Custom contract types

If none of the built in contract types serve your needs, you can define a custom contract type. The steps to do this are:

  1. Somewhere in your Python path, create a module that implements a Contract class for your supplied type.

  2. Register the contract type in your configuration file.

  3. Define one or more contracts of your custom type, also in your configuration file.

Step one: implementing a Contract class

You define a custom contract type by subclassing importlinter.Contract and implementing the following methods:

  • check(graph: ImportGraph, verbose: bool) -> ContractCheck:

    Given an import graph of your project, return a ContractCheck describing whether the contract was adhered to.

    • graph: a Grimp ImportGraph of your project, which can be used to inspect / analyse any dependencies. For full details of how to use this, see the Grimp documentation.

    • verbose: Whether we’re in verbose mode. You can use this flag to determine whether to output text during the check, using output.verbose_print, as in the example below.

    • An importlinter.ContractCheck instance. This is a simple dataclass with two attributes, kept (a boolean indicating if the contract was kept) and metadata (a dictionary of data about the check). The metadata can contain anything you want, as it is only used in the render_broken_contract method that you also define in this class.

  • render_broken_contract(check: ContractCheck) -> None:

    Renders the results of a broken contract check. For output, this should use the importlinter.output module.

    • check: the ContractCheck instance returned by the check method above.

Contract fields

A contract will usually need some further configuration. This can be done using fields. For an example, see importlinter.contracts.layers.

Example custom contract

from importlinter import Contract, ContractCheck, fields, output

class ForbiddenImportContract(Contract):
    Contract that defines a single forbidden import between
    two modules.
    importer = fields.StringField()
    imported = fields.StringField()

    def check(self, graph, verbose):
            f"Getting import details from {self.importer} to {self.imported}..."
        forbidden_import_details = graph.get_import_details(
        import_exists = bool(forbidden_import_details)

        return ContractCheck(
            kept=not import_exists,
                'forbidden_import_details': forbidden_import_details,

    def render_broken_contract(self, check):
            f'{self.importer} is not allowed to import {self.imported}:',
        for details in check.metadata['forbidden_import_details']:
            line_number = details['line_number']
            line_contents = details['line_contents']
            output.print_error(f'{self.importer}:{line_number}: {line_contents}')

Step two: register the contract type

In the [importlinter] section of your configuration file, include a list of contract_types that map type names onto the Python path of your custom class:

root_package_name = mypackage
contract_types =
    forbidden_import: somepackage.contracts.ForbiddenImportContract

Step three: define your contracts

You may now use the type name defined in the previous step to define a contract:

name = My custom contract
type = forbidden_import
importer =
imported =