10.1. Type Annotations¶

10.1.1. Rationale¶

Since Python 3.5: PEP 484 -- Type Hints
Also known as Type Hinting
Accessed by __annotations__ attribute
No type checking happens at runtime
Use separate off-line type checker which users can run over their source code voluntarily
Supports unions, generic types, and a special type named
Any is consistent with (i.e. assignable to and from) all types
This latter feature is taken from the idea of gradual typing.
PEP 483 -- The Theory of Type Hints
https://mypy.readthedocs.io/en/stable/cheat_sheet_py3.html

It should also be emphasized that Python will remain a dynamically typed language, and the authors have no desire to ever make type hints mandatory, even by convention.

10.1.2. Glossary¶

Generic¶: A type that can be parameterized, typically a container. Also known as a parametric type or a generic type. For example: dict.
Parameterized Generic¶: A specific instance of a generic with the expected types for container elements provided. Also known as a parameterized type. For example: dict[str, int].

10.1.3. Data types¶

name: str = 'Jan Twardowski'
leet: int = 1337
pi: float = 3.14

10.1.4. Final¶

Since Python 3.8: PEP 591 -- Adding a final qualifier to typing

from typing import final

@final
class Base:
    ...

class Derived(Base):  # Error: Cannot inherit from final class "Base"
    ...

from typing import Final


ID: Final = 1
ID: Final[float] = 1

from typing import Final

class Window:
    BORDER_WIDTH: Final = 2.5

class ListView(Window):
    BORDER_WIDTH = 3  # Error: can't override a final attribute

from typing import Final

class ImmutablePoint:
    x: Final[int]
    y: Final[int]  # Error: final attribute without an initializer

    def __init__(self) -> None:
        self.x = 1  # Good

from typing import Final

RATE: Final = 3000

class Base:
    DEFAULT_ID: Final = 0

RATE = 300  # Error: can't assign to final attribute
Base.DEFAULT_ID = 1  # Error: can't override a final attribute

10.1.5. New Features¶

Since Python 3.9: PEP 585 -- Type Hinting Generics In Standard Collections
In type annotations you can now use built-in collection types such as list and dict as generic types instead of importing the corresponding capitalized types (e.g. List or Dict) from typing.
Some other types in the standard library are also now generic, for example queue.Queue.
Importing those from typing is deprecated.
Due to :pep`563` and the intention to minimize the runtime impact of typing, this deprecation will not generate DeprecationWarnings.
Instead, type checkers may warn about such deprecated usage when the target version of the checked program is signalled to be Python 3.9 or newer.
The deprecated functionality will be removed from the typing module in the first Python version released 5 years after the release of Python 3.9.0.

def greet_all(names: list[str]) -> None:
    for name in names:
        print("Hello", name)

You can try this feature since Python 3.7 with from __future__ import annotations:

from __future__ import annotations

def find(haystack: dict[str, list[int]]) -> int:
    ...

tuple instead of typing.Tuple
list instead of typing.List
dict instead of typing.Dict
set instead of typing.Set
frozenset instead of typing.FrozenSet
type instead of typing.Type
collections.deque
collections.defaultdict
collections.OrderedDict
collections.Counter
collections.ChainMap
collections.abc.Awaitable
collections.abc.Coroutine
collections.abc.AsyncIterable
collections.abc.AsyncIterator
collections.abc.AsyncGenerator
collections.abc.Iterable
collections.abc.Iterator
collections.abc.Generator
collections.abc.Reversible
collections.abc.Container
collections.abc.Collection
collections.abc.Callable
collections.abc.Set instead of typing.AbstractSet
collections.abc.MutableSet
collections.abc.Mapping
collections.abc.MutableMapping
collections.abc.Sequence
collections.abc.MutableSequence
collections.abc.ByteString
collections.abc.MappingView
collections.abc.KeysView
collections.abc.ItemsView
collections.abc.ValuesView
contextlib.AbstractContextManager instead of typing.ContextManager
contextlib.AbstractAsyncContextManager instead of typing.AsyncContextManager
re.Pattern instead of typing.Pattern, typing.re.Pattern
re.Match instead of typing.Match, typing.re.Match

l = list[str]()
# []

list is list[str]
# False

list == list[str]
# False

list[str] == list[str]
# True

list[str] == list[int]
# False

isinstance([1, 2, 3], list[str])
# Traceback (most recent call last):
# TypeError: isinstance() arg 2 cannot be a parameterized generic

issubclass(list, list[str])
# Traceback (most recent call last):
# TypeError: issubclass() arg 2 cannot be a parameterized generic

isinstance(list[str], types.GenericAlias)
# True