Invariance vs covariance

Most mutable generic collections are invariant, and mypy considers alluser-defined generic classes invariant by default(see Variance of generic types for motivation). This could lead to someunexpected errors when combined with type inference. For example:

class A: ...
class B(A): ...
 
lst = [A(), A()]  # Inferred type is List[A]
new_lst = [B(), B()]  # inferred type is List[B]
lst = new_lst  # mypy will complain about this, because List is invariant

Possible strategies in such situations are:

• Use an explicit type annotation:

new_lst: List[A] = [B(), B()]
lst = new_lst  # OK

• Make a copy of the right hand side:

lst = list(new_lst) # Also OK

• Use immutable collections as annotations whenever possible:

def f_bad(x: List[A]) -> A:
    return x[0]
f_bad(new_lst) # Fails
 
def f_good(x: Sequence[A]) -> A:
    return x[0]
f_good(new_lst) # OK