Using List/Tuple/etc. from typing vs directly referring type as list/tuple/etc


What’s the difference of using List, Tuple, etc. from typing module:

from typing import Tuple

def f(points: Tuple):
    return map(do_stuff, points)

As opposed to referring to Python’s types directly:

def f(points: tuple):
    return map(do_stuff, points)

And when should I use one over the other?

Asked By: Markus Meskanen



Until Python 3.9 added support for type hinting using standard collections, you had to use typing.Tuple and 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, tuple and 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 float values.

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 tuple with floats.

For 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 __class_getitem__).

Answered By: Martijn Pieters

From Python 3.9 (PEP 585) onwards tuple, 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 don’t 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.Tuple, typing.List and other generics are deprecated and will be removed in a later version of Python.

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. It’s 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.

Answered By: Peilonrayz