Plugin for mypy to support zope.interface

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")

animal: IAnimal = Cow()
animal.say()

The interface IAnimal will be treated as superclass of the implementation
Cow: you will be able to pass an implementation to functions accepting an
interface and all the usual polymorphism tricks.

It is also possible to declare the implementation using classImplements
function with the same effect as @imlementer decorator. This is useful if
you do not control the code that defines the implementation class.

classImplements(Cow, IAnimal)

animal: IAnimal = Cow()

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 IAnimal
type. 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.

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

Declaration of overloaded methods in interfaces

Similarly to regular overloaded
functions,
@overload declarations are supported in interfaces as well:

class IAnimal(zope.interface.Interface):
    @overload
    def say() -> str:
        ...

    @overload
    def say(count: int) -> List[str]:
        ...

    def say(count: int = None) -> Union[str, List[str]]:
        pass


@zope.interface.implementer(IAnimal)
class Cow(object):
    @overload
    def say(self) -> str:
        ...

    @overload
    def say(self, count: int) -> List[str]:
        ...

    def say(self, count: int = None) -> Union[str, List[str]]:
        if count is None:
            return "Mooo"
        return ["Mooo"] * count

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.

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
Interface compatibility checker will not type-check non-method attributes

Ready to use!

Currently, the project is used in production in various substantially large
projects and considered production-grade, however there still might be subtle
bugs around. Suggestions and pull requests are welcomed!

Dependency	Optional
mypy	false
zope.interface	false
zope.schema	false
lxml	true
pytest	true
pytest-cov	true

Package profile

mypy-zope

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