Python

Bazelle includes a custom Python extension that generates py_library, py_binary, and py_test rules.

Generated Rules

Source Pattern	Generated Rule
`*.py` files (non-test)	`py_library`
`_test.py`, `test_.py`	`py_test`
Files with `if __name__ == "__main__":`	`py_binary`

Configuration

Enable Python

# In your root BUILD.bazel
# gazelle:python_enabled true

Available Directives

# Enable/disable Python extension
# gazelle:python_enabled true

# Custom macros (optional)
# gazelle:python_library_macro py_library
# gazelle:python_test_macro py_test
# gazelle:python_binary_macro py_binary

# Visibility for generated targets
# gazelle:python_visibility //visibility:public

# Custom load path for macros
# gazelle:python_load //my/macros:defs.bzl

# Test framework: "pytest" (default) or "unittest"
# gazelle:python_test_framework pytest

# Custom stdlib modules file (optional)
# gazelle:python_stdlib_modules_file //:stdlib_modules.txt

Example

Given these Python source files:

def hello(name: str) -> str:
    return f"Hello, {name}!"

from src.greeter.greeter import hello

def test_hello():
    assert hello("World") == "Hello, World!"

Bazelle generates:

load("@rules_python//python:defs.bzl", "py_library", "py_test")

py_library(
    name = "greeter",
    srcs = glob(
        ["*.py"],
        exclude = ["*_test.py", "test_*.py"],
    ),
    visibility = ["//visibility:public"],
)

py_test(
    name = "greeter_test",
    srcs = glob(["*_test.py", "test_*.py"]),
    deps = [":greeter"],
)

Binary Detection

Files containing if __name__ == "__main__": blocks are detected and generate py_binary rules:

def main():
    print("Hello from CLI!")

if __name__ == "__main__":
    main()

Generates:

py_binary(
    name = "main",
    srcs = ["main.py"],
    main = "main.py",
)

Test Detection

Test files are detected by filename patterns:

*_test.py - e.g., greeter_test.py
test_*.py - e.g., test_greeter.py
Files in /tests/ directories

Import Parsing

The Python parser extracts imports to determine dependencies:

import os                      # Standard library (ignored)
import requests               # Third-party (needs manual dep)
from mypackage import utils   # Internal (resolved to //mypackage:utils)
from . import sibling         # Relative (internal)

Stdlib Modules

Bazelle includes a built-in list of Python standard library modules to avoid creating dependencies on them. You can provide a custom list:

Limitations

Current limitations of the Python extension:

No automatic pip dependency resolution (manual deps needed)
No requirements.txt auto-update
No type stub (.pyi) handling
Relative imports are skipped during dependency resolution
No namespace package support