What does -> mean in Python function definitions?

Question

I\'ve recently noticed something interesting when looking at Python 3.3 grammar specification:

funcdef: \'def\' NAME parameters [\'->\' test] \':\' suite
         


        

Answer 1

In the following code:

def f(x) -> int:
    return int(x)

the -> int just tells that f() returns an integer (but it doesn't force the function to return an integer). It is called a return annotation, and can be accessed as f.__annotations__['return'].

Python also supports parameter annotations:

def f(x: float) -> int:
    return int(x)

: float tells people who read the program (and some third-party libraries/programs, e. g. pylint) that x should be a float. It is accessed as f.__annotations__['x'], and doesn't have any meaning by itself. See the documentation for more information:

https://docs.python.org/3/reference/compound_stmts.html#function-definitions https://www.python.org/dev/peps/pep-3107/