Pass empty argument to Python function?

Question

When a function I'm calling has a lot of parameters, and there's one I'd like to include conditionally, do I have to have two separate calls to the function, or is there someway to pass nothing (almost like None) so that I'm not passing in any argument for a particular parameter?

For example, I want to pass an argument for the parameter sixth sometimes, but other times I want to not pass anything for that parameter. This code works, but it feels like I'm duplicating more than I should have to.

The function I'm calling is in a third-party library, so I can't change how it handles the received arguments. If I pass in None for sixth, the function raises an exception. I need to either pass my 'IMPORTANT_VALUE' or not pass in anything.

What I'm doing currently:

def do_a_thing(stuff, special=False):

    if special:
        response = some.library.func(
            first=os.environ['first'],
            second=stuff['second'],
            third=stuff['third']
            fourth='Some Value',
            fifth=False,
            sixth='IMPORTANT_VALUE',
            seventh='example',
            eighth=True
        )
    else:
        response = some.library.func(
            first=os.environ['first'],
            second=stuff['second'],
            third=stuff['third']
            fourth='Some Value',
            fifth=False,
            seventh='example',
            eighth=True
        )

    return response

What I'd like to do:

def do_a_thing(stuff, special=False):
    special_value = 'IMPORTANT_VALUE' if special else EMPTY_VALUE

    response = some.library.func(
        first=os.environ['first'],
        second=stuff['second'],
        third=stuff['third']
        fourth='Some Value',
        fifth=False,
        sixth=special_value,
        seventh='example',
        eighth=True
    )

    return response

The way to do this is look at some.library.func and see what the default value for the argument you want to omit is, then pass that in when you don't want the sixth argument. — kindall
– kindall, Commented Feb 23, 2018 at 15:57
If I pass in None for sixth, the function raises an exception What is the exception? Does it give you any clues? — John Gordon
– John Gordon, Commented Feb 23, 2018 at 16:02
It returns an invalid parameter error when I pass in None. It wants only nothing passed in, or str values, and even then, only particular strings. The library function is a boto3 call to boto3.client('cognito-idp').admin_create_user(), but I was trying to keep the question more general. boto3.readthedocs.io/en/latest/reference/services/… — WhiteHotLoveTiger
– WhiteHotLoveTiger, Commented Feb 23, 2018 at 19:51

Antwane · Accepted Answer · 2018-02-23 15:55:24Z

7

One solution could be building a dict with values you want to pass to the function, modified depending on special value. Then use python unpacking to expand it as a list of named arguments to the function you want to call:

def do_a_thing(stuff, special=False):

    kwargs = dict(
        first=os.environ['first'],
        second=stuff['second'],
        third=stuff['third']
        fourth='Some Value',
        fifth=False,
        seventh='example',
        eighth=True
    )

    if special:
        kwargs['sixth'] = 'IMPORTANT_VALUE'

    return some.library.func(**kwargs)

answered Feb 23, 2018 at 15:55

Antwane

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Comments

randyjp · Accepted Answer · 2018-02-23 16:18:00Z

2

Since functions are first class objects, you can pass that function to a wrapper that will construct the params you want properly. This will make that piece of code reusable but also gives you more flexibility, like handling errors is needed.

def wrapper(some_library_func, special=False):
# you could have try catch here if needed. 
  kwards = {
    'first': os.environ['first'],
    'second': stuff['second'],
    'third': stuff['third'],
    'fourth': 'Some Value',
    'fifth': False,
    'seventh': 'example',
    'eighth': True
  }

  if special:
    kwards['sixth'] = 'IMPORTANT_VALUE'

  return some_library_func(**kwards)

answered Feb 23, 2018 at 16:18

randyjp

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Comments

Levi Lesches · Accepted Answer · 2018-02-23 15:59:11Z

-2

I don't see anything wrong with your way of doing it, using a function to define the variable. You should declare special_value in global scope, then include global special_value at the top of your function.

answered Feb 23, 2018 at 15:59

Levi Lesches

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4 Comments

John Gordon Over a year ago

I don't see anything wrong with your way of doing it Nothing wrong except that he said it doesn't work...

Levi Lesches Over a year ago

His second solution ("What I'd like to do...") looks fine and he didn't say it returned an error

WhiteHotLoveTiger Over a year ago

Sorry I wasn't more clear. EMPTY_VALUE is a placeholder for a literal empty value of not passing anything. I don't know if there is such a thing in Python.

Levi Lesches Over a year ago

I'm starting to realize the scope of the issue. I suggest finding the default value for this function (as you noted, you didn't make it), and supply it in the else part of your do_a_thing function

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