Count how many arguments passed as positional

Question

If I have a function

def foo(x, y):     pass 

how can I tell, from inside the function, whether y was passed positionally or with its keyword?

I'd like to have something like

def foo(x, y):     if passed_positionally(y):         print('y was passed positionally!')     else:         print('y was passed with its keyword') 

so that I get

>>> foo(3, 4) y was passed positionally >>> foo(3, y=4) y was passed with its keyword 

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I realise I didn't originally specify this, but is it possible to do this whilst preserving type annotations? The top answer so far suggests using a decorator - however, that does not preserve the return type

Answer 1

You can create a decorator, like this:

def checkargs(func):     def inner(*args, **kwargs):         if 'y' in kwargs:             print('y passed with its keyword!')         else:             print('y passed positionally.')         result = func(*args, **kwargs)         return result     return inner  >>>  @checkargs ...: def foo(x, y): ...:     return x + y  >>> foo(2, 3) y passed positionally. 5  >>> foo(2, y=3) y passed with its keyword! 5 

Of course you can improve this by allowing the decorator to accept arguments. Thus you can pass the parameter you want to check for. Which would be something like this:

def checkargs(param_to_check):     def inner(func):         def wrapper(*args, **kwargs):             if param_to_check in kwargs:                 print('y passed with its keyword!')             else:                 print('y passed positionally.')             result = func(*args, **kwargs)             return result         return wrapper     return inner  >>>  @checkargs(param_to_check='y') ...: def foo(x, y): ...:     return x + y  >>> foo(2, y=3) y passed with its keyword! 5 

I think adding functools.wraps would preserve the annotations, following version also allows to perform the check over all arguments (using inspect):

from functools import wraps import inspect  def checkargs(func):     @wraps(func)     def inner(*args, **kwargs):         for param in inspect.signature(func).parameters:             if param in kwargs:                 print(param, 'passed with its keyword!')             else:                 print(param, 'passed positionally.')         result = func(*args, **kwargs)         return result     return inner  >>>  @checkargs ...: def foo(x, y, z) -> int: ...:     return x + y  >>> foo(2, 3, z=4) x passed positionally. y passed positionally. z passed with its keyword! 9  >>> inspect.getfullargspec(foo) FullArgSpec(args=[], varargs='args', varkw='kwargs', defaults=None,  kwonlyargs=[], kwonlydefaults=None, annotations={'return': <class 'int'>})                                              _____________HERE____________