Custom checkers

Writing your own checker

You can find some simple examples in the examples directory of the distribution ( and

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. For each module, Pylint’s engine is doing the following:

  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

A checker is composed from multiple components, which needs to be given in order for it to work properly:

  1. a name. The name is internally for a couple of things, one of them being used for generating a special configuration section of the checker, in case in has provided options.

  2. a priority that needs to be lower than 0. The checkers are ordered by the priority, from the most negative to the most positive.

  3. a message dictionary. Each checker is being used for finding problems in your code, the problems being displayed to the user through messages. The message dictionary should specify what messages the said checker is going to emit. It has the following format:

    msgs = {'message-id': ('displayed-message', 'message-symbol', 'message-help')}

    The message id should be a 5-digits number, prefixed with a message category. There are multiple message categories, these being C, W, E, F, R, standing for Convention, Warning, Error, Fatal and Refactoring. The rest of the 5 digits should not conflict with existing checkers and they should be consistent across the checker. For instance, the first two digits should not be different across the checker.

    The displayed message is used for displaying the message to the user, once it is emitted. The message symbol is an alias of the message id and it can be used wherever the message id can be used. The message help is used when calling pylint --help-msg.

  4. An options list (optional)