Writing Plugins For Flake8 2 and 3

Plugins have existed for Flake8 2.x for a few years. There are a number of these on PyPI already. While it did not seem reasonable for Flake8 to attempt to provide a backwards compatible shim for them, we did decide to try to document the easiest way to write a plugin that’s compatible across both versions.


If your plugin does not register options, it should Just Work.

The only two breaking changes in Flake8 3.0 is the fact that we no longer check the option parser for a list of strings to parse from a config file and we no longer patch pep8 or pycodestyle’s stdin_get_value functions. On Flake8 2.x, to have an option parsed from the configuration files that Flake8 finds and parses you would have to do something like:

parser.add_option('-X', '--example-flag', type='string',

For Flake8 3.0, we have added three arguments to the add_option() method you will call on the parser you receive:

  • parse_from_config which expects True or False

    When True, Flake8 will parse the option from the config files Flake8 finds.

  • comma_separated_list which expects True or False

    When True, Flake8 will split the string intelligently and handle extra whitespace. The parsed value will be a list.

  • normalize_paths which expects True or False

    When True, Flake8 will:

    • remove trailing path separators (i.e., os.path.sep)
    • return the absolute path for values that have the separator in them

All three of these options can be combined or used separately.

Parsing Options from Configuration Files

The example from Flake8 2.x now looks like:

parser.add_option('-X', '--example-flag', type='string',

Parsing Comma-Separated Lists

Now let’s imagine that the option we want to add is expecting a comma-separatd list of values from the user (e.g., --select E123,W503,F405). Flake8 2.x often forced users to parse these lists themselves since pep8 special-cased certain flags and left others on their own. Flake8 3.0 adds comma_separated_list so that the parsed option is already a list for plugin authors. When combined with parse_from_config this means that users can also do something like:

example-flag =

And Flake8 will just return the list:

["first", "second", "third", "fourth", "fifth"]

Normalizing Values that Are Paths

Finally, let’s imagine that our new option wants a path or list of paths. To ensure that these paths are semi-normalized (the way Flake8 2.x used to work) we need only pass normalize_paths=True. If you have specified comma_separated_list=True then this will parse the value as a list of paths that have been normalized. Otherwise, this will parse the value as a single path.

Option Handling on Flake8 2 and 3

To ease the transition, the Flake8 maintainers have released flake8-polyfill. flake8-polyfill provides a convenience function to help users transition between Flake8 2 and 3 without issue. For example, if your plugin has to work on Flake8 2.x and 3.x but you want to take advantage of some of the new options to add_option, you can do

from flake8_polyfill import options

class MyPlugin(object):
    def add_options(cls, parser):
            '--application-names', default='', type='string',
            help='Names of the applications to be checked.',
            '--style-name', default='', type='string',
            help='The name of the style convention you want to use',
            '--application-paths', default='', type='string',
            help='Locations of the application code',

    def parse_options(cls, parsed_options):
        cls.application_names = parsed_options.application_names
        cls.style_name = parsed_options.style_name
        cls.application_paths = parsed_options.application_paths

flake8-polyfill will handle these extra options using callbacks to the option parser. The project has direct replications of the functions that Flake8 uses to provide the same functionality. This means that the values you receive should be identically parsed whether you’re using Flake8 2.x or 3.x.

flake8_polyfill.options.register(parser, *args, **kwargs)[source]

Register an option for the Option Parser provided by Flake8.

  • parser – The option parser being used by Flake8 to handle command-line options.
  • *args – Positional arguments that you might otherwise pass to add_option.
  • **kwargs – Keyword arguments you might otherwise pass to add_option.

Standard In Handling on Flake8 2.5, 2.6, and 3

After releasing Flake8 2.6, handling standard-in became a bit trickier for some plugins. Flake8 2.5 and earlier had started monkey-patching pep8’s stdin_get_value function. 2.6 switched to pycodestyle and only monkey-patched that. 3.0 has its own internal implementation and uses that but does not directly provide anything for plugins using pep8 and pycodestyle’s stdin_get_value function. flake8-polyfill provides this functionality for plugin developers via its flake8_polyfill.stdin module.

If a plugin needs to read the content from stdin, it can do the following:

from flake8_polyfill import stdin

stdin.monkey_patch('pep8')  # To monkey-patch only pep8
stdin.monkey_patch('pycodestyle')  # To monkey-patch only pycodestyle
stdin.monkey_patch('all')  # To monkey-patch both pep8 and pycodestyle

Further, when using all, flake8-polyfill does not require both packages to be installed but will attempt to monkey-patch both and will silently ignore the fact that pep8 or pycodestyle is not installed.


Monkey-patch the specified module with the appropriate stdin.

On Flake8 2.5 and lower, Flake8 would would monkey-patch pep8.stdin_get_value for everyone. This avoided problems where stdin might be exhausted.

On Flake8 2.6, Flake8 stopped patching pep8 and started monkey-patching pycodestyle.stdin_get_value.

On Flake8 3.x, Flake8 has no need to monkey patch either pep8 or pycodestyle.

This function accepts three parameters:

  • pep8
  • pycodestyle
  • all

“all” is a special value that will monkey-patch both “pep8” and “pycodestyle”.

Parameters:which (str) – The name of the module to patch.
Return type:NoneType