What do ** (double star/asterisk) and * (star/asterisk) mean in a function call?
argument-unpacking, iterable-unpacking, parameter-passing, python, syntax
Solution
A single star `*` unpacks a sequence or collection into positional arguments. Suppose we have
def add(a, b):
return a + b
values = (1, 2)
Using the `*` unpacking operator, we can write `s = add(*values)`, which will be equivalent to writing `s = add(1, 2)`.
The double star `**` does the same thing for a dictionary, providing values for named arguments:
values = { 'a': 1, 'b': 2 }
s = add(**values) # equivalent to add(a=1, b=2)
Both operators can be used for the same function call. For example, given:
def sum(a, b, c, d):
return a + b + c + d
values1 = (1, 2)
values2 = { 'c': 10, 'd': 15 }
then `s = add(*values1, **values2)` is equivalent to `s = sum(1, 2, c=10, d=15)`.
See also the relevant section of the tutorial in the Python documentation.
Similarly, `*` and `**` can be used for parameters. Using `*` allows a function to accept any number of positional arguments, which will be collected into a single parameter:
def add(*values):
s = 0
for v in values:
s = s + v
return s
Now when the function is called like `s = add(1, 2, 3, 4, 5)`, `values` will be the tuple `(1, 2, 3, 4, 5)` (which, of course, produces the result `15`).
Similarly, a parameter marked with `**` will receive a `dict`:
def get_a(**values):
return values['a']
s = get_a(a=1, b=2) # returns 1
this allows for specifying a large number of optional parameters without having to declare them.
Again, both can be combined:
def add(*values, **options):
s = 0
for i in values:
s = s + i
if "neg" in options:
if options["neg"]:
s = -s
return s
s = add(1, 2, 3, 4, 5) # returns 15
s = add(1, 2, 3, 4, 5, neg=True) # returns -15
s = add(1, 2, 3, 4, 5, neg=False) # returns 15
Problem
In code like `zip(*x)` or `f(**k)`, what do the `*` and `**` respectively mean? How does Python implement that behaviour, and what are the performance implications? See also: Expanding tuples into arguments. Please use that one to close questions where OP needs to use `*` on an argument and doesn't know it exists. Similarly, use Converting Python dict to kwargs? for the case of using `**`. See What does ** (double star/asterisk) and * (star/asterisk) do for parameters? for the complementary question about parameters.