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

Question

Asked By – Markus Meskanen

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?

Now we will see solution for issue: Using List/Tuple/etc. from typing vs directly referring type as list/tuple/etc


Answer

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__).

This question is answered By – Martijn Pieters

This answer is collected from stackoverflow and reviewed by FixPython community admins, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0