How to use Python decorators to check function arguments?

Question

I would like to define some generic decorators to check arguments before calling some functions.

Something like:

@checkArguments(types = [\'int\', \'         


        

Answer 1

On Python 3.3, you can use function annotations and inspect:

import inspect

def validate(f):
    def wrapper(*args):
        fname = f.__name__
        fsig = inspect.signature(f)
        vars = ', '.join('{}={}'.format(*pair) for pair in zip(fsig.parameters, args))
        params={k:v for k,v in zip(fsig.parameters, args)}
        print('wrapped call to {}({})'.format(fname, params))
        for k, v in fsig.parameters.items():
            p=params[k]
            msg='call to {}({}): {} failed {})'.format(fname, vars, k, v.annotation.__name__)
            assert v.annotation(params[k]), msg
        ret = f(*args)
        print('  returning {} with annotation: "{}"'.format(ret, fsig.return_annotation))
        return ret
    return wrapper

@validate
def xXy(x: lambda _x: 10<_x<100, y: lambda _y: isinstance(_y,float)) -> ('x times y','in X and Y units'):
    return x*y

xy = xXy(10,3)
print(xy)

If there is a validation error, prints:

AssertionError: call to xXy(x=12, y=3): y failed )

If there is not a validation error, prints:

wrapped call to xXy({'y': 3.0, 'x': 12})
  returning 36.0 with annotation: "('x times y', 'in X and Y units')"

You can use a function rather than a lambda to get a name in the assertion failure.