Darglint

A functional docstring linter which checks whether a docstring's description matches the actual function/method implementation. Darglint expects docstrings to be formatted using the Google Python Style Guide, or Sphinx Style Guide, or Numpy Style Guide.

Feel free to submit an issue/pull request if you spot a problem or would like a feature in darglint.

Table of Contents:

• Installation
• Configuration
• Usage
• Scope
• Sphinx
• Numpy
• Integrations
• Flake8
• Roadmap
• Contribution

Installation

To install darglint, use pip.

pip install darglint

Or, clone the repository, cd to the directory, and

pip install .

Configuration

darglint can be configured using a configuration file. The configuration file must be named either .darglint, setup.cfg, or tox.ini. It must also have a section starting with the section header, [darglint]. Finally, the configuration file must be located either in the directory darglint is called from, or from a parent directory of that working directory.

Currently, the configuration file allows us to ignore errors, to specify message templates, to specify the strictness of checks and to ignore common exceptions.

Error Configuration

If we would like to ignore ExcessRaiseErrors (because we know that an underlying function will raise an exception), then we would add its error code to a file named .darglint:

[darglint]
ignore=DAR402

We can ignore multiple errors by using a comma-separated list:

[darglint]
ignore=DAR402,DAR103

Instead of specifying error codes to ignore in general one can also specify a regex to exclude certain function names from tests. For example, the following configuration would disable linting on all private methods.

[darglint]
ignore_regex=^_(.*)

Message Template Configuration

If we would like to specify a message template, we may do so as follows:

[darglint]
message_template={msg_id}@{path}:{line}

Which will produce a message such as [email protected]:72.

Finally, we can specify the docstring style type using docstring_style ("google" by default):

[darglint]
docstring_style=sphinx

Strictness Configuration

Strictness determines how lax darglint will be when checking docstrings. There are three levels of strictness available:

• short: One-line descriptions are acceptable; anything more and the docstring will be fully checked.

• long: One-line descriptions and descriptions without arguments/returns/yields/etc. sections will be allowed. Anything more, and the docstring will be fully checked.

• full: (Default) Docstrings will be fully checked.

For example, if we have the following function:

def double(x):
    # <docstring>
    return x * 2

Then the following table describes which errors will be raised for each of the docstrings (rows) when checked against each of the configurations (columns):

┌──────────────────────────────┬──────────────────┬────────────────┬──────────────────┐
│ Docstring                    │  short           │  long          │  full            │
├──────────────────────────────┼──────────────────┼────────────────┼──────────────────┤
│ """Doubles the argument."""  │ None             │ None           │ Missing argument │
│                              │                  │                │ Missing return   │
│                              │                  │                │                  │
│                              │                  │                │                  │
├──────────────────────────────┼──────────────────┼────────────────┼──────────────────┤
│ """Doubles the argument.     │ Missing argument │ None           │ Missing argument │
│                              │ Missing return   │                │ Missing return   │
│ Not very pythonic.           │                  │                │                  │
│                              │                  │                │                  │
│ """                          │                  │                │                  │
│                              │                  │                │                  │
├──────────────────────────────┼──────────────────┼────────────────┼──────────────────┤
│ """Doubles the argument.     │ Missing return   │ Missing return │ Missing return   │
│                              │                  │                │                  │
│ Args:                        │                  │                │                  │
│     x: The number to double. │                  │                │                  │
│                              │                  │                │                  │
│ """                          │                  │                │                  │
└──────────────────────────────┴──────────────────┴────────────────┴──────────────────┘

In short, if you want to be able to have single-line docstrings, and check all other docstrings against their described parameters, you would specify

[darglint]
strictness=short

In your configuration file.

Ignoring common exceptions

We can specify a list of exceptions that don't need to be documented in the raises section of a docstring. For example,

[darglint]
ignore_raise=ValueError,MyCustomError

Logging

When darglint fails unexpectedly, you can try to gather more information when submitting a bug by running with logging. For example,

darglint --log-level=INFO unexpected_failures.py

Darglint accepts the levels, DEBUG, INFO, WARNING, ERROR, and CRITICAL.

Usage

Command Line use

Given a python source file, serializers.py, you would check the docstrings as follows:

darglint serializers.py

You can give an optional verbosity setting to darglint. For example,

darglint -v 2 *.py

Would give a description of the error along with information as to this specific instance. The default verbosity is 1, which gives the filename, function name, line number, error code, and some general hints.

To use an arbitrary error format, you can pass a message template, which is a python format string. For example, if we pass the message template

darglint -m "{path}:{line} -> {msg_id}" darglint/driver.py

Then we would get back error messages like

darglint/driver.py :61 -> DAR101

The following attributes can be passed to the format string:

• line: The line number,
• msg: The error message,
• msg_id: The error code,
• obj: The function/method name,
• path: The relative file path.

The message template can also be specified in the configuration file as the value message_template.

darglint is particularly useful when combined with the utility, find. This allows us to check all of the files in our project at once. For example, when eating my own dogfood (as I tend to do), I invoke darglint as follows:

find . -name "*.py" | xargs darglint

Where I'm searching all files ending in ".py" recursively from the current directory, and calling darglint on each one in turn.

Ignoring Errors in a Docstring

You can ignore specific errors in a particular docstring. The syntax is much like that of pycodestyle, etc. It generally takes the from of:

# noqa: <error> <argument>

Where <error> is the particular error to ignore (DAR402, or DAR201 for example), and <argument> is what (if anything) the ignore statement refers to (if nothing, then it is not specified).

Let us say that we want to ignore a missing return statement in the following docstring:

def we_dont_want_a_returns_section():
  """Return the value, 3.

  # noqa: DAR201

  """
  return 3

We put the noqa anywhere in the top level of the docstring. However, this won't work if we are missing something more specific, like a parameter. We may not want to ignore all missing parameters, either, just one particular one. For example, we may be writing a function that takes a class instance as self. (Say, in a bound celery task.) Then we would do something like:

def a_bound_function(self, arg1):
  """Do something interesting.

  Args:
    arg1: The first argument.

  # noqa: DAR101 arg1

  """
  arg1.execute(self)

So, the argument comes to the right of the error.

We may also want to mark excess documentation as being okay. For example, we may not want to explicitly catch and raise a ZeroDivisionError. We could do the following:

def always_raises_exception(x):
    """Raise a zero division error or type error.o

    Args:
      x: The argument which could be a number or could not be.

    Raises:
      ZeroDivisionError: If x is a number.  # noqa: DAR402
      TypeError: If x is not a number.  # noqa: DAR402

    """
    x / 0

So, in this case, the argument for noqa is really all the way to the left. (Or whatever description we are parsing.) We could also have put it on its own line, as # noqa: DAR402 ZeroDivisionError.

Type Annotations

Darglint parses type annotations in docstrings, and can, optionally, compare the documented type to the actual type annotation. This can be useful when migrating a codebase to use type annotations.

In order to make these comparisons, Darglint only accepts types accepted by Python (see PEP 484.) That is, it does not accept parentheses in type signatures. (If parentheses are used in the type signature, Darglint will mark that argument as missing. See Issue #90.)

Error Codes

• DAR001: The docstring was not parsed correctly due to a syntax error.
• DAR002: An argument/exception lacks a description
• DAR003: A line is under-indented or over-indented.
• DAR004: The docstring contains an extra newline where it shouldn't.
• DAR005: The item contains a type section (parentheses), but no type.
• DAR101: The docstring is missing a parameter in the definition.
• DAR102: The docstring contains a parameter not in function.
• DAR103: The docstring parameter type doesn't match function.
• DAR104: (disabled) The docstring parameter has no type specified
• DAR105: The docstring parameter type is malformed.
• DAR201: The docstring is missing a return from definition.
• DAR202: The docstring has a return not in definition.
• DAR203: The docstring parameter type doesn't match function.
• DAR301: The docstring is missing a yield present in definition.
• DAR302: The docstring has a yield not in definition.
• DAR401: The docstring is missing an exception raised.
• DAR402: The docstring describes an exception not explicitly raised.
• DAR501: The docstring describes a variable which is not defined.

The number in the hundreds narrows the error by location in the docstring:

• 000: Syntax, formatting, and style
• 100: Args section
• 200: Returns section
• 300: Yields section
• 400: Raises section
• 500: Variables section

You can enable disabled-by-default exceptions in the configuration file using the enable option. It accepts a comma-separated list of error codes.

[darglint]
enable=DAR104

Scope

Darglint's primary focus is to identify incorrect and missing documentationd of a function's signature. Checking style is a stretch goal, and is supported on a best-effort basis. Darglint does not check stylistic preferences expressed by tools in the Python Code Quality Authority (through tools such as pydocstyle). So when using Darglint, it may be a good idea to also use pydocstyle, if you want to enforce style. (For example, pydocstyle requires the short summary to be separated from other sections by a line break. Darglint makes no such check.)

Sphinx

Darglint can handle sphinx-style docstrings, but imposes some restrictions on top of the Sphinx style. For example, all fields (such as :returns:) must be the last items in the docstring. They must be together, and all indents should be four spaces. These restrictions may be loosened at a later date.

To analyze Sphinx-style docstrings, pass the style flag to the command:

darglint -s sphinx example.py
darglint --docstring-style sphinx example.py

Alternatively, you can specify the style in the configuration file using the setting, "docstring_style":

[darglint]
docstring_style=sphinx

Numpy

Darglint now has an initial implementation for Numpy-style docstrings. Similarly to Sphinx-style docstrings, you can pass a style flag to the command:

darglint -s numpy example.py
darglint --docstring-style numpy example.py

Or set it in a configuration file:

[darglint]
docstring_style=numpy

The numpy parser and error reporter are not yet fully stabilized. Add issues or suggestions to the tracking bug, Issue #69.

Integrations

Flake8

Darglint can be used in conjunction with Flake8 as a plugin. The only setup necessary is to install Flake8 and Darglint in the same environment. Darglint will pull its configuration from Flake8. So, if you would like to lint Sphinx-style comments, then you should have docstring_style=sphinx in a Flake8 configuration file in the project directory. The settings would be entered under the flake8 configuration, not a separate configuration for Darglint. E.g.:

[flake8]
strictness=short
docstring_style=sphinx

To see which options are exposed through Flake8, you can check the Flake8 tool:

flake8 --help | grep --before-context=2 Darglint

SublimeLinter

A plugin for SublimeLinter can be found here

Pre-commit

Download pre-commit and install it. Once it is installed, add this to .pre-commit-config.yaml in your repository:

repos:
-   repo: https://github.com/terrencepreilly/darglint
    rev: master
    hooks:
    - id: darglint

Then run pre-commit install and you're ready to go. Before commiting, darglint will be run on the staged files. If it finds any errors, the user is notified and the commit is aborted. Store necessary configuration (such as error formatting) in .darglint, setup.cfg or tox.ini.

Roadmap

Below are some of the current features or efforts. Where a milestone or issue is associated with the idea, it will be mentioned. Some of these ideas are moonshots and may not get implemented. They are ordered roughly according to current priority/feasibility.

• Expose command-line options through sphinx.
• Robust logging for errors caused/encountered by darglint.
• Check class docstrings (See Issue #25).
• Autoformatting docstrings. (See Milestone #3).
• Optional aggressive style checking through command line flag.
• ALE support.
• Syntastic support. (Syntastic is not accepting new checkers until their next API stabilizes, so this may take some time.)

Development and Contributions

Development Setup

Install darglint. First, clone the repository:

git clone https://github.com/terrencepreilly/darglint.git

cd into the directory, create a virtual environment (optional), then setup:

cd darglint/
virtualenv -p python3.6 .env
source .env/bin/activate
pip install -e .

You can install dependencies using

pip install poetry
poetry install

You can run the tests using

python setup.py test

Or, install pytest manually, cd to the project's root directory, and run

pytest

This project tries to conform by the styles imposed by pycodestyle and pydocstyle, as well as by darglint itself.

A dockerfile exists for testing with Python3.4. Although it's not officially supported (only 3.6+), it's nice to try to make minor version numbers support it. You would build the dockerfile and test using something like

pushd docker-build
docker build -t darglint-34 -f Dockerfile.test34 .
popd
docker run -it --rm -v $(pwd):/code darglint-34 pytest

Contribution

If you would like to tackle an issue or feature, email me or comment on the issue to make sure it isn't already being worked on. Contributions will be accepted through pull requests. New features should include unit tests, and, of course, properly formatted documentation.

Also, check out the wiki prior to updating the grammar. It includes a description of darglint's parsing pipline.