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:
intis duck type compatible withfloatandcomplex.floatis duck type compatible withcomplex.- In Python 2,
stris duck type compatible withunicode.
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!
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.