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.
  • In Python 2, str is duck type compatible with unicode.

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).

Note

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.