Validated Function Arguments in Python

Question

I have a function

def func(a,b,c,d):
    ...

and I am trying to write a decorator that understands the arguments and logs some of them to a different system.

def decorator(func):
    def new_func(*args, **kwargs):
        if (func.__name__ == 'func'):
            a = ?
            b = ?
            c = ?
            d = ?
        else:
            a = ?
            b = ?
            c = ?
            d = ?
        log_to_system(a, b, c, d)
        return func(*args, **kwargs)
return new_func

The problem is that the decorator doesn't have an easy way to extract the a,b,c,d values from both the args and the kwargs since the user can pass these in using either positional or keyword arguments. I would also like to keep this generic since this decorator could be used on various different functions.

Is there a library or a utility that can extract the values of the parameters from args and kwargs easily?

Answer 1

A simple approach is to make your log_to_system function accept variable parameters and variable keyword parameters in addition to the known parameters that it will actually log, so that you can simply pass on the variable arguments and variable keyword arguments from the decorated function to log_to_system and let the interpreter extract the parameters a, b, c and d for you:

def log_to_system(a, b, c, d, *args, **kwargs):
    print(a, b, c, d)

def decorator(func):
    def new_func(*args, **kwargs):
        log_to_system(*args, **kwargs)
        return func(*args, **kwargs)
    return new_func

@decorator
def func(a, b, c, d, e):
    pass

func(1, 2, c=3, d=4, e=5)

This outputs:

1 2 3 4

Alternatively, you can use inspect.signature to obtain a dict of arguments after binding the given variable arguments and keyword arguments to the decorated function's signature, so that you can call log_to_system with just the parameters it needs:

import inspect

def log_to_system(a, b, c, d):
    print(a, b, c, d)

def decorator(func):
    sig = inspect.signature(func)
    def new_func(*args, **kwargs):
        arguments = sig.bind(*args, **kwargs).arguments
        log_to_system(**{k: arguments[k] for k in log_to_system.__code__.co_varnames})
        return func(*args, **kwargs)
    return new_func

@decorator
def func(a, b, c, d, e):
    pass

func(1, 2, c=3, d=4, e=5)

This outputs:

1 2 3 4