# What does the star and doublestar operator mean in a function call?

The single star `*` unpacks the sequence/collection into positional arguments, so you can do this:

``````def sum(a, b):
return a + b

values = (1, 2)

s = sum(*values)
``````

This will unpack the tuple so that it actually executes as:

``````s = sum(1, 2)
``````

The double star `**` does the same, only using a dictionary and thus named arguments:

``````values = { 'a': 1, 'b': 2 }
s = sum(**values)
``````

You can also combine:

``````def sum(a, b, c, d):
return a + b + c + d

values1 = (1, 2)
values2 = { 'c': 10, 'd': 15 }
s = sum(*values1, **values2)
``````

will execute as:

``````s = sum(1, 2, c=10, d=15)
``````

Also see section 4.7.4 – Unpacking Argument Lists of the Python documentation.

Additionally you can define functions to take `*x` and `**y` arguments, this allows a function to accept any number of positional and/or named arguments that aren’t specifically named in the declaration.

Example:

``````def sum(*values):
s = 0
for v in values:
s = s + v
return s

s = sum(1, 2, 3, 4, 5)
``````

or with `**`:

``````def get_a(**values):
return values['a']

s = get_a(a=1, b=2)      # returns 1
``````

this can allow you to specify a large number of optional parameters without having to declare them.

And again, you can combine:

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