Decorator to enable changing the values of default arguments in python functions.

defaults.py

from functools import wraps
 
class Default(object):
    def __init__(self, name):
        super(Default, self).__init__()
 
        self.name = name
 
 
def set_defaults(defaults):
    def decorator(f):
        @wraps(f)
        def wrapper(*args, **kwargs):
            # Backup original function defaults.
            original_defaults = f.func_defaults
 
            # Replace every `Default("...")` argument with its current value.
            function_defaults = []
            for default_value in f.func_defaults:
                if isinstance(default_value, Default):
                    function_defaults.append(defaults[default_value.name])
                else:
                    function_defaults.append(default_value)
 
            # Set the new function defaults.
            f.func_defaults = tuple(function_defaults)
 
            return_value = f(*args, **kwargs)
 
            # Restore original defaults (required to keep this trick working.)
            f.func_defaults = original_defaults
 
            return return_value
 
        return wrapper
 
    return decorator
 
 
my_defaults = dict(USER_INPUT=0)
 
 
@set_defaults(my_defaults)
def DoSomething(var_of_data, user_input=Default("USER_INPUT")):
    return var_of_data, user_input
 
 
def main():
    print DoSomething("This")
 
    my_defaults["USER_INPUT"] = 1
 
    print DoSomething("Thing")
 
    my_defaults["USER_INPUT"] = 2
 
    print DoSomething("Actually")
    print DoSomething("Works", 3)
 
 
if __name__ == "__main__":
    main()

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