Confused about ** parameters in python function calls

Question

For reference I am referring to the answer in this post

The author of the answer gives the following code

def sum(*values, **options):
    s = 0
    for i in values:
        s = s + i
    if "neg" in options:
        if neg:
            s = -s
    return s

s = sum(1, 2, 3, 4, 5)            # returns 15
s = sum(1, 2, 3, 4, 5, neg=True)  # returns -15
s = sum(1, 2, 3, 4, 5, neg=False) # returns 15

However when I run on mine I get the following error

NameError: global name 'neg' is not defined

Can anyone explain this. And in general, how does the function know when values ends and when options begins

John Kugelman · Accepted Answer · 2013-08-07T14:46:54.380

4

if neg:

That line is buggy. It should be:

if options["neg"]:

How does the function know when values ends and when options begins?

Unnamed values go in *values. Keyword arguments go in **options.

edited Aug 07 '13 at 14:46

answered Aug 07 '13 at 14:37

John Kugelman

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`if options.get("neg", False):` includes both `if` conditions. – glglgl Aug 07 '13 at 14:40
@glglgl The shorter code seems to be slower by a factor of about 2.4. I'm guessing it's due to the `dict.get` method call. – RussW Aug 07 '13 at 14:58
@RussW Oups! Ok, but in the case of a multi-threading program, it is free of a race condition. – glglgl Aug 07 '13 at 15:17
@glglgl I guess you got me there. +1 – RussW Aug 07 '13 at 15:25

Sukrit Kalra · Answer 2 · 2013-08-07T14:42:45.277

You have made a small mistake. Change your code to the following and it should work. Just get the value of "neg" from the options dictionary, (values holds the unnamed arguments and options holds the keyword arguments)

>>> def sum(*values, **options):
        s = 0
        for i in values:
            s = s + i
        if "neg" in options:
            if options["neg"]:
                s = -s
        return s

>>> s = sum(1, 2, 3, 4, 5, neg=True)
>>> s
-15
>>> sum(1, 2, 3, 4, 5)
15
>>> sum(1, 2, 3, 4, 5, neg=True)
-15
>>> sum(1, 2, 3, 4, 5, neg=False)
15

Although, as @glglgl pointed out, changing your code to the following consumes both the if statements into one.

>>> def sum(*values, **options):
    s = 0
    for i in values:
        s = s + i
    if options.get("neg", False):
            s = -s
    return s

How does get(...) work?

If the options dictionary doesn't have a key "neg", (as handled by your first if condition), then, get(...) returns the default value of False and s is not negated, and if options contains "neg", then it's value is returned, in which case, s is negated depending on the value in the dictionary.

`if options.get("neg", False):` includes both `if` conditions. — glglgl, Aug 07 '13 at 14:40
@glglgl : True. Thanks. Added to the answer and referenced to you. :) — Sukrit Kalra, Aug 07 '13 at 14:43

Confused about ** parameters in python function calls

2 Answers2