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

Values set at the command line have highest priority, then those in the
project configuration file, and finally there are the defaults. However,
there are additional command line options which can alter this.


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]
    extend-ignore = E203
    exclude = .git,__pycache__,docs/source/conf.py,old,build,dist
    max-complexity = 10

This is equivalent to:

.. prompt:: bash

    flake8 --extend-ignore E203 \
             --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]
    extend-ignore = E203
    exclude =
        .git,
        __pycache__,
        docs/source/conf.py,
        old,
        build,
        dist
    max-complexity = 10

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

.. code-block:: ini

    [flake8]
    extend-ignore = E203
    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::

    Following the recommended settings for
    `Python's configparser <https://docs.python.org/3/library/configparser.html#customizing-parser-behaviour>`_,
    |Flake8| does not support inline comments for any of the keys. So while
    this is fine:

    .. code-block:: ini

        [flake8]
        per-file-ignores =
            # imported but unused
            __init__.py: F401

    this is not:

    .. code-block:: ini

        [flake8]
        per-file-ignores =
            __init__.py: F401 # imported but unused


.. 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 a project's Flake8 configuration in their ``tox.ini``:

.. code-block:: ini

    [flake8]
    # it's not a bug that we aren't using all of hacking, ignore:
    # 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
    extend-ignore = 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
    extend-ignore =
        # 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 appropriately handle
these situations.


Using Local Plugins
-------------------

.. versionadded:: 3.5.0

|Flake8| allows users to write plugins that live locally in a project. These
plugins do not need to use setuptools or any of the other overhead associated
with plugins distributed on PyPI. To use these plugins, users must specify
them in their configuration file (i.e., ``.flake8``, ``setup.cfg``, or
``tox.ini``). This must be configured in a separate INI section named
``flake8:local-plugins``.

Users may configure plugins that check source code, i.e., ``extension``
plugins, and plugins that report errors, i.e., ``report`` plugins.

An example configuration might look like:

.. code-block:: ini

    [flake8:local-plugins]
    extension =
        MC1 = project.flake8.checkers:MyChecker1
        MC2 = project.flake8.checkers:MyChecker2
    report =
        MR1 = project.flake8.reporters:MyReporter1
        MR2 = project.flake8.reporters:MyReporter2

|Flake8| will also, however, allow for commas to separate the plugins for
example:

.. code-block:: ini

    [flake8:local-plugins]
    extension =
        MC1 = project.flake8.checkers:MyChecker1,
        MC2 = project.flake8.checkers:MyChecker2
    report =
        MR1 = project.flake8.reporters:MyReporter1,
        MR2 = project.flake8.reporters:MyReporter2

These configurations will allow you to select your own custom reporter plugin
that you've designed or will utilize your new check classes.

If your package is installed in the same virtualenv that |Flake8| will run
from, and your local plugins are part of that package, you're all set; |Flake8|
will be able to import your local plugins. However, if you are working on a
project that isn't set up as an installable package, or |Flake8| doesn't run
from the same virtualenv your code runs in, you may need to tell |Flake8| where
to import your local plugins from. You can do this via the ``paths`` option in
the ``local-plugins`` section of your config:

.. code-block:: ini

    [flake8:local-plugins]
    extension =
      MC1 = myflake8plugin:MyChecker1
    paths =
      ./path/to

Relative paths will be interpreted relative to the config file. Multiple paths
can be listed (comma separated just like ``exclude``) as needed. If your local
plugins have any dependencies, it's up to you to ensure they are installed in
whatever Python environment |Flake8| runs in.

.. note::

    These plugins otherwise follow the same guidelines as regular plugins.