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python – What does the star operator mean?

python – What does the star operator mean?

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

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