Fix Python – Forced naming of parameters in Python


Asked By – Mark Mayo

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:

info(odbchelper, 12)                
info(odbchelper, collapse=0)        
info(spacing=15, object=odbchelper)

thanks to Python’s allowing of any-order arguments, so long as they’re named.

The problem we’re having is as some of our larger functions grow, people might be adding parameters between spacing and collapse, meaning that the wrong values may be going to parameters that aren’t named. In addition sometimes it’s not always clear as to what needs to go in. We’re after a way to force people to name certain parameters – not just a coding standard, but ideally a flag or pydev plugin?

so that in the above 4 examples, only the last would pass the check as all the parameters are named.

Odds are we’ll only turn it on for certain functions, but any suggestions as to how to implement this – or if it’s even possible would be appreciated.

Now we will see solution for issue: Forced naming of parameters in Python


In Python 3 – Yes, you can specify * in the argument list.

From docs:

Parameters after “*” or “*identifier” are keyword-only parameters and
may only be passed used keyword arguments.

>>> def foo(pos, *, forcenamed):
...   print(pos, forcenamed)
>>> foo(pos=10, forcenamed=20)
10 20
>>> foo(10, forcenamed=20)
10 20
>>> foo(10, 20)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: foo() takes exactly 1 positional argument (2 given)

This can also be combined with **kwargs:

def foo(pos, *, forcenamed, **kwargs):

To complete example:

def foo(pos, *, forcenamed ):
    print(pos, forcenamed)

foo(pos=10, forcenamed=20)
foo(10, forcenamed=20)
# basically you always have to give the value!


Traceback (most recent call last):
  File "/Users/brando/anaconda3/envs/metalearning/lib/python3.9/site-packages/IPython/core/", line 3444, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-12-ab74191b3e9e>", line 7, in <module>
TypeError: foo() missing 1 required keyword-only argument: 'forcenamed'

So you are forced to always give the value. If you don’t call it you don’t have to do anything else named argument forced.

This question is answered By – Eli Bendersky

This answer is collected from stackoverflow and reviewed by FixPython community admins, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0