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. This is 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.

  • bytearray and memoryview are duck type compatible with bytes.

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:

import math

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

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

You can also often use Protocols and structural subtyping to achieve a similar effect in a more principled and extensible fashion. Protocols don’t apply to cases like int being compatible with float, since float is not a protocol class but a regular, concrete class, and many standard library functions expect concrete instances of float (or int).