add-trailing-comma

Common errors

• ModuleNotFoundError: No module named 'add_trailing_comma'

  cause This library is a command-line utility and is not designed for programmatic 'import' into Python scripts.
  fix Run the tool directly from your terminal using `add-trailing-comma <files>` or `python -m add_trailing_comma <files>`.

• add-trailing-comma: command not found

  cause The `add-trailing-comma` executable is not in your system's PATH, or the package was not successfully installed.
  fix Ensure the package is installed (`pip install add-trailing-comma`) and your Python environment's script directory is in your PATH. If using a virtual environment, make sure it's activated. Alternatively, run via `python -m add_trailing_comma <files>`.

• ERROR: add-trailing-comma requires Python '>=3.10' but the running Python is X.Y

  cause You are attempting to install or run `add-trailing-comma` version 4.0.0+ with a Python interpreter older than 3.10.
  fix Switch to a Python 3.10 or newer environment. If you absolutely need to use an older Python environment, you must install an older version of `add-trailing-comma` (e.g., `pip install 'add-trailing-comma<4.0.0'`) that is compatible with your Python version.

Warnings

• breaking As of version 4.0.0, the `add-trailing-comma` tool itself now requires Python 3.10 or newer to run. Attempting to install or execute it with older Python versions will result in an error.
  Fix: Ensure you are running `add-trailing-comma` within a Python 3.10+ environment. If you need to use an older Python environment, you must install an older version of `add-trailing-comma` (e.g., `<4.0.0`).
• breaking The command-line flags `--py36-plus` and `--py310-plus` were removed in version 4.0.0. The tool now automatically infers the target Python version of the code it's formatting based on the Python environment it's running in.
  Fix: Remove these flags from your command-line invocations or pre-commit configurations. The tool will now handle the appropriate Python version inference.
• gotcha `add-trailing-comma` modifies files in-place without creating backups. It is strongly recommended to use this tool within a version-controlled repository (e.g., Git) to easily review and revert any unintended changes.
  Fix: Always commit your code before running `add-trailing-comma` or ensure you have a robust backup strategy. Integrate it as part of a pre-commit hook for automated version control management.
• gotcha When combining `add-trailing-comma` with other code formatters (e.g., Black, Ruff), it should generally be run *before* the other formatters in your pipeline (e.g., pre-commit hooks). Some opinionated formatters might remove trailing commas in certain contexts, potentially undoing `add-trailing-comma`'s work if run out of order.
  Fix: Configure your pre-commit hooks or formatting scripts to execute `add-trailing-comma` before other formatters that might conflict with trailing comma placement.

Install

• pip install add-trailing-comma Install latest version

Imports

• add-trailing-comma (CLI)
  wrong:

  import add_trailing_comma

  correct:

  add-trailing-comma <filenames>...

  This library is a command-line tool and is not intended for programmatic import or direct use as a Python module.

Quickstart

First, create a Python file with some function calls or literals. Then, run `add-trailing-comma` on that file from your terminal. The tool will modify the file in-place, adding trailing commas where applicable.

# Create a Python file, e.g., 'example.py'
# -- example.py BEFORE --
# def my_function(arg1, arg2):
#     pass
# 
# my_list = [
#     1,
#     2
# ]
# 
# my_dict = {
#     'key1': 'value1',
#     'key2': 'value2'
# }

# Run the tool from your terminal:
# $ add-trailing-comma example.py

# -- example.py AFTER --
# def my_function(arg1, arg2,):
#     pass
# 
# my_list = [
#     1,
#     2,
# ]
# 
# my_dict = {
#     'key1': 'value1',
#     'key2': 'value2',
# }