Insertion Sort in PythonPublished on 29 January 2019 (Updated: 02 May 2020)
In this article, we’ll be tackling Insertion Sort in Python.
How to Implement the Solution
At this point, let’s dig into the code a bit. The following sections break down the Insertion Sort in Python functionality.
#!/usr/bin/env python import sys from itertools import takewhile def insertion_sort(xs): new_xs =  for x in xs: new_xs = insert(x, new_xs) return new_xs def insert(x, xs): left = list(takewhile(lambda i: i < x, xs)) right = xs[len(left):] if xs else xs return left + [x] + right def input_list(list_str): return [int(x.strip(" "), 10) for x in list_str.split(',')] def exit_with_error(): print('Usage: please provide a list of at least two integers to sort in the format “1, 2, 3, 4, 5”') sys.exit(1) def main(args): try: xs = input_list(args) if len(xs) <= 1: exit_with_error() print(insertion_sort(xs)) except (IndexError,ValueError): raise exit_with_error() if __name__ == "__main__": main(sys.argv[1:])
The Main Function
Breaking down this solution bottom up,
if __name__ == "__main__": main(sys.argv[1:])
This bit of code checks to see if this is the main module run. If it is, it then calls the main
function and passes user input to it. In this case the user input would be a string of numbers
"2, 1, 10, 5, 3" (to sort).
def main(args): try: xs = input_list(args) if len(xs) <= 1: exit_with_error() print(insertion_sort(xs)) except (IndexError,ValueError): exit_with_error()
This is the main function of this file. It parses the input, then calls our insertion sort function (and prints the results). It also deals with any errors raised.
Transform Input Parameters
def input_list(list_str): return [int(x.strip(" "), 10) for x in list_str.split(',')]
This function takes a string like
"2, 1, 10, 5, 3", and turns into a list of numbers.
It does this using a list comprehension. First, we need to convert our string into a
list_str.split(',') which is a list of strings split by comma (,).
So our original input string becomes
["2", " 1", " 10", " 5", " 3"]. Then for each
element in the list
for x in ... , we do something to it.
In this example we convert it into a decimal integer,
int(x.strip(" "), 10). Then,
removes any whitespace so
" 1" becomes
"1". After that,
converts the string
"1" into a decimal number in this case
1. This is done
for every item in the list so our original input of
"2, 1, 10, 5, 3" becomes
[2, 1, 10, 5, 3].
def exit_with_error(): print('Usage: please provide a list of at least two integers to sort in the format “1, 2, 3, 4, 5”') sys.exit(1)
This function prints a message and then exits the script with an error,
If any non-zero value is returned then the program didn’t complete properly. This function is called
if the user input isn’t correct.
def insertion_sort(xs): new_xs =  for x in xs: new_xs = insert(x, new_xs) return new_xs
Let’s take a look at the part of the program that sorts our list. The unsorted list is passed to the
insertion_sort method as the parameter
new_xs =  is our new sorted listed which is
empty to begin with.
We loop through every element in the unsorted list
for x in xs. Then we call the
function to add
x to the
new_xs (the sorted list) in the correct position.
Each time the
insert() function is called it returns a sorted list which we assign
new_xs. Finally, when we’ve looped through every item in
xs, we return the sorted list
new_xs which will then get printed on the terminal to the user.
Taking a look at an example where
xs = [5, 3, 10]
x = 5
new_xs = 
x = 3
new_xs = [3, 5]
x = 10
insert(10, [3, 5])
new_xs = [3, 5, 10]
def insert(x, xs): left = list(takewhile(lambda i: i < x, xs)) right = xs[len(left):] return left + [x] + right
This function takes two parameters
x, which is an element from our unsorted list, and
xs, which is the list to add
x to such that
xs remains sorted.
A High-level overview of this function is that the
left variable will be a list that stores
all elements less than
right will store all elements greater than (or equal)
x. That way we can “insert”
x between these two lists. Hence the return statement
return left + [x] + right and this returned list will therefore be sorted.
Let’s take a look at the first line
left = list(takewhile(lambda i: i < x, xs)) it looks a bit
complicated so let’s break it down. The first part
lambda i: i < x, xs is a lambda function
which is a small anonymous unnamed function.
In this case
i is an element of
xs and we want all
i’s less than
Then, we call
takewhile(lambda i: i < x, xs) which takes a predicate (our lambda function) and a list.
It stops iterating over our list as soon as the lambda function evaluates to False. It then returns
all the elements up to that index.
Lets take a look at an example where
xs = [4, 7, 10] and
x = 8.
The first two items of
xs would evaluate as True since 4 and 7 are
less than 8, so takewhile would store 4 and 7 but not 10.
takewhile() function returns a
takewhile object but we want a list so we convert that into a
list(takewhile(lambda i: i < x, xs)). So the
left variable will store all numbers
xs is already sorted.
right = xs[len(left):],
len(left) returns the length of the left list.
Then we do some index splicing; you can learn more about that
Index splicing is used to get part of a list in Python. In this case we are getting every element in
the list that’s not already in
left. We can do this because we know
xs is already sorted.
xs = [1, 4, 6, 8] and
x = 7 then
left = [1, 4, 6] (all elements < 7). Then
len(left) = 3
right = xs[3:]. Where
[3:] gets all elements from
xs not including the first
right = . Finally, we
return left + [x] + right as we can simply “slot”
x into the
correct position. We convert
x to a list
[x] first so we can do list concatenation using the
Lets take a look at an example,
xs = [3, 4, 6, 8, 11, 15, 18] and
5. The variable
will add consecutive elements from
lambda i: i < x (i < x) evaluates as False.
In this case
left = [3, 4] as 6 > 5. Then we get the length of
left which is
len(left) = 2,
slice so we don’t include the first two elements
xs[2:] = [6, 8, 11, 15, 18]. Then we return
left + [x] + right = [3, 4] +  + [6, 8, 11, 15, 18] or
[3, 4, 5, 6, 8, 11, 15, 18].
How to Run Solution
If we want to run this program, we should probably download a copy of Insertion Sort in Python. After that, we should make sure we have the latest Python interpreter. From there, we can run the following command in the terminal:
python insertion-sort.py "3, 2, 10, 6, 1, 7"
Alternatively, we can copy the solution into an online Python interpreter and hit run.
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