Forced naming of parameters in Python

Question

In Python you may have a function definition:

def info(object, spacing=10, collapse=1)

which could be called in any of the following ways:<

Answer 1

The python3 keyword-only arguments (*) can be simulated in python2.x with **kwargs

Consider the following python3 code:

def f(pos_arg, *, no_default, has_default='default'):
    print(pos_arg, no_default, has_default)

and its behaviour:

>>> f(1, 2, 3)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: f() takes 1 positional argument but 3 were given
>>> f(1, no_default='hi')
1 hi default
>>> f(1, no_default='hi', has_default='hello')
1 hi hello
>>> f(1)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: f() missing 1 required keyword-only argument: 'no_default'
>>> f(1, no_default=1, wat='wat')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: f() got an unexpected keyword argument 'wat'

This can be simulated using the following, note I've taken the liberty of switching TypeError to KeyError in the "required named argument" case, it wouldn't be too much work to make that the same exception type as well

def f(pos_arg, **kwargs):
    no_default = kwargs.pop('no_default')
    has_default = kwargs.pop('has_default', 'default')
    if kwargs:
        raise TypeError('unexpected keyword argument(s) {}'.format(', '.join(sorted(kwargs))))

    print(pos_arg, no_default, has_default)

And behaviour:

>>> f(1, 2, 3)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: f() takes exactly 1 argument (3 given)
>>> f(1, no_default='hi')
(1, 'hi', 'default')
>>> f(1, no_default='hi', has_default='hello')
(1, 'hi', 'hello')
>>> f(1)
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 2, in f
KeyError: 'no_default'
>>> f(1, no_default=1, wat='wat')
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 6, in f
TypeError: unexpected keyword argument(s) wat

The recipe works equally as well in python3.x, but should be avoided if you are python3.x only