path: root/docs/intro.rst

.. currentmodule:: pep8

Introduction
============

pep8 is a tool to check your Python code against some of the style
conventions in `PEP 8`_.

.. contents::
   :local:


Features
--------

* Plugin architecture: Adding new checks is easy.

* Parseable output: Jump to error location in your editor.

* Small: Just one Python file, requires only stdlib.  You can use just
  the pep8.py file for this purpose.

* Comes with a comprehensive test suite.


Disclaimer
----------

This utility does not enforce every single rule of PEP 8.  It helps to
verify that some coding conventions are applied but it does not intend
to be exhaustive.  Some rules cannot be expressed with a simple algorithm,
and other rules are only guidelines which you could circumvent when you
need to.

Always remember this statement from `PEP 8`_:

  *A style guide is about consistency. Consistency with this style guide is
  important. Consistency within a project is more important. Consistency
  within one module or function is most important.*


Among other things, these features are currently not in the scope of
the ``pep8`` library:

* **naming conventions**: this kind of feature is supported through plugins.
  Install `flake8 <https://pypi.python.org/pypi/flake8>`_ and the
  `pep8-naming extension <https://pypi.python.org/pypi/pep8-naming>`_ to use
  this feature.
* **docstring conventions**: they are not in the scope of this library;
  see the `pep257 project <https://github.com/GreenSteam/pep257>`_.
* **automatic fixing**: see the section *PEP8 Fixers* in the
  :ref:`related tools <related-tools>` page.


Installation
------------

You can install, upgrade, uninstall pep8.py with these commands::

  $ pip install pep8
  $ pip install --upgrade pep8
  $ pip uninstall pep8

There's also a package for Debian/Ubuntu, but it's not always the
latest version::

  $ sudo apt-get install pep8


Example usage and output
------------------------

::

  $ pep8 --first optparse.py
  optparse.py:69:11: E401 multiple imports on one line
  optparse.py:77:1: E302 expected 2 blank lines, found 1
  optparse.py:88:5: E301 expected 1 blank line, found 0
  optparse.py:222:34: W602 deprecated form of raising exception
  optparse.py:347:31: E211 whitespace before '('
  optparse.py:357:17: E201 whitespace after '{'
  optparse.py:472:29: E221 multiple spaces before operator
  optparse.py:544:21: W601 .has_key() is deprecated, use 'in'

You can also make pep8.py show the source code for each error, and
even the relevant text from PEP 8::

  $ pep8 --show-source --show-pep8 testsuite/E40.py
  testsuite/E40.py:2:10: E401 multiple imports on one line
  import os, sys
           ^
      Imports should usually be on separate lines.

      Okay: import os\nimport sys
      E401: import sys, os


Or you can display how often each error was found::

  $ pep8 --statistics -qq Python-2.5/Lib
  232     E201 whitespace after '['
  599     E202 whitespace before ')'
  631     E203 whitespace before ','
  842     E211 whitespace before '('
  2531    E221 multiple spaces before operator
  4473    E301 expected 1 blank line, found 0
  4006    E302 expected 2 blank lines, found 1
  165     E303 too many blank lines (4)
  325     E401 multiple imports on one line
  3615    E501 line too long (82 characters)
  612     W601 .has_key() is deprecated, use 'in'
  1188    W602 deprecated form of raising exception

You can also make pep8.py show the error text in different formats by using --format having options default/pylint/custom::

  $ pep8 testsuite/E40.py --format=default
  testsuite/E40.py:2:10: E401 multiple imports on one line

  $ pep8 testsuite/E40.py --format=pylint
  testsuite/E40.py:2: [E401] multiple imports on one line

  $ pep8 testsuite/E40.py --format='%(path)s|%(row)d|%(col)d| %(code)s %(text)s'
  testsuite/E40.py|2|10| E401 multiple imports on one line

Variables in the ``custom`` format option

+----------------+------------------+
|   Variable     |   Significance   |
+================+==================+
| ``path``       | File name        |
+----------------+------------------+
| ``row``        | Row number       |
+----------------+------------------+
| ``col``        | Column number    |
+----------------+------------------+
| ``code``       | Error code       |
+----------------+------------------+
| ``text``       | Error text       |
+----------------+------------------+

Quick help is available on the command line::

  $ pep8 -h
  Usage: pep8 [options] input ...

  Options:
    --version            show program's version number and exit
    -h, --help           show this help message and exit
    -v, --verbose        print status messages, or debug with -vv
    -q, --quiet          report only file names, or nothing with -qq
    --first              show first occurrence of each error
    --exclude=patterns   exclude files or directories which match these comma
                         separated patterns (default: .svn,CVS,.bzr,.hg,.git)
    --filename=patterns  when parsing directories, only check filenames matching
                         these comma separated patterns (default: *.py)
    --select=errors      select errors and warnings (e.g. E,W6)
    --ignore=errors      skip errors and warnings (e.g. E4,W)
    --show-source        show source code for each error
    --show-pep8          show text of PEP 8 for each error (implies --first)
    --statistics         count errors and warnings
    --count              print total number of errors and warnings to standard
                         error and set exit code to 1 if total is not null
    --max-line-length=n  set maximum allowed line length (default: 79)
    --hang-closing       hang closing bracket instead of matching indentation of
                         opening bracket's line
    --format=format      set the error format [default|pylint|<custom>]
    --diff               report only lines changed according to the unified diff
                         received on STDIN

    Testing Options:
      --benchmark        measure processing speed

    Configuration:
      The project options are read from the [pep8] section of the tox.ini
      file or the setup.cfg file located in any parent folder of the path(s)
      being processed.  Allowed options are: exclude, filename, select,
      ignore, max-line-length, hang-closing, count, format, quiet, show-pep8,
      show-source, statistics, verbose.

      --config=path      user config file location (default: ~/.config/pep8)


Configuration
-------------

The behaviour may be configured at two levels, the user and project levels.

At the user level, settings are read from the following locations:

If on Windows:
    ``~\.pep8``

Otherwise, if the :envvar:`XDG_CONFIG_HOME` environment variable is defined:
    ``XDG_CONFIG_HOME/pep8``

Else if :envvar:`XDG_CONFIG_HOME` is not defined:
    ``~/.config/pep8``

Example::

  [pep8]
  ignore = E226,E302,E41
  max-line-length = 160

At the project level, a ``setup.cfg`` file or a ``tox.ini`` file is read if
present (``.pep8`` file is also supported, but it is deprecated).  If none of
these files have a ``[pep8]`` section, no project specific configuration is
loaded.

If the ``ignore`` option is not in the configuration and not in the arguments,
only the error codes ``E123/E133``, ``E226`` and ``E241/E242`` are ignored
(see below).


Error codes
-----------

This is the current list of error and warning codes:

+----------+----------------------------------------------------------------------+
| code     | sample message                                                       |
+==========+======================================================================+
| **E1**   | *Indentation*                                                        |
+----------+----------------------------------------------------------------------+
| E101     | indentation contains mixed spaces and tabs                           |
+----------+----------------------------------------------------------------------+
| E111     | indentation is not a multiple of four                                |
+----------+----------------------------------------------------------------------+
| E112     | expected an indented block                                           |
+----------+----------------------------------------------------------------------+
| E113     | unexpected indentation                                               |
+----------+----------------------------------------------------------------------+
| E114     | indentation is not a multiple of four (comment)                      |
+----------+----------------------------------------------------------------------+
| E115     | expected an indented block (comment)                                 |
+----------+----------------------------------------------------------------------+
| E116     | unexpected indentation (comment)                                     |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| E121 (*^)| continuation line under-indented for hanging indent                  |
+----------+----------------------------------------------------------------------+
| E122 (^) | continuation line missing indentation or outdented                   |
+----------+----------------------------------------------------------------------+
| E123 (*) | closing bracket does not match indentation of opening bracket's line |
+----------+----------------------------------------------------------------------+
| E124 (^) | closing bracket does not match visual indentation                    |
+----------+----------------------------------------------------------------------+
| E125 (^) | continuation line with same indent as next logical line              |
+----------+----------------------------------------------------------------------+
| E126 (*^)| continuation line over-indented for hanging indent                   |
+----------+----------------------------------------------------------------------+
| E127 (^) | continuation line over-indented for visual indent                    |
+----------+----------------------------------------------------------------------+
| E128 (^) | continuation line under-indented for visual indent                   |
+----------+----------------------------------------------------------------------+
| E129 (^) | visually indented line with same indent as next logical line         |
+----------+----------------------------------------------------------------------+
| E131 (^) | continuation line unaligned for hanging indent                       |
+----------+----------------------------------------------------------------------+
| E133 (*) | closing bracket is missing indentation                               |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| **E2**   | *Whitespace*                                                         |
+----------+----------------------------------------------------------------------+
| E201     | whitespace after '('                                                 |
+----------+----------------------------------------------------------------------+
| E202     | whitespace before ')'                                                |
+----------+----------------------------------------------------------------------+
| E203     | whitespace before ':'                                                |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| E211     | whitespace before '('                                                |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| E221     | multiple spaces before operator                                      |
+----------+----------------------------------------------------------------------+
| E222     | multiple spaces after operator                                       |
+----------+----------------------------------------------------------------------+
| E223     | tab before operator                                                  |
+----------+----------------------------------------------------------------------+
| E224     | tab after operator                                                   |
+----------+----------------------------------------------------------------------+
| E225     | missing whitespace around operator                                   |
+----------+----------------------------------------------------------------------+
| E226 (*) | missing whitespace around arithmetic operator                        |
+----------+----------------------------------------------------------------------+
| E227     | missing whitespace around bitwise or shift operator                  |
+----------+----------------------------------------------------------------------+
| E228     | missing whitespace around modulo operator                            |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| E231     | missing whitespace after ',', ';', or ':'                            |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| E241 (*) | multiple spaces after ','                                            |
+----------+----------------------------------------------------------------------+
| E242 (*) | tab after ','                                                        |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| E251     | unexpected spaces around keyword / parameter equals                  |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| E261     | at least two spaces before inline comment                            |
+----------+----------------------------------------------------------------------+
| E262     | inline comment should start with '# '                                |
+----------+----------------------------------------------------------------------+
| E265     | block comment should start with '# '                                 |
+----------+----------------------------------------------------------------------+
| E266     | too many leading '#' for block comment                               |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| E271     | multiple spaces after keyword                                        |
+----------+----------------------------------------------------------------------+
| E272     | multiple spaces before keyword                                       |
+----------+----------------------------------------------------------------------+
| E273     | tab after keyword                                                    |
+----------+----------------------------------------------------------------------+
| E274     | tab before keyword                                                   |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| **E3**   | *Blank line*                                                         |
+----------+----------------------------------------------------------------------+
| E301     | expected 1 blank line, found 0                                       |
+----------+----------------------------------------------------------------------+
| E302     | expected 2 blank lines, found 0                                      |
+----------+----------------------------------------------------------------------+
| E303     | too many blank lines (3)                                             |
+----------+----------------------------------------------------------------------+
| E304     | blank lines found after function decorator                           |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| **E4**   | *Import*                                                             |
+----------+----------------------------------------------------------------------+
| E401     | multiple imports on one line                                         |
+----------+----------------------------------------------------------------------+
| E402     | module level import not at top of file                               |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| **E5**   | *Line length*                                                        |
+----------+----------------------------------------------------------------------+
| E501 (^) | line too long (82 > 79 characters)                                   |
+----------+----------------------------------------------------------------------+
| E502     | the backslash is redundant between brackets                          |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| **E7**   | *Statement*                                                          |
+----------+----------------------------------------------------------------------+
| E701     | multiple statements on one line (colon)                              |
+----------+----------------------------------------------------------------------+
| E702     | multiple statements on one line (semicolon)                          |
+----------+----------------------------------------------------------------------+
| E703     | statement ends with a semicolon                                      |
+----------+----------------------------------------------------------------------+
| E704 (*) | multiple statements on one line (def)                                |
+----------+----------------------------------------------------------------------+
| E711 (^) | comparison to None should be 'if cond is None:'                      |
+----------+----------------------------------------------------------------------+
| E712 (^) | comparison to True should be 'if cond is True:' or 'if cond:'        |
+----------+----------------------------------------------------------------------+
| E713     | test for membership should be 'not in'                               |
+----------+----------------------------------------------------------------------+
| E714     | test for object identity should be 'is not'                          |
+----------+----------------------------------------------------------------------+
| E721     | do not compare types, use 'isinstance()'                             |
+----------+----------------------------------------------------------------------+
| E731     | do not assign a lambda expression, use a def                         |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| **E9**   | *Runtime*                                                            |
+----------+----------------------------------------------------------------------+
| E901     | SyntaxError or IndentationError                                      |
+----------+----------------------------------------------------------------------+
| E902     | IOError                                                              |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| **W1**   | *Indentation warning*                                                |
+----------+----------------------------------------------------------------------+
| W191     | indentation contains tabs                                            |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| **W2**   | *Whitespace warning*                                                 |
+----------+----------------------------------------------------------------------+
| W291     | trailing whitespace                                                  |
+----------+----------------------------------------------------------------------+
| W292     | no newline at end of file                                            |
+----------+----------------------------------------------------------------------+
| W293     | blank line contains whitespace                                       |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| **W3**   | *Blank line warning*                                                 |
+----------+----------------------------------------------------------------------+
| W391     | blank line at end of file                                            |
+----------+----------------------------------------------------------------------+
+----------+----------------------------------------------------------------------+
| **W6**   | *Deprecation warning*                                                |
+----------+----------------------------------------------------------------------+
| W601     | .has_key() is deprecated, use 'in'                                   |
+----------+----------------------------------------------------------------------+
| W602     | deprecated form of raising exception                                 |
+----------+----------------------------------------------------------------------+
| W603     | '<>' is deprecated, use '!='                                         |
+----------+----------------------------------------------------------------------+
| W604     | backticks are deprecated, use 'repr()'                               |
+----------+----------------------------------------------------------------------+


**(*)** In the default configuration, the checks **E121**, **E123**, **E126**,
**E133**, **E226**, **E241**, **E242** and **E704** are ignored because they
are not rules unanimously accepted, and `PEP 8`_ does not enforce them.  The
check **E133** is mutually exclusive with check **E123**.  Use switch ``--hang-
closing`` to report **E133** instead of **E123**.

**(^)** These checks can be disabled at the line level using the ``# noqa``
special comment.  This possibility should be reserved for special cases.

  *Special cases aren't special enough to break the rules.*


Note: most errors can be listed with such one-liner::

  $ python pep8.py --first --select E,W testsuite/ --format '%(code)s: %(text)s'


.. _related-tools:

Related tools
-------------

The `flake8 checker <https://flake8.readthedocs.org>`_ is a wrapper around
``pep8`` and similar tools. It supports plugins.

Other tools which use ``pep8`` are referenced in the Wiki: `list of related
tools <https://github.com/jcrocholl/pep8/wiki/RelatedTools>`_.

.. _PEP 8: http://www.python.org/dev/peps/pep-0008/