Option and Configuration Handling

Option Management

Command-line options are often also set in configuration files for Flake8. While not all options are meant to be parsed from configuration files, many default options are also parsed from configuration files as well as most plugin options.

In Flake8 2, plugins received a optparse.OptionParser instance and called optparse.OptionParser.add_option() to register options. If the plugin author also wanted to have that option parsed from config files they also had to do something like:

parser.config_options.extend(['config_opt1', 'config_opt2'])

This was previously undocumented and led to a lot of confusion about why registered options were not automatically parsed from configuration files.

Since Flake8 3 was rewritten from scratch, we decided to take a different approach to configuration file parsing. Instead of needing to know about an undocumented attribute that pep8 looks for, Flake8 3 now accepts a parameter to add_option, specifically parse_from_config which is a boolean value.

Flake8 does this by creating its own abstractions on top of argparse. The first abstraction is the flake8.options.manager.Option class. The second is the flake8.options.manager.OptionManager. In fact, we add three new parameters:

  • parse_from_config

  • comma_separated_list

  • normalize_paths

The last two are not specifically for configuration file handling, but they do improve that dramatically. We found that there were options that, when specified in a configuration file, often necessitated being split across multiple lines and those options were almost always comma-separated. For example, let’s consider a user’s list of ignored error codes for a project:

ignore =
    # Reasoning
    # Reasoning
    # Reasoning
    # Reasoning
    # Reasoning
    # Reasoning
    # Reasoning
    # Reasoning

It makes sense here to allow users to specify the value this way, but, the standard library’s configparser.RawConfigParser class does returns a string that looks like

"\nE111,  \nE711,  \nE712,  \nE121,  \nE122,  \nE123,  \nE131,  \nE251  "

This means that a typical call to str.split() with ',' will not be sufficient here. Telling Flake8 that something is a comma-separated list (e.g., comma_separated_list=True) will handle this for you. Flake8 will return:

["E111", "E711", "E712", "E121", "E122", "E123", "E131", "E251"]

Next let’s look at how users might like to specify their exclude list. Presently OpenStack’s Nova project has this line in their tox.ini:

exclude = .venv,.git,.tox,dist,doc,*openstack/common/*,*lib/python*,*egg,build,tools/xenserver*,releasenotes

We think we can all agree that this would be easier to read like this:

exclude =

In this case, since these are actually intended to be paths, we would specify both comma_separated_list=True and normalize_paths=True because we want the paths to be provided to us with some consistency (either all absolute paths or not).

Now let’s look at how this will actually be used. Most plugin developers will receive an instance of OptionManager so to ease the transition we kept the same API as the optparse.OptionParser object. The only difference is that add_option() accepts the three extra arguments we highlighted above.

Configuration File Management

In Flake8 2, configuration file discovery and management was handled by pep8. In pep8’s 1.6 release series, it drastically broke how discovery and merging worked (as a result of trying to improve it). To avoid a dependency breaking Flake8 again in the future, we have created our own discovery and management in 3.0.0. In 4.0.0 we have once again changed how this works and we removed support for user-level config files.

  • Project files (files stored in the current directory) are read next and merged on top of the user file. In other words, configuration in project files takes precedence over configuration in user files.

  • New in 3.0.0 The user can specify --append-config <path-to-file> repeatedly to include extra configuration files that should be read and take precedence over user and project files.

  • New in 3.0.0 The user can specify --config <path-to-file> to so this file is the only configuration file used. This is a change from Flake8 2 where pep8 would simply merge this configuration file into the configuration generated by user and project files (where this takes precedence).

  • New in 3.0.0 The user can specify --isolated to disable configuration via discovered configuration files.

To facilitate the configuration file management, we’ve taken a different approach to discovery and management of files than pep8. In pep8 1.5, 1.6, and 1.7 configuration discovery and management was centralized in 66 lines of very terse python which was confusing and not very explicit. The terseness of this function (Flake8 3.0.0’s authors believe) caused the confusion and problems with pep8’s 1.6 series. As such, Flake8 has separated out discovery, management, and merging into a module to make reasoning about each of these pieces easier and more explicit (as well as easier to test).

Configuration file discovery and raw ini reading is managed by load_config(). This produces a loaded RawConfigParser and a config directory (which will be used later to normalize paths).

Next, parse_config() parses options using the types in the OptionManager.

Most of this is done in aggregate_options().

Aggregating Configuration File and Command Line Arguments

aggregate_options() accepts an instance of OptionManager and does the work to parse the command-line arguments.

After parsing the configuration file, we determine the default ignore list. We use the defaults from the OptionManager and update those with the parsed configuration files. Finally we parse the user-provided options one last time using the option defaults and configuration file values as defaults. The parser merges on the command-line specified arguments for us so we have our final, definitive, aggregated options.

API Documentation