# pandas code style guide

Table of contents:

- Patterns

- Testing

  - Failing tests

  - Do not use `pytest.xfail`

  - Using `pytest.mark.xfail`

- Miscellaneous

  - Reading from a url

pandas follows the PEP8 standard and uses Black and Flake8 to ensure a consistent code format throughout the project. We encourage you to use pre-commit to automatically run `black`, `flake8`, `isort`, and related code checks when you make a git commit.

## Patterns

We use a `flake8` plugin, pandas-dev-flaker, to check our codebase for unwanted patterns. See its `README` for the up-to-date list of rules we enforce.

## Testing

### Failing tests

See https://docs.pytest.org/en/latest/skipping.html for background.

### Do not use `pytest.xfail`

Do not use this method. It has the same behavior as `pytest.skip`, namely it immediately stops the test and does not check if the test will fail. If this is the behavior you desire, use `pytest.skip` instead.

### Using `pytest.mark.xfail`

Use this method if a test is known to fail but the manner in which it fails is not meant to be captured. It is common to use this method for a test that exhibits buggy behavior or a non-implemented feature. If the failing test has flaky behavior, use the argument `strict=False`. This will make it so pytest does not fail if the test happens to pass.

Prefer the decorator `@pytest.mark.xfail` and the argument `pytest.param` over usage within a test so that the test is appropriately marked during the collection phase of pytest. For xfailing a test that involves multiple parameters, a fixture, or a combination of these, it is only possible to xfail during the testing phase. To do so, use the `request` fixture:

    import pytest

    def test_xfail(request):
        mark = pytest.mark.xfail(raises=TypeError, reason="Indicate why here")
        request.node.add_marker(mark)

xfail is not to be used for tests involving failure due to invalid user arguments. For these tests, we need to verify the correct exception type and error message is being raised, using `pytest.raises` instead.

## Miscellaneous

### Reading from a url

Good:

    from pandas.io.common import urlopen

    with urlopen("http://www.google.com") as url:
        raw_text = url.read()