Add a Warn-Only mode that falls back on the system Python (#12)

Adds a new flag. BAZEL_PYTHON_WARN_ONLY=true will cause it to print a debug message and then carry on with the system Python if no bazel-python installation is found. This allows the user to do a 'fast build' in cases where Python is not especially important (e.g., build just the C++ server of a project).

3 files changed, 41 insertions, 11 deletions

diff --git a/README.md b/README.md
index a48de6b..8a11e8b 100644
--- a/README.md
+++ b/README.md
@@ -112,6 +112,13 @@ the repository for which you want to compute coverage. To use it, after running
 `bazel test //...` you should be able to run `bazel run coverage_report` to
 produce an `htmlcov` directory with the coverage report.
 
+#### Selectively Disabling Bazel-Python
+In some scenarios, it may suffice to use the system Python. For example, if a
+backend server only needs Python for one or two setup operations. In this case,
+one may pass `--define BAZEL_PYTHON_ONLY_WARN=true` to instruct Bazel-Python to
+fallback on the system Python installation if a version-specific one is not
+found. It will still print a debug message, but will not fail the build.
+
 ## Known Issues
 #### Missing Modules
 If you get errors about missing modules (e.g., `pytest not found`), please
diff --git a/bazel_python.bzl b/bazel_python.bzl
index 60950be..ccde609 100644
--- a/bazel_python.bzl
+++ b/bazel_python.bzl
@@ -73,21 +73,35 @@ def _bazel_python_venv_impl(ctx):
 
     Also installs requirements specified by @ctx.attr.requirements_file.
     """
-    if "BAZEL_PYTHON_DIR" not in ctx.var:
-        fail("You must run setup_python.sh for " + ctx.attr.python_version)
-    python_parent_dir = ctx.var.get("BAZEL_PYTHON_DIR")
     python_version = ctx.attr.python_version
-    python_dir = python_parent_dir + "/" + python_version
+    use_system = False
+    only_warn = ctx.var.get("BAZEL_PYTHON_ONLY_WARN", "false").lower() == "true"
+    if "BAZEL_PYTHON_DIR" not in ctx.var:
+        if only_warn:
+            print("A bazel-python installation was not found. Falling back to the system python. For reproducibility, please run setup_python.sh for " + python_version)
+            use_system = True
+        else:
+            fail("You must run setup_python.sh for " + python_version)
+    if use_system:
+        python_dir = ""
+    else:
+        python_parent_dir = ctx.var.get("BAZEL_PYTHON_DIR")
+        python_dir = python_parent_dir + "/" + python_version
 
     # TODO: Fail if python_dir does not exist.
     venv_dir = ctx.actions.declare_directory("bazel_python_venv_installed")
     inputs = []
-    command = """
-        export PATH={py_dir}/bin:$PATH
-        export PATH={py_dir}/include:$PATH
-        export PATH={py_dir}/lib:$PATH
-        export PATH={py_dir}/share:$PATH
-        export PYTHON_PATH={py_dir}:{py_dir}/bin:{py_dir}/include:{py_dir}/lib:{py_dir}/share
+    if use_system:
+        command = ""
+    else:
+        command = """
+            export PATH={py_dir}/bin:$PATH
+            export PATH={py_dir}/include:$PATH
+            export PATH={py_dir}/lib:$PATH
+            export PATH={py_dir}/share:$PATH
+            export PYTHON_PATH={py_dir}:{py_dir}/bin:{py_dir}/include:{py_dir}/lib:{py_dir}/share
+        """
+    command += """
         python3 -m venv {out_dir} || exit 1
         source {out_dir}/bin/activate || exit 1
     """
diff --git a/pywrapper.sh b/pywrapper.sh
index 6a7e99d..0059881 100755
--- a/pywrapper.sh
+++ b/pywrapper.sh
@@ -1,4 +1,13 @@
 #!/bin/bash
 
-source bazel_python_venv_installed/bin/activate || exit 1
+# If python is run from the 'main' workspace, then we will have
+# bazel_python_venv_installed available right in the current directory. But if
+# it's run from a dependency (e.g., GRPC) then it will be under
+# bazel_out/.../[mainworkspace]. This searches for the first matching path then
+# exits, so it should be reasonably fast in most cases.
+# https://unix.stackexchange.com/questions/68414/only-find-first-few-matched-files-using-find
+venv_path=$((find . -path "*/bazel_python_venv_installed/bin/activate" & ) | head -n 1)
+# If venv_path was not found it will be empty and the below will throw an
+# error, alerting Bazel something went wrong.
+source $venv_path || exit 1
 python $@