python – Using List/Tuple/etc. from typing vs directly referring type as list/tuple/etc
Until Python 3.9 added support for type hinting using standard collections, you had to use
typing.List if you wanted to document what type the contents of the containers needed to be:
def f(points: Tuple[float, float]): return map(do_stuff, points)
Up until Python 3.8,
list did not support being used as generic types. The above example documents that the function
f requires the
points argument to be a tuple with two
typing.Tuple is special here in that it lets you specify a specific number of elements expected and the type of each position. Use ellipsis if the length is not set and the type should be repeated:
Tuple[float, ...] describes a variable-length
typing.List and other sequence types you generally only specify the type for all elements;
List[str] is a list of strings, of any size. Note that functions should preferentially take
typing.Sequence as arguments and
typing.List is typically only used for return types; generally speaking most functions would take any sequence and only iterate, but when you return a
list, you really are returning a specific, mutable sequence type.
If you still need to support Python 3.8 or older code, you should always pick the
typing generics even when you are not currently restricting the contents. It is easier to add that constraint later with a generic type as the resulting change will be smaller.
If you are implementing a custom container type and want that type to support generics, you can implement a
__class_getitem__ hook or inherit from
typing.Generic (which in turn implements
From Python 3.9 (PEP 585) onwards
list and various other classes are now generic types. Using these rather than their
typing counterpart is now preferred. From Python 3.9 you can now just do:
def f(points: tuple[float, float]): return map(do_stuff, points)
If you dont need to evaluate your type hints then you can use this syntax in Python 3.7+ due to PEP 563.
from __future__ import annotations def f(points: tuple[float, float]): return map(do_stuff, points)
You should always pick then non-
typing generic whenever possible as the old
typing.List and other generics are deprecated and will be removed in a later version of Python.
Importing those from
typingis 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. Its recommended to allow for those warnings to be silenced on a project-wide basis.
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.