An Example of Covariance, Contravariance and Invariance in Python Type Hints
Date: 12/12/2023 · Tags: #python, #dev, #functional-programmingQiangning Hong (@hongqn) share a Python type annotation tip on X (Twitter), and raised discussions about covariance, contravariance and invariance in Python
from collections.abc import Sequence
a: list[str] =["a"]
def f1(a: list[int | str]): pass
def f2(a: Sequence[int | str]): pass
f1(a) # incompatible type assignment
f2(a) # compatibleSo why?
The key point is that list[int | str] is not covariant with list[str], while
Sequence[int | str] is covariant with list[str].
The reason of Sequence is immutable and is also covariant, most immutable
containers are covariant, so f2 cannot modify the list even if it is passed as
a list.
List is mutable, so if the parameter is declared as a list type, list[str]
and list[int | str] are two different types. In type system, mutable types
usually are invariant (e.g. MutableSequence, list, set in Python).
It would be more clear that we add an function to accept Sequence type, and it
will be compatible with variable a:
def f3(a: Sequence[str]): pass
f3(a) # compatibleRefs: