Issue
What does the *
operator mean in Python, such as in code like zip(*x)
or f(**k)
?
- How is it handled internally in the interpreter?
- Does it affect performance at all? Is it fast or slow?
- When is it useful and when is it not?
- Should it be used in a function declaration or in a call?
Solution
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
Answered By - Lasse V. Karlsen Answer Checked By - Katrina (PHPFixing Volunteer)
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