Duck type compatibility

In Python, certain types are compatible even though they aren’t subclasses of each other. For example, int objects are valid whenever float objects are expected. Mypy supports this idiom via duck type compatibility. As of now, this is only supported for a small set of built-in types:

  • int is duck type compatible with float and complex.
  • float is duck type compatible with complex.
  • In Python 2, str is duck type compatible with unicode.


Mypy support for Python 2 is still work in progress.

For example, mypy considers an int object to be valid whenever a float object is expected. Thus code like this is nice and clean and also behaves as expected:

def degrees_to_radians(x: float) -> float:
    return math.pi * degrees / 180

n = 90  # Inferred type 'int'
print(degrees_to_radians(n))   # Okay!


Note that in Python 2 a str object with non-ASCII characters is often not valid when a unicode string is expected. The mypy type system does not consider a string with non-ASCII values as a separate type so some programs with this kind of error will silently pass type checking. In Python 3 str and bytes are separate, unrelated types and this kind of error is easy to detect. This a good reason for preferring Python 3 over Python 2!

See Text and AnyStr for details on how to enforce that a value must be a unicode string in a cross-compatible way.