3.13. Typing Extra

  • https://github.com/python/typeshed

  • Typeshed contains external type annotations for the Python standard library and Python builtins, as well as third party packages as contributed by people external to those projects.

3.13.1. TypeGuard

https://docs.python.org/3/library/typing.html#typing.TypeGuard

3.13.2. TypeAlias

  • Since Python 3.10 PEP 613 -- TypeAlias Annotation

Type annotations can be written in both str and identifier form. Both are equivalent.

>>> age: int
>>> age: 'int'

Now consider creating variable with common value such as word 'Any'. Is this just a variable with some more or less random text, or a type alias to typing.Any? This is very problematic for a type checker.

>>> from typing import Any
>>>
>>>
>>> x = Any
>>> x = 'Any'

In order to avoid such ambiguity we can use TypeAlias annotation and therefore explicitly declare a type alias.

>>> from typing import Any, TypeAlias
>>>
>>>
>>> x: TypeAlias = Any
>>> x: TypeAlias = 'Any'

3.13.3. TypeVar

SetUp:

>>> from typing import TypeVar

Definition:

>>> T = TypeVar('T', int, float)
>>>
>>> def add(a: T, b: T) -> T:
...     return a + b

Usage:

>>> add(1, 2)
3

>>> add(1.0, 2.0)
3.0

3.13.4. NewType

You can create new types which are identical to some other type, and those new values made with the new type will have access to all the methods and properties as the original type.

>>> from typing import NewType
>>>
>>>
>>> class Person:
...     ...
>>>
>>> Astronaut = NewType('Astronaut', Person)
>>> Cosmonaut = NewType('Cosmonaut', Person)
>>>
>>> def fly_to_space(who: Astronaut):
...     ...

3.13.5. Use Case - 0x01

>>> from functools import wraps
>>> from typing import get_type_hints, get_origin, get_args, Annotated
>>>
>>>
>>> class MaxLen:
...     def __init__(self, value):
...         self.value = value
>>>
>>>
>>> def check_annotations(func):
...     @wraps(func)
...     def wrapped(**kwargs):
...         type_hints = get_type_hints(func, include_extras=True)
...         for param, hint in type_hints.items():
...             if get_origin(hint) is not Annotated:
...                 continue
...             hint_type, *hint_args = get_args(hint)
...             if hint_type is str or get_origin(hint_type) is str:
...                 for arg in hint_args:
...                     if isinstance(arg, MaxLen):
...                         max_len = arg.value
...                         actual_len = len(kwargs[param])
...                         if actual_len > max_len:
...                             raise ValueError(f"Parameter '{param}' cannot have a length "
...                                              f"larger than {max_len} (got length {actual_len}).")
...         return func(**kwargs)
...     return wrapped
>>>
>>>
>>> word = Annotated[str, MaxLen(10)]
>>>
>>> @check_annotations
... def echo(text: word):
...     return text
>>>
>>>
>>> echo(text='abcdefghij')
'abcdefghij'
>>>
>>> echo(text='abcdefghijk')
Traceback (most recent call last):
ValueError: Parameter 'text' cannot have a length larger than 10 (got length 11).

3.13.6. Use Case - 0x02

>>> from typing import TypeVar
>>>
>>>
>>> T = TypeVar('T', int, float)
>>> Vector = tuple[T, T]
>>>
>>>
>>> def product(data: Vector[T]) -> T:
...     return sum(x*y for x,y in data)