August 03, 2019     4min read

The Asterisk (*) operator - Advanced Python 19

The asterisk sign (*) can be used for different cases in Python:

  • Multiplication and power operations
  • Creation of list, tuple, or string with repeated elements
  • *args , **kwargs , and keyword-only parameters
  • Unpacking lists/tuples/dictionaries for function arguments
  • Unpacking containers
  • Merging containers into list / Merge dictionaries

Multiplication and power operations

# multiplication
result = 7 * 5
print(result)

# power operation
result = 2 ** 4
print(result)
    35
    16

Creation of list, tuple, or string with repeated elements

# list
zeros = [0] * 10
onetwos = [1, 2] * 5
print(zeros)
print(onetwos)

# tuple
zeros = (0,) * 10
onetwos = (1, 2) * 5
print(zeros)
print(onetwos)

# string
A_string = "A" * 10
AB_string = "AB" * 5
print(A_string)
print(AB_string)
    [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
    [1, 2, 1, 2, 1, 2, 1, 2, 1, 2]
    (0, 0, 0, 0, 0, 0, 0, 0, 0, 0)
    (1, 2, 1, 2, 1, 2, 1, 2, 1, 2)
    AAAAAAAAAA
    ABABABABAB

*args , **kwargs , and keyword-only arguments

  • Use *args for variable-length arguments
  • Use **kwargs for variable-length keyword arguments
  • Use *, followed by more function parameters to enforce keyword-only arguments
def my_function(*args, **kwargs):
    for arg in args:
        print(arg)
    for key in kwargs:
        print(key, kwargs[key])
        
my_function("Hey", 3, [0, 1, 2], name="Alex", age=8)

# Parameters after '*' or '*identifier' are keyword-only parameters and may only be passed using keyword arguments.
def my_function2(name, *, age):
    print(name)
    print(age)

# my_function2("Michael", 5) --> this would raise a TypeError
my_function2("Michael", age=5)
    Hey
    3
    [0, 1, 2]
    name Alex
    age 8
    Michael
    5

Unpacking for function arguments

  • Lists/tuples/sets/strings can be unpacked into function arguments with one * if the length matches the parameters.
  • Dictionaries can be unpacked with two ** if the length and the keys match the parameters.
def foo(a, b, c):
    print(a, b, c)

# length must match
my_list = [1, 2, 3]
foo(*my_list)

my_string = "ABC"
foo(*my_string)

# length and keys must match
my_dict = {'a': 4, 'b': 5, 'c': 6}
foo(**my_dict)
    1 2 3
    A B C
    4 5 6

Unpacking containers

Unpack the elements of a list, tuple, or set into single and multiple remaining elements. Note that multiple elements are combined in a list, even if the unpacked container is a tuple or a set.

numbers = (1, 2, 3, 4, 5, 6, 7, 8)

*beginning, last = numbers
print(beginning)
print(last)

print()

first, *end = numbers
print(first)
print(end)

print()
first, *middle, last = numbers
print(first)
print(middle)
print(last)
    [1, 2, 3, 4, 5, 6, 7]
    8

    1
    [2, 3, 4, 5, 6, 7, 8]

    1
    [2, 3, 4, 5, 6, 7]
    8

Merge iterables into a list / Merge dictionaries

This is possible since Python 3.5 thanks to PEP 448 (https://www.python.org/dev/peps/pep-0448/).

# dump iterables into a list and merge them
my_tuple = (1, 2, 3)
my_set = {4, 5, 6}
my_list = [*my_tuple, *my_set]
print(my_list)

# merge two dictionaries with dict unpacking
dict_a = {'one': 1, 'two': 2}
dict_b = {'three': 3, 'four': 4}
dict_c = {**dict_a, **dict_b}
print(dict_c)
    [1, 2, 3, 4, 5, 6]
    {'one': 1, 'two': 2, 'three': 3, 'four': 4}

But be careful with the following merging solution. It does not work if the dictionary has any non-string keys:
https://stackoverflow.com/questions/38987/how-to-merge-two-dictionaries-in-a-single-expression/39858#39858

dict_a = {'one': 1, 'two': 2}
dict_b = {3: 3, 'four': 4}
dict_c = dict(dict_a, **dict_b)
print(dict_c)

# this works:
# dict_c = {**dict_a, **dict_b}
    ---------------------------------------------------------------------------
    TypeError                                 Traceback (most recent call last)
    <ipython-input-52-2660fb90a60f> in <module>
          1 dict_a = {'one': 1, 'two': 2}
          2 dict_b = {3: 3, 'four': 4}
    ----> 3 dict_c = dict(dict_a, **dict_b)
          4 print(dict_c)
          5 
    TypeError: keywords must be strings

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