class A:
x = None # type: float
def __init__(self, x: float) -> None:
"""
self should not be annotated
init should be annotated to return None
"""
self.x = x
@classmethod
def from_int(cls, x: int) -> 'A':
"""
cls should not be annotated
Use forward reference to refer to current class with string literal 'A'
"""
return cls(float(x))
Forward reference of the current class is needed since annotations are evaluated when the function is defined. Forward references can also be used when referring to a class that would cause a circular import if imported.