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Plugin for mypy to support zope.interface
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README.md

Plugin for mypy to support zope.interface

Build Status Coverage Status Checked with mypy

The goal is to be able to make zope interfaces to be treated as types in mypy sense.

Usage

Install both mypy and mypy-zope:

pip install mypy-zope

Edit mypy.ini file in your project to enable the plugin:

[mypy]
namespace_packages=True
plugins=mypy_zope:plugin

You're done! You can now check your project with mypy:

mypy your-project-dir

What is supported?

You can browse sample files to get some sense on what features are supported and how they are handled.

Interface declarations

You can define the interface and provide implementation:

class IAnimal(zope.interface.Interface):
    def say() -> None:
        pass

@zope.interface.implementer(IAnimal)
class Cow(object):
    def say(self) -> None:
        print("Moooo")

The interface IAnimal will be treated as an abstract base class of the implementation Cow: you will not be able to instantiate Cow if interface implementation is incomplete. Incompatible method signatures will be reported as well.

Schema field type inference

A limited support for defining attributes as zope.schema.Fields is supported too:

class IAnimal(zope.interface.Interface):
    number_of_legs = zope.schema.Int(title="Number of legs")

@zope.interface.implementer(IAnimal)
class Cow(object):
    number_of_legs = 4

In context of an interface, some known zope.schema field types are automatically translated to python types, so the number_of_legs attributes is getting the type int in the example above. That means mypy will report an error if you try to assign string to that attribute on an instance of a Cow class. Custom fields or fields not recognized by plugin are given type Any.

Field properties

Support for zope.schema.FieldProperty is limited, because type information is not transferred from an interface to implementation attribute, but mypy doesn't report errors on sources like this:

class IAnimal(zope.interface.Interface):
    number_of_legs = zope.schema.Int(title="Number of legs")

@zope.interface.implementer(IAnimal)
class Cow(object):
    number_of_legs = zope.schema.FieldProperty(IAnimal['number_of_legs'])

The type of Cow.number_of_legs will become Any in this case, even though IAnimal.number_of_legs would be inferred as int.

Adaptation pattern

Zope interfaces can be "called" to lookup an adapter, like this:

class IEUPowerSocket(zope.interface.Interface):
    def fit():
        pass

adapter = IEUPowerSocket(us_plug)
adapter.fit()

Type of the adapter variable will be set to IEUPowerSocket.

Type stubs for zope.interface and zope.schema

mypy-zope ships with type stubs (*.pyi files) for zope.interface and zope.schema packages. They are enabled automatically as soon as plugin is enabled.

Conditional type inference

When using zope.interface's implementedBy() and providedBy() methods in an if statement, mypy will know which type it is inside those statements.

if IAnimal.providedBy(ob):
    ob.number_of_legs += 2

What is not supported?

These zope.interface features are not supported:

  • Declaring modules as interface implementers.
  • Type inference for zope.schema.List and zope.schema.Dict fields.
  • Stub files are largely incomplete

Under development!

Currently the project is in a very early stage of development and might not be practically usable yet. Suggestions and pull requests are welcomed!

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