What are good uses for Python3’s “Function Annotations”?

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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.

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