Question

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?

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_{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.}

Answer 1

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.

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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