/
+
+static PyTypeObject PyUpb_SymbolTableType;
+
+#define Check_SymbolTable(o, badret) \
+  (void*)(((PyUpb_ObjWrapper*)o)->obj); do { \
+    if(!PyObject_TypeCheck(o, &PyUpb_SymbolTableType)) { \
+      PyErr_SetString(PyExc_TypeError, "must be a upb.MessageDef"); \
+      return badret; \
+    } \
+  } while(0)
+
+static PyObject *PyUpb_SymbolTable_new(PyTypeObject *subtype,
+                                       PyObject *args, PyObject *kwds) {
+  return PyUpb_ObjCacheGet(upb_symtab_new(), subtype);
+}
+
+static int PyUpb_SymbolTable_init(PyObject *self, PyObject *args, PyObject *kwds) {
+  return 0;
+}
+
+static void PyUpb_SymbolTable_dealloc(PyObject *obj) {
+  PyUpb_ObjWrapper *wrapper = (void*)obj;
+  upb_symtab_unref((upb_symtab*)wrapper->obj);
+  obj->ob_type->tp_free(obj);
+}
+
+// narg is a lua table containing a list of defs to add.
+static PyObject *PyUpb_SymbolTable_add_defs(PyObject *o, PyObject *defs) {
+  upb_symtab *s = Check_SymbolTable(o, NULL);
+  if (!PySequence_Check(defs)) return PyUpb_Error("Must be a sequence");
+  Py_ssize_t n = PySequence_Length(defs);
+
+  // Prevent stack overflow.
+  if (n > 2048) return PyUpb_Error("Too many defs");
+  upb_def *cdefs[n];
+
+  int i = 0;
+  for (i = 0; i < n; i++) {
+    PyObject *pydef = PySequence_GetItem(defs, i);
+    upb_def *def = Check_MessageDef(pydef, NULL);
+    cdefs[i++] = def;
+    upb_msgdef *md = upb_dyncast_msgdef(def);
+    if (!md) continue;
+    upb_msg_iter j;
+    for(j = upb_msg_begin(md); !upb_msg_done(j); j = upb_msg_next(md, j)) {
+      upb_fielddef *f = upb_msg_iter_field(j);
+      upb_fielddef_setaccessor(f, PyUpb_AccessorForField(f));
+    }
+    upb_msgdef_layout(md);
+  }
+
+  upb_status status = UPB_STATUS_INIT;
+  upb_symtab_add(s, cdefs, n, &status);
+  PyUpb_CheckStatus(&status);
+  return Py_None;
+}
+
+static PyObject *PyUpb_SymbolTable_add_def(PyObject *o, PyObject *def) {
+  PyObject *defs = PyList_New(1);
+  PyList_SetItem(defs, 0, def);
+  return PyUpb_SymbolTable_add_defs(o, defs);
+}
+
+// TODO: update to allow user to choose type of defs.
+static PyObject *PyUpb_SymbolTable_defs(PyObject *o, PyObject *none) {
+  upb_symtab *s = Check_SymbolTable(o, NULL);
+  int count;
+  upb_def **defs = upb_symtab_getdefs(s, &count, UPB_DEF_ANY);
+  PyObject *ret = PyList_New(count);
+  int i;
+  for(i = 0; i < count; i++)
+    PyList_SetItem(ret, i, PyUpb_Def_GetOrCreate(defs[i]));
+  return ret;
+}
+
+static PyObject *PyUpb_SymbolTable_lookup(PyObject *o, PyObject *arg) {
+  upb_symtab *s = Check_SymbolTable(o, NULL);
+  const char *name = PyString_AsString(arg);
+  upb_def *def = upb_symtab_lookup(s, name);
+  return PyUpb_Def_GetOrCreate(def);
+}
+
+static PyMethodDef PyUpb_SymbolTable_methods[] = {
+  {"add_def", &PyUpb_SymbolTable_add_def, METH_O, NULL},
+  {"add_defs", &PyUpb_SymbolTable_add_defs, METH_O, NULL},
+  {"defs", &PyUpb_SymbolTable_defs, METH_NOARGS, NULL},
+  {"lookup", &PyUpb_SymbolTable_lookup, METH_O, NULL},
+  {NULL, NULL}
+};
+
+static PyTypeObject PyUpb_SymbolTableType = {
+  PyObject_HEAD_INIT(NULL)
+  0,                                      /* ob_size */
+  "upb.SymbolTable",                      /* tp_name */
+  sizeof(PyUpb_ObjWrapper),               /* tp_basicsize */
+  0,                                      /* tp_itemsize */
+  &PyUpb_SymbolTable_dealloc,             /* tp_dealloc */
+  0,                                      /* tp_print */
+  0,                                      /* tp_getattr */
+  0,                                      /* tp_setattr */
+  0,                                      /* tp_compare */
+  0, /* TODO */                           /* tp_repr */
+  0,                                      /* tp_as_number */
+  0,                                      /* tp_as_sequence */
+  0,                                      /* tp_as_mapping */
+  0,                                      /* tp_hash */
+  0,                                      /* tp_call */
+  0,                                      /* tp_str */
+  0,                                      /* tp_getattro */
+  0,                                      /* tp_setattro */
+  0,                                      /* tp_as_buffer */
+  Py_TPFLAGS_DEFAULT,                     /* tp_flags */
+  0,                                      /* tp_doc */
+  0,                                      /* tp_traverse */
+  0,                                      /* tp_clear */
+  0,                                      /* tp_richcompare */
+  offsetof(PyUpb_ObjWrapper, weakreflist),/* tp_weaklistoffset */
+  0,                                      /* tp_iter */
+  0,                                      /* tp_iternext */
+  PyUpb_SymbolTable_methods,              /* tp_methods */
+  0,                                      /* tp_members */
+  0,                                      /* tp_getset */
+  0,                                      /* tp_base */
+  0,                                      /* tp_dict */
+  0,                                      /* tp_descr_get */
+  0,                                      /* tp_descr_set */
+  0,                                      /* tp_dictoffset */
+  &PyUpb_SymbolTable_init,                /* tp_init */
+  0,                                      /* tp_alloc */
+  &PyUpb_SymbolTable_new,                 /* tp_new */
+  0,                                      /* tp_free */
+};
+
+
+/* Accessor and PyUpb_Message *************************************************/
+
+typedef struct {
+  PyTypeObject type;
+  PyTypeObject *alt_type;
+} PyUpb_MessageType;
+
+typedef struct {
+  PyObject_HEAD;
+  PyObject *msgdef;
+  char data[1];
+} PyUpb_Message;
+
+PyObject **PyUpb_Accessor_GetPtr(PyObject *_m, upb_value fval) {
+  PyUpb_Message *m = (PyUpb_Message*)_m;
+  upb_fielddef *f = upb_value_getfielddef(fval);
+  return (PyObject**)&m->data[f->offset];
+}
+
+static upb_sflow_t PyUpb_Message_StartSequence(void *m, upb_value fval) {
+  PyObject **seq = PyUpb_Accessor_GetPtr(m, fval);
+  PyTypeObject *type = ((PyUpb_MessageType*)Py_TYPE(m))->alt_type;
+  if (!*seq) *seq = type->tp_alloc(type, 0);
+  upb_stdmsg_sethas(m, fval);
+  return UPB_CONTINUE_WITH(*seq);
+}
+
+static upb_sflow_t PyUpb_Message_StartSubmessage(void *m, upb_value fval) {
+  PyObject **submsg = PyUpb_Accessor_GetPtr(m, fval);
+  PyTypeObject *type = Py_TYPE(m);
+  if (!*submsg) *submsg = type->tp_alloc(type, 0);
+  upb_stdmsg_sethas(m, fval);
+  return UPB_CONTINUE_WITH(*submsg);
+}
+
+static upb_sflow_t PyUpb_Message_StartRepeatedSubmessage(void *a, upb_value fval) {
+  (void)fval;
+  PyObject **elem = upb_stdarray_append(a, sizeof(void*));
+  PyTypeObject *type = ((PyUpb_MessageType*)Py_TYPE(a))->alt_type;
+  if (!*elem) *elem = type->tp_alloc(type, 0);
+  return UPB_CONTINUE_WITH(*elem);
+}
+
+static upb_flow_t PyUpb_Message_StringValue(void *m, upb_value fval, upb_value val) {
+  PyObject **str = PyUpb_Accessor_GetPtr(m, fval);
+  if (*str) Py_DECREF(*str);
+  *str = PyString_FromStringAndSize(NULL, upb_value_getstrref(val)->len);
+  upb_strref_read(upb_value_getstrref(val), PyString_AsString(*str));
+  upb_stdmsg_sethas(m, fval);
+  return UPB_CONTINUE;
+}
+
+static upb_flow_t PyUpb_Message_AppendStringValue(void *a, upb_value fval, upb_value val) {
+  (void)fval;
+  PyObject **elem = upb_stdarray_append(a, sizeof(void*));
+  *elem = PyString_FromStringAndSize(NULL, upb_value_getstrref(val)->len);
+  upb_strref_read(upb_value_getstrref(val), PyString_AsString(*elem));
+  return UPB_CONTINUE;
+}
+
+#define STDMSG(type, size) static upb_accessor_vtbl vtbl = { \
+    &PyUpb_Message_StartSubmessage, \
+    &upb_stdmsg_set ## type, \
+    &PyUpb_Message_StartSequence, \
+    &PyUpb_Message_StartRepeatedSubmessage, \
+    &upb_stdmsg_set ## type ## _r, \
+    &upb_stdmsg_has, \
+    &upb_stdmsg_getptr, \
+    &upb_stdmsg_get ## type, \
+    &upb_stdmsg_seqbegin, \
+    &upb_stdmsg_ ## size ## byte_seqnext, \
+    &upb_stdmsg_seqget ## type};
+
+#define RETURN_STDMSG(type, size) { STDMSG(type, size); return &vtbl; }
+
+static upb_accessor_vtbl *PyUpb_AccessorForField(upb_fielddef *f) {
+  switch (f->type) {
+    case UPB_TYPE(DOUBLE): RETURN_STDMSG(double, 8)
+    case UPB_TYPE(FLOAT): RETURN_STDMSG(float, 4)
+    case UPB_TYPE(UINT64):
+    case UPB_TYPE(FIXED64): RETURN_STDMSG(uint64, 8)
+    case UPB_TYPE(INT64):
+    case UPB_TYPE(SFIXED64):
+    case UPB_TYPE(SINT64): RETURN_STDMSG(int64, 8)
+    case UPB_TYPE(INT32):
+    case UPB_TYPE(SINT32):
+    case UPB_TYPE(ENUM):
+    case UPB_TYPE(SFIXED32): RETURN_STDMSG(int32, 4)
+    case UPB_TYPE(UINT32):
+    case UPB_TYPE(FIXED32): RETURN_STDMSG(uint32, 4)
+    case UPB_TYPE(BOOL): { STDMSG(bool, 1); return &vtbl; }
+    case UPB_TYPE(GROUP):
+    case UPB_TYPE(MESSAGE): RETURN_STDMSG(ptr, 8)  // TODO: 32-bit
+    case UPB_TYPE(STRING):
+    case UPB_TYPE(BYTES): {
+      STDMSG(ptr, 8);
+      vtbl.set = &PyUpb_Message_StringValue;
+      vtbl.append = &PyUpb_Message_AppendStringValue;
+      return &vtbl;
+    }
+  }
+  return NULL;
+}
+
+
+/* Toplevel *******************************************************************/
+
+static PyMethodDef methods[] = {
+  {NULL, NULL}
+};
+
+// PyModule_AddObject steals a ref, but our object is statically allocated
+// and must not be deleted.
+#define PyUpb_AddType(mod, name, type) \
+  if (PyType_Ready(type) < 0) return; \
+  Py_INCREF(type); \
+  PyModule_AddObject(mod, name, (PyObject*)type);
+
+PyMODINIT_FUNC initupb(void) {
+  PyObject *mod = Py_InitModule("upb", methods);
+
+  PyUpb_AddType(mod, "FieldDef", &PyUpb_FieldDefType);
+  PyUpb_AddType(mod, "MessageDef", &PyUpb_MessageDefType);
+  PyUpb_AddType(mod, "SymbolTable", &PyUpb_SymbolTableType);
+
+  PyModule_AddIntConstant(mod, "LABEL_OPTIONAL", UPB_LABEL(OPTIONAL));
+  PyModule_AddIntConstant(mod, "LABEL_REQUIRED", UPB_LABEL(REQUIRED));
+  PyModule_AddIntConstant(mod, "LABEL_REPEATED", UPB_LABEL(REPEATED));
+
+  PyModule_AddIntConstant(mod, "TYPE_DOUBLE", UPB_TYPE(DOUBLE));
+  PyModule_AddIntConstant(mod, "TYPE_FLOAT", UPB_TYPE(FLOAT));
+  PyModule_AddIntConstant(mod, "TYPE_INT64", UPB_TYPE(INT64));
+  PyModule_AddIntConstant(mod, "TYPE_UINT64", UPB_TYPE(UINT64));
+  PyModule_AddIntConstant(mod, "TYPE_INT32", UPB_TYPE(INT32));
+  PyModule_AddIntConstant(mod, "TYPE_FIXED64", UPB_TYPE(FIXED64));
+  PyModule_AddIntConstant(mod, "TYPE_FIXED32", UPB_TYPE(FIXED32));
+  PyModule_AddIntConstant(mod, "TYPE_BOOL", UPB_TYPE(BOOL));
+  PyModule_AddIntConstant(mod, "TYPE_STRING", UPB_TYPE(STRING));
+  PyModule_AddIntConstant(mod, "TYPE_GROUP", UPB_TYPE(GROUP));
+  PyModule_AddIntConstant(mod, "TYPE_MESSAGE", UPB_TYPE(MESSAGE));
+  PyModule_AddIntConstant(mod, "TYPE_BYTES", UPB_TYPE(BYTES));
+  PyModule_AddIntConstant(mod, "TYPE_UINT32", UPB_TYPE(UINT32));
+  PyModule_AddIntConstant(mod, "TYPE_ENUM", UPB_TYPE(ENUM));
+  PyModule_AddIntConstant(mod, "TYPE_SFIXED32", UPB_TYPE(SFIXED32));
+  PyModule_AddIntConstant(mod, "TYPE_SFIXED64", UPB_TYPE(SFIXED64));
+  PyModule_AddIntConstant(mod, "TYPE_SINT32", UPB_TYPE(SINT32));
+  PyModule_AddIntConstant(mod, "TYPE_SINT64", UPB_TYPE(SINT64));
+
+  obj_cache = PyDict_New();
+  reverse_cache = PyDict_New();
+  static PyMethodDef method = {
+        "WeakRefCallback", &PyUpb_ObjCacheDeleteCallback, METH_O, NULL};
+  PyObject *pyname = PyString_FromString(method.ml_name);
+  weakref_callback = PyCFunction_NewEx(&method, NULL, pyname);
+  Py_DECREF(pyname);
+}
diff --git a/bindings/python/upb/__init__.py b/bindings/python/upb/__init__.py
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/bindings/python/upb/__init__.py