path: root/doc/faq.rst

.. -*- coding: utf-8 -*-

.. _faq:

==========================
Frequently Asked Questions
==========================

1. About Pylint
===============

1.1 What is Pylint?
--------------------

Pylint is a `static code checker`_, meaning it can analyse your code without
actually running it. Pylint checks for errors, tries to enforce a coding
standard, and tries to enforce a coding style.

.. _`static code checker`: http://en.wikipedia.org/wiki/Static_code_analysis

1.2 How is Pylint different from Pychecker?
-------------------------------------------

A major difference between Pylint and Pychecker_ is that Pylint checks for
style issues, while Pychecker explicitly does not. There are a few other
differences, such as the fact that Pylint does not import live modules while
Pychecker does (see `6.2 Why does Pychecker catch problems with imports that
Pylint doesn't?`_).

.. _Pychecker: http://pychecker.sf.net

1.3 Who wrote Pylint?
---------------------

Pylint's main author and maintainer for the first ten years of its life has been
Sylvain Thénault, while he worked at Logilab_ where the project was born. For a
full list of contributors, see the "Contributors" section of Pylint's README
file.

.. _Logilab: http://www.logilab.fr/

1.4 Who uses Pylint?
--------------------

Everybody knows someone who uses Pylint.

2. Installation
===============

2.1 How do I install Pylint?
----------------------------

Everything should be explained on http://docs.pylint.org/installation

2.2 What kind of versioning system does Pylint use?
---------------------------------------------------

Pylint uses the Mercurial_ distributed version control system. The URL of the
repository is: https://bitbucket.org/logilab/pylint. To get the latest version of
Pylint from the repository, simply invoke ::

    hg clone https://bitbucket.org/logilab/pylint

.. _Mercurial: http://mercurial.selenic.com/

2.3 What are Pylint's dependencies?
-----------------------------------

Pylint requires the latest `astroid`_ and `logilab-common`_ packages. It should be
compatible with any Python version greater than 2.7.0.

.. _`astroid`: https://bitbucket.org/logilab/astroid
.. _`logilab-common`: http://www.logilab.org/project/logilab-common

2.4 What versions of Python is Pylint supporting?
--------------------------------------------------

Since Pylint 1.4, we support only Python 2.7+ and Python 3.3+.
Using this strategy really helps in maintaining a code base compatible
with both versions and from this benefits not only the maintainers,
but the end users as well, because it's easier to add and test
new features.
If support for Python 2.6 is absolutely required, then the version
from pylint-1.3 branch can be used. It will receive backports of
bug fixes for a while.


3. Running Pylint
=================

3.1 Can I give pylint a file as an argument instead of a module?
-----------------------------------------------------------------

Pylint expects the name of a package or module as its argument. As a
convenience,
you can give it a file name if it's possible to guess a module name from
the file's path using the python path. Some examples :

"pylint mymodule.py" should always work 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.

"pylint /whatever/directory/mymodule.py" will work if either:

	- "/whatever/directory" is in the python path

	- your cwd is "/whatever/directory"

	- "directory" is a python package and "/whatever" is in the python
          path

	- "directory" is a python package and your cwd is "/whatever" and so
          on...

3.2 Where is the persistent data stored to compare between successive runs?
----------------------------------------------------------------------------

Analysis data are stored as a pickle file in a directory which is
localized using the following rules:

* value of the PYLINTHOME environment variable if set

* ".pylint.d" subdirectory of the user's home directory if it is found
	(not always findable on Windows platforms)

* ".pylint.d" directory in the current directory

3.3 How do I find the option name (for pylintrc) corresponding to a specific command line option?
--------------------------------------------------------------------------------------------------------

You can always generate a sample pylintrc file with --generate-rcfile
Every option present on the command line before this will be included in
the rc file

For example::

	pylint --disable=bare-except,invalid-name --class-rgx='[A-Z][a-z]+' --generate-rcfile

3.4 I'd rather not run Pylint from the command line. Can I integrate it with my editor?
---------------------------------------------------------------------------------------

Much probably. Read http://docs.pylint.org/ide-integration

4. Message Control
==================

4.1 Is it possible to locally disable a particular message?
-----------------------------------------------------------

Yes, this feature has been added in Pylint 0.11. This may be done by
adding "#pylint: disable=some-message,another-one" at the desired block level
or at the end of the desired line of code

4.2 Is there a way to disable a message for a particular module only?
---------------------------------------------------------------------

Yes, you can disable or enable (globally disabled) messages at the
module level by adding the corresponding option in a comment at the
top of the file: ::

	# pylint: disable=wildcard-import, method-hidden
	# pylint: enable=too-many-lines

4.3 How can I tell Pylint to never check a given module?
--------------------------------------------------------

With Pylint < 0.25, add "#pylint: disable-all" at the beginning of the
module. Pylint 0.26.1 and up have renamed that directive to
"#pylint: skip-file" (but the first version will be kept for backward
compatibility).

In order to ease finding which modules are ignored a Information-level message
`file-ignored` is emited. With recent versions of Pylint, if you use the old
syntax, an additional `deprecated-disable-all` message is emited.

4.4 Do I have to remember all these numbers?
--------------------------------------------

No, starting from 0.25.3, you can use symbolic names for messages::

    # pylint: disable=fixme, line-too-long


4.5 I have a callback function where I have no control over received arguments. How do I avoid getting unused argument warnings?
----------------------------------------------------------------------------------------------------------------------------------

Prefix (ui) the callback's name by `cb_`, as in cb_onclick(...). By
doing so arguments usage won't be checked. Another solution is to
use one of the names defined in the "dummy-variables" configuration
variable for unused argument ("_" and "dummy" by default).

4.6 What is the format of the configuration file?
---------------------------------------------------

Pylint uses ConfigParser from the standard library to parse the configuration
file.  It means that if you need to disable a lot of messages, you can use
tricks like: ::

    # disable wildcard-import, method-hidden and too-many-lines because I do
    # not want it
    disable= wildcard-import,
     method-hidden,
     too-many-lines

5. Classes and Inheritance
==========================


5.1 When is Pylint considering a class as an abstract class?
-------------------------------------------------------------

A class is considered as an abstract class if at least one of its
methods is doing nothing but raising NotImplementedError.

5.2 How do I avoid "access to undefined member" messages in my mixin classes?
-------------------------------------------------------------------------------

To do so you have to set the ignore-mixin-members option to
"yes" (this is the default value) and to name your mixin class with
a name which ends with "mixin" (whatever case).


6. Troubleshooting
==================

6.1 Pylint gave my code a negative rating out of ten. That can't be right!
--------------------------------------------------------------------------

Even though the final rating Pylint renders is nominally out of ten, there's no
lower bound on it. By default, the formula to calculate score is ::

    10.0 - ((float(5 * error + warning + refactor + convention) / statement) * 10)

However, this option can be changed in the Pylint rc file. If having negative
values really bugs you, you can set the formula to be the maximum of 0 and the
above expression.


6.2 Why does Pychecker catch problems with imports that Pylint doesn't?
------------------------------------------------------------------------

Pychecker and Pylint use different approaches.  pychecker
imports the modules and rummages around in the result, hence it sees my
mangled sys.path.  Pylint doesn't import any of the candidate modules and
thus doesn't include any of import's side effects (good and bad).  It
traverses an AST representation of the code.

6.3 Pylint keeps crashing with `Maximum recursion depth exceeded`
-----------------------------------------------------------------

Pylint can crash with this error if you have a string in your analyzed
program, created by joining a lot of strings with the addition operator.
Due to how Pylint works, visiting nodes on a AST tree and due to how
the BinOp node is represented (the node which represents the string '1+1'
for instance), the same visit method will be called over and over again, leading
to a maximum recursion error. You can alleviate this problem by passing
the flag `--optimize-ast=y` to Pylint. This will activate an optimization
which will transform such AST subtrees into the final resulting string.
This flag is off by default. If this is not the case, please report a bug!

6.4 I think I found a bug in Pylint. What should I do?
-------------------------------------------------------

Read http://docs.pylint.org/contribute#bug-reports-feedback

6.5 I have a question about Pylint that isn't answered here.
------------------------------------------------------------

Read http://docs.pylint.org/contribute#mailing-lists