Python Engineer

Free Python and Machine Learning Tutorials

Become A Patron and get exclusive content! Get access to ML From Scratch notebooks, join a private Slack channel, get priority response, and more! I really appreciate the support!

back to course overview

The Asterisk (*) operator - Advanced Python 19

03 Aug 2019

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

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

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

Recommended further readings: - https://treyhunner.com/2018/10/asterisks-in-python-what-they-are-and-how-to-use-them/ - https://treyhunner.com/2016/02/how-to-merge-dictionaries-in-python/