=================
Pylint Quickstart
=================

:Author: Alexandre Fayolle
:Organization: Logilab
:Version: $Revision: 1.10 $
:Date: $Date: 2005-04-15 10:40:17 $

.. contents::


This document is meant to get you started with Pylint. It assumes that
you have installed pylint following the instructions in the README
document found in the source documentation.


What is pylint?
---------------

Pylint is a tool that checks for errors in python code, tries to
enforce a coding standard and looks for smelling code . This is
similar but nevertheless different from what pychecker_ provides,
especially since pychecker explicitly does not bother with coding
style. The default coding style used by pylint is close to 
`Guido's style guide`_. For more information about code smells, refer
to Martin Fowler's `refactoring book`_

Pylint will display a number of errors and warnings as it analyzes the
code, as well as some statistics about the number of warnings and
errors found in different files. If you run pylint twice, it will
display the statistics from the previous run together with the ones
from the current run, so that you can see if the code has improved or
not.

Last but not least, the code is given an overall mark, based on the
number an severity of the warnings and errors. This has proven to
be very motivating for programmers.


Invoking pylint
---------------

Pylint is meant to be called from the command line. The usage is ::

   pylint [options] module_or_package

You should give pylint the name of a Python package or module. Pylint
will ``import`` this package or module, so you should pay attention to
your ``PYTHONPATH``, since it is a common error to analyze an
installed version of a module instead of the development version.

It is also possible to analyze python files, with a few
restrictions. The thing to keep in mind is that pylint will try to 
convert the file name to a module name, and only be able to process
the file if it succeeds.  ::

  pylint mymodule.py 

should always works since the current working
directory is automatically added on top  of the python path ::

  pylint directory/mymodule.py

will work if "directory" is a python package (i.e. has an __init__.py
file) or if "directory" is in the python path.

For more details on this see the FAQ_.

You can also start a thin gui around pylint (require TkInter) by
typing ::

  pylint-gui

This should open a window where you can enter the name of the package
or module to check, at pylint messages will be displayed in the user
interface.


Pylint output
-------------

The default format for the output is raw text. But passing pylint the
``--output-format=html`` option will produce an HTML document.

There are several sections in pylint's output. 

Source code analysis section
''''''''''''''''''''''''''''
   
For each python module,
pylint will first display a few '*' characters followed by the name
of the module. Then, a number of messages with the following
format: ::

  MESSAGE_TYPE: LINE_NUM:[OBJECT:] MESSAGE

You can get another output format, useful since it's recognized by
most editors or other development tools using the ``--parseable=y``
option.

The message type can be:

  * [R]efactor for a "good practice" metric violation
  * [C]onvention for coding standard violation
  * [W]arning for stylistic problems, or minor programming issues
  * [E]rror for important programming issues (i.e. most probably bug)
  * [F]atal for errors which prevented further processing

Sometimes the line of code which caused the error is displayed with
a caret pointing to the error. This may be generalized in future
versions of pylint.

Example (extracted from a run of pylint on itself...): 

::

  ************* Module pylint.checkers.format
  W: 50: Too long line (86/80)
  W:108: Operator not followed by a space
       print >>sys.stderr, 'Unable to match %r', line
              ^
  W:141: Too long line (81/80)
  W: 74:searchall: Unreachable code
  W:171:FormatChecker.process_tokens: Redefining built-in (type)
  W:150:FormatChecker.process_tokens: Too many local variables (20/15)
  W:150:FormatChecker.process_tokens: Too many branchs (13/12)


Reports section
''''''''''''''' 

Following the analysis message, pylint will display a set of reports,
each one focusing on a particular aspect of the project, such as number
of messages by categories, modules dependancies...

For instance, the metrics report displays summaries gathered from the
current run. 

  * the number of processed modules
  * for each module, the percentage of errors and warnings
  * the total number of errors and warnings
  * percentage of classes, functions and modules with docstrings, and
    a comparison from the previous run 
  * percentage of classes, functions and modules with correct name
    (according the the coding standard), and a comparison from the
    previous run 
  * a list of external dependencies found in the code, and where they appear

Also, a global evaluation for the code is computed, and an
optional witty comment is displayed (if ``--comment=y`` was
specified on the command line).


Command line options
--------------------

First of all, we have two basic (but useful) options. 

--version             show program's version number and exit
-h, --help            show help about the command line options

Pylint is architectured around several checkers. By default all
checkers are enabled. You can disable a specific checker by specifying
``--enable-<checker>=n``, or disable all checkers using
``--disable-all`` and afterwards enable specific checkers with
``--enable-<checker>=y``. See the list of available features_ for a
description of provided checkers with their functionalities.

Each checker has some specific options, which can take either a yes/no
value, an integer, a python regular expression, or a comma separated
list of values (which are generally used to override a regular
expression in special cases). For a full list of options, use ``--help``

Specifying all the options suitable for your setup and coding
standards can be tedious, so it is possible to use a rc file to
specify the default values. Pylint looks for /etc/pylintrc and
~/.pylintrc. The ``--generate-rcfile`` option will generate a
commented configuration file according to the current configuration on
standard output and exit. You can put other options before this one to
use them in the configuration, or start with the default values and
hand tune the configuration.

Other useful global options include:

--zope                  Initialize Zope products before starting
--ignore=file           Add <file> (may be a directory) to the black
                        list. It should be a base name, not a path. 
                        You may set this option multiple times.
--statistics=y_or_n     Compute statistics on collected data.
--persistent=y_or_n     Pickle collected data for later comparisons.
--comment=y_or_n        Add a comment according to your evaluation note.
--parseable=y_or_n      Use a parseable output format.
--html=y_or_n           Use HTML as output format instead of text.
--enable-msg=msgids     Enable the given messages.
--disable-msg=msgids    Disable the given messages.
--enable-msg-cat=cats   Enable all messages in the given categories.
--disable-msg-cat=cats  Disable all messages in the given categories.



Bug reports
-----------

You think you have found a bug in Pylint? Well, this may be the case
since Pylint is under development. Please take the time to send a bug
report to python-projects@logilab.org. This mailing list is also a
nice place to discuss Pylint issues.


.. _pychecker: http://pychecker.sf.net
.. _features: features.html
.. _FAQ: FAQ.html
.. _`Guido's style guide`: http://www.python.org/doc/essays/styleguide.html
.. _`refactoring book`: http://www.refactoring.com/