.. _configuration:

====================
 Configuring Flake8
====================

Once you have learned how to :ref:`invoke <invocation>` |Flake8|, you will soon
want to learn how to configure it so you do not have to specify the same
options every time you use it.

This section will show you how to make

.. prompt:: bash

    flake8

Remember that you want to specify certain options without writing

.. prompt:: bash

    flake8 --select E123,W456 --enable-extensions H111


Configuration Locations
=======================

|Flake8| supports storing its configuration in the following places:

- Your top-level user directory

- In your project in one of ``setup.cfg``, ``tox.ini``, or ``.flake8``.


"User" Configuration
--------------------

|Flake8| allows a user to use "global" configuration file to store preferences.
The user configuration file is expected to be stored somewhere in the user's
"home" directory.

- On Windows the "home" directory will be something like
  ``C:\\Users\sigmavirus24``, a.k.a, ``~\``.

- On Linux and other Unix like systems (including OS X) we will look in
  ``~/``.

Note that |Flake8| looks for ``~\.flake8`` on Windows and ``~/.config/flake8``
on Linux and other Unix systems.

User configuration files use the same syntax as Project Configuration files.
Keep reading to see that syntax.


Project Configuration
---------------------

|Flake8| is written with the understanding that people organize projects into
sub-directories. Let's take for example |Flake8|'s own project structure

.. code::

    flake8
    ├── docs
    │   ├── build
    │   └── source
    │       ├── _static
    │       ├── _templates
    │       ├── dev
    │       ├── internal
    │       └── user
    ├── flake8
    │   ├── formatting
    │   ├── main
    │   ├── options
    │   └── plugins
    └── tests
        ├── fixtures
        │   └── config_files
        ├── integration
        └── unit

In the top-level ``flake8`` directory (which contains ``docs``, ``flake8``,
and ``tests``) there's also ``tox.ini`` and ``setup.cfg`` files. In our case,
we keep our |Flake8| configuration in ``tox.ini``. Regardless of whether you
keep your config in ``.flake8``, ``setup.cfg``, or ``tox.ini`` we expect you
to use INI to configure |Flake8| (since each of these files already uses INI
as a format). This means that any |Flake8| configuration you wish to set needs
to be in the ``flake8`` section, which means it needs to start like so:

.. code-block:: ini

    [flake8]

Each command-line option that you want to specify in your config file can
be named in either of two ways:

#. Using underscores (``_``) instead of hyphens (``-``)

#. Simply using hyphens (without the leading hyphens)

.. note::

    Not every |Flake8| command-line option can be specified in the
    configuration file. See :ref:`our list of options <options-list>` to
    determine which options will be parsed from the configuration files.

Let's actually look at |Flake8|'s own configuration section:

.. code-block:: ini

    [flake8]
    ignore = D203
    exclude = .git,__pycache__,docs/source/conf.py,old,build,dist
    max-complexity = 10

This is equivalent to:

.. prompt:: bash

    flake8 --ignore D203 \
             --exclude .git,__pycache__,docs/source/conf.py,old,build,dist \
             --max-complexity 10

In our case, if we wanted to, we could also do

.. code-block:: ini

    [flake8]
    ignore = D203
    exclude =
        .git,
        __pycache__,
        docs/source/conf.py,
        old,
        build,
        dist
    max-complexity = 10

This would allow us to add comments for why we're excluding items, e.g.,

.. code-block:: ini

    [flake8]
    ignore = D203
    exclude =
        # No need to traverse our git directory
        .git,
        # There's no value in checking cache directories
        __pycache__,
        # The conf file is mostly autogenerated, ignore it
        docs/source/conf.py,
        # The old directory contains Flake8 2.0
        old,
        # This contains our built documentation
        build,
        # This contains builds of flake8 that we don't want to check
        dist
    max-complexity = 10

.. note::

    If you're using Python 2, you will notice that we download the
    :mod:`configparser` backport from PyPI. That backport enables us to
    support this behaviour on all supported versions of Python.

    Please do **not** open issues about this dependency to |Flake8|.

.. note::

    You can also specify ``--max-complexity`` as ``max_complexity = 10``.

This is also useful if you have a long list of error codes to ignore. Let's
look at a portion of OpenStack's Swift `project configuration`_:

.. code-block:: ini

    [flake8]
    # it's not a bug that we aren't using all of hacking, ignore:
    # F812: list comprehension redefines ...
    # H101: Use TODO(NAME)
    # H202: assertRaises Exception too broad
    # H233: Python 3.x incompatible use of print operator
    # H301: one import per line
    # H306: imports not in alphabetical order (time, os)
    # H401: docstring should not start with a space
    # H403: multi line docstrings should end on a new line
    # H404: multi line docstring should start without a leading new line
    # H405: multi line docstring summary not separated with an empty line
    # H501: Do not use self.__dict__ for string formatting
    ignore = F812,H101,H202,H233,H301,H306,H401,H403,H404,H405,H501

They use the comments to describe the check but they could also write this as:

.. code-block:: ini

    [flake8]
    # it's not a bug that we aren't using all of hacking
    ignore =
        # F812: list comprehension redefines ...
        F812,
        # H101: Use TODO(NAME)
        H101,
        # H202: assertRaises Exception too broad
        H202,
        # H233: Python 3.x incompatible use of print operator
        H233,
        # H301: one import per line
        H301,
        # H306: imports not in alphabetical order (time, os)
        H306,
        # H401: docstring should not start with a space
        H401,
        # H403: multi line docstrings should end on a new line
        H403,
        # H404: multi line docstring should start without a leading new line
        H404,
        # H405: multi line docstring summary not separated with an empty line
        H405,
        # H501: Do not use self.__dict__ for string formatting
        H501

Or they could use each comment to describe **why** they've ignored the check.
|Flake8| knows how to parse these lists and will appropriatey handle
these situations.

.. _project configuration:
    https://github.com/openstack/swift/blob/3944d820387f08372c1a29444f4af7d8e6090ae9/tox.ini#L66..L81
