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.

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?).

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.

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

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 2.5.0.

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

You can show these symbols in the output with the -sy option.

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

4.7 Why do I get a lot of spurious “unused variables messages” when using psyobj from psyco?

This is actually due to a bug in psyco, making the locals() function for objects inheriting from psyobj returning an empty dictionary. For the moment, the only way to fix this is to use the PYLINT_IMPORT environment variable to not use psyco during Pylint checking. Sample code

import os
try:
        if os.environ.has_key('PYLINT_IMPORT'):
                raise ImportError()
from psyco.classes import psyobj
except ImportError:
        class psyobj:
                pass

NOTICE: this problem should not occur with Pylint >= 0.5 since from this version Pylint is not looking anymore for information in living objects (i.e. it no longer imports analysed modules)

5. Classes and Inheritance

5.1 When is Pylint considering a class as an interface?

A class is considered as an interface if there is a class named “Interface” somewhere in its inheritance tree.

5.2 When is Pylint considering that a class is implementing a given interface?

Pylint is using the Zope 2 interfaces conventions, and so is considering that a class is implementing interfaces listed in its __implements__ attribute.

5.3 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.

5.4 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 minimum 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 I think I found a bug in Pylint. What should I do?

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

6.4 I have a question about Pylint that isn’t answered here.

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

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