Receiving Information For A Check Plugin
Plugins to Flake8 have a great deal of information that they can request
FileProcessor instance. Historically,
Flake8 has supported two types of plugins:
classes that accept parsed abstract syntax trees (ASTs)
functions that accept a range of arguments
Flake8 now does not distinguish between the two types of plugins. Any plugin can accept either an AST or a range of arguments. Further, any plugin that has certain callable attributes can also register options and receive parsed options.
Indicating Desired Data
Flake8 inspects the plugin’s signature to determine what parameters it
flake8.plugins.finder.LoadedPlugin.parameters caches the values so that
each plugin makes that fairly expensive call once per plugin. When processing
a file, a plugin can ask for any of the following:
Some properties are set once per file for plugins which iterate itself over the data instead of being called on each physical or logical line.
These parameters can also be supplied to plugins working on each line separately.
Plugins that depend on
logical_line are run on each
physical or logical line once. These parameters should be the first in the
list of arguments (with the exception of
self). Plugins that need an AST
(e.g., PyFlakes and McCabe) should depend on
tree. These plugins will run
once per file. The parameters listed above can be combined with
Any plugin that has callable attributes
parse_options can parse option information and register new options.
add_options function should expect to receive an instance of
OptionManager instance behaves very similarly to
optparse.OptionParser. It, however, uses the layer that Flake8 has
developed on top of
argparse to also handle configuration file parsing.
add_option() creates an
which accepts the same parameters as
optparse as well as three extra
The command-line option should also be parsed from config files discovered by Flake8.
This takes the place of appending strings to a list on the
The value provided to this option is a comma-separated list. After parsing the value, it should be further broken up into a list. This also allows us to handle values like:
E123,E124, E125, E126
The value provided to this option is a path. It should be normalized to be an absolute path. This can be combined with
comma_separated_listto allow a comma-separated list of paths.
Each of these options works individually or can be combined. Let’s look at a
couple examples from Flake8. In each example, we will have
option_manager which is an instance of
'--max-line-length', type='int', metavar='n',
help='Maximum allowed line length for the entirety of this run. '
Here we are adding the
--max-line-length command-line option which is
always an integer and will be parsed from the configuration file. Since we
provide a default, we take advantage of
argparse‘s willingness to
display that in the help text with
'--select', metavar='errors', default='',
help='Comma-separated list of errors and warnings to enable.'
' For example, ``--select=E4,E51,W234``. (Default: %(default)s)',
In adding the
--select command-line option, we’re also indicating to the
OptionManager that we want the value parsed from the config files and parsed
as a comma-separated list.
'--exclude', metavar='patterns', default=defaults.EXCLUDE,
help='Comma-separated list of files or directories to exclude.'
Finally, we show an option that uses all three extra flags. Values from
--exclude will be parsed from the config, converted from a comma-separated
list, and then each item will be normalized.
Accessing Parsed Options
When a plugin has a callable
parse_options attribute, Flake8 will call
it and attempt to provide the
OptionManager instance, the parsed options
which will be an instance of
argparse.Namespace, and the extra
arguments that were not parsed by the
OptionManager. If that fails, we will
just pass the
argparse.Namespace. In other words, your
parse_options callable will have one of the following signatures:
def parse_options(option_manager, options, args):