What are good uses for Python3's "Function Annotations"

Question

Function Annotations: PEP-3107

I ran across a snippet of code demonstrating Python3's function annotations. The concept is simple but I can't think of why these were implemented in Python3 or any good uses for them. Perhaps SO can enlighten me?

How it works:

def foo(a: 'x', b: 5 + 6, c: list) -> max(2, 9):
    ... function body ...

Everything following the colon after an argument is an 'annotation', and the information following the -> is an annotation for the function's return value.

foo.func_annotations would return a dictionary:

{'a': 'x',
 'b': 11,
 'c': list,
 'return': 9}

What's the significance of having this available?

Answer 1

Function annotations are what you make of them.

They can be used for documentation:

def kinetic_energy(mass: 'in kilograms', velocity: 'in meters per second'):
     ...

They can be used for pre-condition checking:

def validate(func, locals):
    for var, test in func.__annotations__.items():
        value = locals[var]
        msg = 'Var: {0}\tValue: {1}\tTest: {2.__name__}'.format(var, value, test)
        assert test(value), msg


def is_int(x):
    return isinstance(x, int)

def between(lo, hi):
    def _between(x):
            return lo <= x <= hi
    return _between

def f(x: between(3, 10), y: is_int):
    validate(f, locals())
    print(x, y)


>>> f(0, 31.1)
Traceback (most recent call last):
   ... 
AssertionError: Var: y  Value: 31.1 Test: is_int

Also see http://www.python.org/dev/peps/pep-0362/ for a way to implement type checking.