Question on keyword arguments

Dan Strohl D.Strohl at
Thu Feb 18 10:39:38 EST 2016

I have approached this in a few different ways, depending on the use case.

I have done the "return_type=list" before, though I don't really like it... it's too easy to forget or mistype the exact name used for the switch.  If you do this, I also recommend setting some class or module variables to the text strings and pass those back and forth.

I have also returned a different type depending on what is being requested... so, if the function user requests data that has multiple objects, return a list, if they request something that has just one object, return that object.  This also has potential problems if the number of returned objects is not consistently predictable, you would have to always test your returned object to see how to handle it.

I like the other suggestions thrown out as well, (having multiple methods and having one method and a converting function).. They are good, easy to read and use methods as long as you know at design time what type of object you want (which is probably the norm).

Another approach that I like (where it makes sense) is to return another (custom) object that can act like either... that way, you have the content in hand and can use it either way.  This approach works well when there is a high overhead in creating the data, that you don't want to have to do again, and you don't mind having a few more objects hanging around until cleaned up.

So, define a return object like:

from collections import UserList
class TestResponse(UserList):
    def __str__(self):
        return '\0xfe%s' % '\0xfe'.join(

...and return that object.

that way, you can iterate over it, treat it as a list, or whatever, or if you do print(str(TestResponse)), you get your r'0xfeONE\0exfeTWO\0xfeTHREE'

In the end, your decision should be made based on:
1.  What would the developer using this EXPECT to get back? (if the user normally expects one or the other, do that, if this is for a "public module", I would expect the list is more likely the common response.  This list is probably not the right place for that question though, unless this is a very general utility, you want to ask the people coding in the domain that would mostly be using your function.)

2.  What is the most likely response type? (if it is common to get it back different ways, or use it multiple times, this is probably mostly the same as #1, but can be different)

3.  What will be the easiest to read and document?  Returning a list is easy to read and expectable, but not if you have to then jump through some loops to convert it to something else immediately.

4.  Is there a large enough benefit in processing that you want to be able to use it multiple ways without having to regenerate the list.  (note that this is the last criteria, unless performance is really critical, readability and predictability is normally much more important than what is probably minor performance tuning.)

Dan Strohl

> -----Original Message-----
> From: Python-list [ at] On
> Behalf Of Peter Otten
> Sent: Thursday, February 18, 2016 6:37 AM
> To: python-list at
> Subject: Re: Question on keyword arguments
> grsmith at wrote:
> > Would this be the correct way to return a list as a default result.
> If it does what you want it to do it is "correct" as in "complies with
> specification".
> > Also, would the list be the preferable result (to a python programmer) ?
> A list as a return value is OK as is any other object, however,
> > def test(command, return_type='LIST'):
> passing the desired result type as an argument is usually not a good idea.
> >     """ Go to database and return data"""
> >     if return_type == 'LIST':
> >         result = ['ONE', 'TWO', 'THREE']
> >     else:
> >         result = r'0xfeONE\0exfeTWO\0xfeTHREE'
> >     return result
> >
> > if __name__ == '__main__':
> >     print(test('cmd'))
> >     print(test('cmd', 'LIST'))
> >     print(test('cmd', None))
> >     print(test('cmd', 'string'))
> Make it separate functions instead:
> def test(command):
>     return ["ONE", "TWO", "THREE"]
> def test_as_string(command, sep=","):
>     return sep.join(test(command))
> if __name__ == '__main__':
>     print(test("cmd"))
>     print(test_as_string("cmd"))
> --

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