How to check at function call if default keyword arguments are used

Question

I`m working on some initialization function/method which should warn you if the default values are used for the kwargs. I can't figure out how to do so and google did not help me either. Any solutions?

To illustrate

class MyClass():
    def __init__(a=0,b=1):
        ''' Should warn you if using default.'''
        self._a = a
        self._b = b

Should act as follows

MyClass(a=1)
>>> 'warning: default b=1 is used.

Why would one want the warning: Defualts are there to illustrate the typical values used. However the end user should set them and not be lazy(!). End user manipulates yamls with kwargs: make end user aware if he/she messes up. Basically I want the (soft) requiredness of args with all the benefits of kwargs.

Answer 1

Use kwargs in __init__ for the warnings and then a dedicated initialize function for the assignment of the variables:

class MyClass():
    def __init__(self, **kwargs):
        defaults = dict(a=0, b=1)
        for key,val in defaults.items():
            if key not in kwargs:
                print("warning: default {}={} is used.".format(key, val))
            kwargs[key]=val

        self.initialize(**kwargs)

    def initialize(self, a=0, b=1):
        self._a = a
        self._b = b

MyClass()
print('-'*50)
MyClass(a=5)
print('-'*50)
MyClass(a=4,b=3)

The output is:

warning: default b=1 is used.
warning: default a=0 is used.
--------------------------------------------------
warning: default b=1 is used.
--------------------------------------------------