How to pass optional python arguments to sub-functions with optional arguments

Question

Given a simple Python function with an optional argument, like:

def wait(seconds=3):
    time.sleep(seconds)

How do I create a function that calls this and passes on an optional argument? For example, this does NOT work:

def do_and_wait(message, seconds=None):
    print(message)
    wait(seconds)

Note: I want to be able to call wait from other functions with the optional seconds argument without having to know and copy the current default seconds value in the underlying wait function to every other function which calls it.

As above, if I call it with the optional argument, like do_and_wait(2) then it works, but trying to rely on wait's default, e.g. calling it like do_and_wait() causes a TypeError because inside wait seconds == None.

Is there a simple and clean way to make this work? I know I can abuse kwargs like this:

def do_and_wait(message, **kwargs):
    print(message)
    wait(**kwargs)

But that seems unclear to the reader and user of this function since there is no useful name on the argument.

Note: This is a stupidly simplified example.

In your example, seconds is not optional, but rather it has a default value that will be used if the argument isn't provided. Since you call wait from do_and_wait, you're always overriding that default with None - you could simply change the default value for seconds in do_and_wait to 3. — Grismar
– Grismar, Commented Aug 7, 2019 at 0:48

Hans · Accepted Answer · 2019-08-07 00:34:54Z

1

I understand you've simplified your question, but I think you mean how one can call a function with optional arguments that could be None. Does the following work for you?

import time

def func1(mess, sec):
  if sec != None:
    time.sleep(sec)
  print(mess)


func1('success', sec=None)

answered Aug 7, 2019 at 0:34

Hans

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2 Comments

simpleuser Over a year ago

thanks - your code helped me come up with the way to do it, below

DeusXMachina Over a year ago

fyi, use if sec is not None instead of the != operator

rsyed0 · Accepted Answer · 2019-08-07 00:27:10Z

1

I don't think you've quite explained your problem completely because I don't expect an answer should be this simple, but I would just use the same default value (and data type) in do_and_wait() as wait() uses, like so:

def do_and_wait(message, seconds=3):
    print(message)
    wait(seconds)

answered Aug 7, 2019 at 0:27

rsyed0

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Comments

simpleuser · Accepted Answer · 2019-08-07 06:20:53Z

0

After thinking a bit more, I came up with something like this; Han's answer suggested this and reminded me that I think PEP even suggests it somewhere. This especially avoids having to know and copy the default value into any function that calls wait and wants to support a variable value for seconds.

def wait(seconds=None):
    time.sleep(seconds if seconds is not None else 3)

def do_and_wait(message, seconds=None):
    print(message)
    wait(seconds)

def something_else(callable, seconds=None):
    callable()
    wait(seconds)

answered Aug 7, 2019 at 6:20

simpleuser

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