Extending Pylint

Writing your own checker

You can find some simple examples in the examples directory of the distribution (custom.py and custom_raw.py). 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.

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