Generic typeclasses

You can define generic typeclasses, just like regular generic functions. You have to ways of doing this:

  • Via raw type variables like in def copy(instance: X) -> X

  • Via instances with type variables like in get_zero_item(instance: Sequence[X]) -> X

Using typevars

When defining typeclasses with generic instances, we will typecheck that all instance definitions match the shape of the typeclass itself:

>>> from typing import TypeVar
>>> from classes import typeclass

>>> X = TypeVar('X')

>>> @typeclass
... def copy(instance: X) -> X:
...     ...

This one will typecheck correctly:

>>> @copy.instance(int)
... def _copy_int(instance: int) -> int:
...     ...

But, this won’t:

def _copy_str(instance: str) -> bool:

# error: Instance callback is incompatible "def (first: builtins.str, second: builtins.float) -> builtins.bool"; expected "def (first: builtins.str, second: builtins.str*) -> builtins.bool"

Using instances

When using instances, you can define type restrictions to limit typeclass instances to only subtypes of a given restriction.

Here’s an example definition:

>>> from typing import Iterable, TypeVar, List
>>> from classes import typeclass

>>> X = TypeVar('X')

>>> @typeclass
... def get_item(instance: Iterable[X], index: int) -> X:
...    ...

This instance will match the definition:

>>> @get_item.instance(list)
... def _get_item_list(instance: List[X], index: int) -> X:
...     ...

reveal_type(get_item([1, 2, 3], 0))  # Revealed type is "*"
reveal_type(get_item(['a', 'b'], 0)) # Revealed type is "builtins.str*"

But, this won’t match and mypy will warn you:

def _get_item_int(instance: int, index: int) -> int:
# error: Instance callback is incompatible "def (instance:, index: ->"; expected "def [X] (instance:, index: -> X`-1"
# error: Instance "" does not match original type "typing.Iterable[X`-1]"

Generic Supports type

You can also use generic Supports type with generic AssociatedType.

To do so, you will need: 1. Declare AssociatedType with type arguments, just like regular Generic 2. Use correct type arguments to define a variable

Let’s get back to get_item example and use a generic Supports type:

>>> from typing import Iterable, List, TypeVar
>>> from classes import AssociatedType, Supports, typeclass

>>> X = TypeVar('X')

>>> class GetItem(AssociatedType[X]):
...     ...

>>> @typeclass(GetItem)
... def get_item(instance: Iterable[X], index: int) -> X:
...     ...

>>> numbers: Supports[GetItem[int]]
>>> strings: Supports[GetItem[str]]

reveal_type(get_item(numbers, 0))  # Revealed type is "*"
reveal_type(get_item(strings, 0))  # Revealed type is "builtins.str*"

Complex concrete generics

There are several advanced techniques in using concrete generic types when working with delegate types.

Here’s the collection of them.



This example only works for Python 3.7 and 3.8 Original bug report.

At first, we need to define a typed dictionary itself:

>>> from typing_extensions import TypedDict
>>> from classes import typeclass

>>> class _User(TypedDict):
...     name: str
...     registered: bool

Then, we need a special class with __instancecheck__ defined. Because original TypedDict just raises a TypeError on isinstance(obj, User).

class _UserDictMeta(type(TypedDict)):
    def __instancecheck__(cls, arg: object) -> bool:
        return (
            isinstance(arg, dict) and
            isinstance(arg.get('name'), str) and
            isinstance(arg.get('registered'), bool)

class UserDict(_User, metaclass=_UserDictMeta):

And finally we can use it! Take a note that we always use the resulting UserDict type, not the base _User.

def get_name(instance) -> str:

def _get_name_user_dict(instance: UserDict) -> str:
    return instance['name']

user: UserDict = {'name': 'sobolevn', 'registered': True}
assert get_name(user) == 'sobolevn'

Tuples of concrete shapes

The logic is the same with concrete Tuple items:

>>> from typing import Tuple
>>> from classes import typeclass

>>> class UserTupleMeta(type):
...     def __instancecheck__(cls, arg: object) -> bool:
...         try:
...             return (
...                 isinstance(arg, tuple) and
...                 isinstance(arg[0], str) and
...                 isinstance(arg[1], bool)
...             )
...         except IndexError:
...             return False

>>> class UserTuple(Tuple[str, bool], metaclass=UserTupleMeta):
...     ...

>>> @typeclass
... def get_name(instance) -> str:
...     ...

>>> @get_name.instance(delegate=UserTuple)
... def _get_name_user_dict(instance: Tuple[str, bool]) -> str:
...     return instance[0]

>>> assert get_name(('sobolevn', True)) == 'sobolevn'
>>> get_name((1, 2))
Traceback (most recent call last):
NotImplementedError: Missing matched typeclass instance for type: tuple