Writing your own checker

You can find some simple examples in the examples directory of the distribution ( and I’ll try to quickly explain the essentials here.

First, there are two kinds of checkers:

  • raw checkers, which are analysing each module as a raw file stream
  • ast checkers, which are working on an ast representation of the module

The ast representation used is an extension of the one provided with the standard Python distribution in the ast package. The extension adds additional information and methods on the tree nodes to ease navigation and code introspection.

An AST checker is a visitor, and should implement visit_<lowered class name> or leave_<lowered class name> methods for the nodes it’s interested in. To get description of the different classes used in an ast tree, look at the ast package documentation. Checkers are ordered by priority. For each module, Pylint’s engine:

  1. give the module source file as a stream to raw checkers
  2. get an ast representation for the module
  3. make a depth first descent of the tree, calling visit_<> on each AST checker when entering a node, and leave_<> on the back traversal

Notice that the source code is probably the best source of documentation, it should be clear and well documented. Don’t hesitate to ask for any information on the code-quality mailing list.

Why write a plugin?

Pylint is a static analysis tool and Python is a dynamically typed language. So there will be cases where Pylint cannot analyze files properly (this problem can happen in statically typed languages also if reflection or dynamic evaluation is used). Plugin is a way to tell Pylint how to handle such cases, since only the user would know what needs to be done.


Let us run Pylint on a module from the Python source: and see what happens:

amitdev$ pylint -E Lib/
E:297,36: Instance of 'WarningMessage' has no 'message' member (no-member)
E:298,36: Instance of 'WarningMessage' has no 'filename' member (no-member)
E:298,51: Instance of 'WarningMessage' has no 'lineno' member (no-member)
E:298,64: Instance of 'WarningMessage' has no 'line' member (no-member)

Did we catch a genuine error? Let’s open the code and look at WarningMessage class:

class WarningMessage(object):

  """Holds the result of a single showwarning() call."""

  _WARNING_DETAILS = ("message", "category", "filename", "lineno", "file",

  def __init__(self, message, category, filename, lineno, file=None,
    local_values = locals()
    for attr in self._WARNING_DETAILS:
      setattr(self, attr, local_values[attr])
    self._category_name = category.__name__ if category else None

  def __str__(self):

Ah, the fields (message, category etc) are not defined statically on the class. Instead they are added using setattr. Pylint would have a tough time figuring this out.

Enter Plugin

We can write a plugin to tell Pylint about how to analyze this properly. A plugin is a module which should have a function register and takes the lint module as input. So a basic hello-world plugin can be implemented as:

# Inside
def register(linter):
  print 'Hello world'

We can run this plugin by placing this module in the PYTHONPATH and invoking as:

amitdev$ pylint -E --load-plugins hello_plugin
Hello world

Back to our example: one way to fix that would be to transform the WarningMessage class and set the attributes using a plugin so that Pylint can see them. This can be done by registering a transform function. We can transform any node in the parsed AST like Module, Class, Function etc. In our case we need to transform a class. It can be done so:

from astroid import MANAGER
from astroid import scoped_nodes

def register(linter):

def transform(cls):
  if == 'WarningMessage':
    import warnings
    for f in warnings.WarningMessage._WARNING_DETAILS:
      cls.locals[f] = [scoped_nodes.Class(f, None)]

MANAGER.register_transform(scoped_nodes.Class, transform)

Let’s go through the plugin. First, we need to register a class transform, which is done via the register_transform function in MANAGER. It takes the node type and function as parameters. We need to change a class, so we use scoped_nodes.Class. We also pass a transform function which does the actual transformation.

transform function is simple as well. If the class is WarningMessage then we add the attributes to its locals (we are not bothered about type of attributes, so setting them as class will do. But we could set them to any type we want). That’s it.

Note: We don’t need to do anything in the register function of the plugin since we are not modifying anything in the linter itself.

Lets run Pylint with this plugin and see:

amitdev$ pylint -E --load-plugins warning_plugin Lib/

All the false positives associated with WarningMessage are now gone. This is just an example, any code transformation can be done by plugins. See nodes and scoped_nodes for details about all node types that can be transformed.