Variables vs type aliases

Mypy has both type aliases and variables with types like Type[…] and it is important to know their difference.

• Variables with type Type[…] should be created by assignments with an explicit type annotations:

class A: ...
tp: Type[A] = A

• Aliases are created by assignments without an explicit type:

class A: ...
Alias = A

• The difference is that aliases are completely known statically and can be used in type context (annotations):

class A: ...
class B: ...
 
if random() > 0.5:
    Alias = A
else:
    Alias = B  # error: Cannot assign multiple types to name "Alias" without an explicit "Type[...]" annotation \
               # error: Incompatible types in assignment (expression has type "Type[B]", variable has type "Type[A]")
 
tp: Type[object]  # tp is a type variable
if random() > 0.5:
    tp = A
else:
    tp = B  # This is OK
 
def fun1(x: Alias) -> None: ...  # This is OK
def fun2(x: tp) -> None: ...  # error: Variable "__main__.tp" is not valid as a type