Lists are the most versatile python data structure and stores an ordered sequence of elements just like your shopping list or to-do-list. In Python, Lists are
mutable, meaning that the elements can be altered unlike
strings.The elements of a
list are called
items and can be any data type.
Creating Lists in Python
Creating lists in python is quite simple, to define the list use square brackets [ ] and separate the items in the list with a comma.
mylist = [ ] # empty list mylist2 = [1, 2] # list containing 2 items with integers # list with 3 items, mixed data types, integers, and a string mylist3 = [1, 2, 'hello']
- A list can be empty or contain any number of items with different data types(integers, float, strings, etc.).
- It is a good practice to use plural names when defining lists, just to make your code readable.
- A list can also hold another list as an item, these are called
nested lists..See this illustration
Programming = [python', ' php ', [1, 2, 3] ] # a nested list
Accessing items in a list
We can access items in a list by using the name of that list followed by the index(position) of the item. Each item in a list has an assigned index value. The first index of an item is 0 and must be an integer.Use the
index operator [ ].To illustrate this, let's create a list holding students names
# Define a list of Students students = ['Cindy', 'Tess', 'Monica', 'Daniel'] # To get the first listed student Cindy first_student = students print(first_student) # To get the second student Tess second_student = students print(second_student) # To get the last student Daniel last_student = students[-1] print(last_student)
Python also supports negative indexing. The negative indexing is useful when you want to get the last item in a list because it starts accessing a list from the end.
- The first item of any non-empty list is .
- The last item of any non-empty list is [-1].
Slicing of a List
Slices are good for getting a subset of items in a list. It uses the
slicing operator : (colon) to extract part of the sequence.
Let's define a list of programming languages to illustrate how to create subsets from it.
# define a list languages = ['java', 'python', 'perl', 'ruby', 'C#'] # slicing everything up to but not including index 3 first_three = languages[:3] print(first_three) # ['java', 'python', 'perl'] # slicing everything from index 3 to the last item third_last = languages[3:] print(third_last) # ['ruby', 'c#'] # elements from beginning to end list_all = languages[:] print(list_all) # ['java', 'python',''perl','ruby', 'c#'] # elements from 1st to 3rd item languages[0:3] print(languages[0:3]) # ['java', 'python', 'perl'] # last three elements list_three = languages[-3:] print(list_three) # ['perl','ruby', 'c#']
Finding the length of a list
Python has useful in-built functions that work with the list. We will discuss them later but for now,
len() function help us in returning the total number of elements in a list.
# define a list languages= ['java', 'python', 'perl', 'ruby', 'c#'] # print the length of the list print(len(languages)) # output 5
Modify elements in a list
At this point, you have learnt how to access individual items in a list. Remember lists are mutable therefore, its possible to change the values, insert elements to it , remove some elements and even sort the elements in list
Change the Value of an Existing Item
Consider the languages list we created in the example above. Change the value of the first index to
kotlin and the last index to anything you want, c++ for me.
# define a list of programming Languages languages = ['java', 'python', 'perl', 'ruby', 'c#'] # access the first index and Change the value to kotlin languages = 'Kotlin' # print the first item to see if it has been modified print(languages) # access the last index and Change the value to kotlin languages[-1] = 'c++' # print the last item to see if it has been modified print(languages[-1]) # print the new list, # pay attention to the first and last index print(languages) # ['Kotlin', 'python', 'perl', 'ruby', 'c++']
How to add items to a list
There are two ways of adding elements to a list. You can either append an item at the end of the list or by inserting at a specific location position.
The default behavior of
list.append(item) is to add items at the end of the list .
# define a list of programming languages languages = ['java', 'python', 'perl', 'ruby', 'c#'] # append c languages.append('c') print(languages) # Output : ['java', 'python' ,'perl', 'ruby', 'c#', 'c'] # try something cool to find the last item, # use **negative index** to find the value of the last item print(languages[-1]) # should give you c
This method will insert an item at the ith position in a list, shifting elements to the right.
# use the languages list we defined in the above example. # This time insert 'c' at the first index # define a list languages = ['java', 'python', 'perl', 'ruby', 'c#'] # insert c in the first position languages.insert(0, 'c') print(languages) # ['c', 'java', 'python', 'perl', 'ruby', 'c#'] # insert kotlin in the third position languages.insert(2, 'Kotlin') print(languages) # ['c', 'java', 'Kotlin', 'python', 'perl', 'ruby', 'c#']
Removing elements in a list
You can remove elements of a list by value or its position. Use the del keywords to delete an item at a specific index(by position)
- single item
# define a list languages= ['Java', 'Python', 'Perl', 'Ruby', 'C#'] del languages # use del keyword to remove Perl print(languages)#removes perl ['java','python', 'ruby', 'c#']
- Multiple items
# define a list languages= ['Java', 'Python', 'Perl', 'Ruby', 'C#'] del languages[1:3] # delete Multiple items by slicing print(languages)#removes ['java', 'ruby', 'C#']
Use list.remove(item) method to remove by value.It will search and remove
only the first occurrence of an item.
# define a list Languages = ['java', 'python', 'perl', 'python', 'ruby', 'c#'] Languages.remove('python') # remove only the **first occurrence** of python
It removes the python after java. Note that the 'python' after Perl is still present
Popping elements in a list
Unlike del() and remove() which do not return a value, pop() returns the object.By default it removes and returns the last item of a list
languages= ['java', 'python', 'perl', 'ruby', 'c#'] print(languages.pop()) # print c#
You can also specify the specific item to remove. list.pop(i). Removes and returns the ith item in a list
Languages= ['java', 'python', 'Perl', 'ruby', 'C#'] print(languages.pop(1)) # removes index 1 item # Output returns Python
Common List Operations and Methods
Removes all items in a list
# define a list of programming languages languages = ['java', 'python', 'perl', 'ruby', 'c#'] # clear languages.clear() print(languages) # 
Returns the number of times an item appears in the list. It takes one argument
Reverse the order of items in a list
The sort method as the name suggest sorts the items in a list in ascending, descending and also as defined by a user. by default, the method will sort the items in ascending order. To sort in descending order pass
reverse=True as a parameter
# Define a list of Students students = ['Cindy', 'Tess', 'Daniel', 'Rachel', 'Alberts'] # sort in ascending order students.sort() print(students) # outputs ['Alberts', 'Cindy', 'Daniel', 'Rachel', 'Tess'] # sort in descending order students.sort(reverse=True) print(students) # outputs ['Tess', 'Rachel', 'Daniel', 'Cindy', 'Alberts']
When lists start to get long, it becomes more difficult to count the items and determine at what index position a certain value is located. use
list.index(item), to return the index in the list where that item is located.
If there is more than one item with value
item, this method will return the first occurrence.
* Operator. Concatenates a list a repeated number of times
mylist= ['strings are cool'] * 2 print(mylist) # output: ['strings are cool', 'strings are cool']
in is used to test if an item is a member of a list.
languages = ['java'', 'python' ,'perl', 'ruby', 'c # '] if 'python' in languages: print('right')
List concatenation is combining lists to create a new list object.You can either use + operator or extend() method to join lists
The extend method concatenates lists by adding the items of the list passed as an argument to an existing list.It does not create a new list.Note that you do not call extend with multiple arguments; it takes in a second list as its argument.
# define a list languages = ['java', 'python', 'perl', 'ruby', 'C#'] Languages2 = ['c++', 'C'] # define a second list languages.extend(Languages2) print(languages.extend(Languages2)) # Output ['Java', 'Python','Perl', 'Ruby', 'C#'']
The extend() does not return any value, instead, it modifies the original list by adding the content of the second list
- Concatenation using the
# define a list languages= ['java', 'python', 'perl', 'ruby', 'c#', ] print(languages + (['C++', 'C'])) # Output: ['java', 'python' ,'perl', 'ruby', 'c#', 'C++', 'C']
Copying a list
use list.copy() or slicing to copy a list.Copying is mostly used to create a temporary version of an existing list so that you can modify it rather than the original list.copy()
Why copy a list using [:]
The best way to copy a list is by using [:]. If you copy a list without using this approach, whatever you do to the copied list will affect the original.
Iteration in Lists
Lists can contain many items and loopings an efficient way to access each item at a time and store it in a temporary variable. The variable name should be the singular version of the list name for consistency and readability. The indented block of code makes up the body of the loop, where you can work with each individual item. Any lines that are not indented run after the loop is completed.
Used to iterate through each element on a list with the keyword
in.The for loop allows you to to perform an action for every element in the list.
languages= ['java', 'python', 'perl', 'ruby', 'c#'] for i in languages: print(i)
This will print all the items in the list one per line as shown below:
java python perl ruby c
random_sum = [2, 7, 8, 9] total= 0 for i in random_sum: total += i print(total)
The above code will print 26, the sum of all the items of the list. The for loop requires a variable to hold the items being iterated and the source.
will first check the condition. If the condition is
true, it will keep iterating and terminates the loop once the condition turns
languages = ['java', 'python', 'perl'' 'ruby', 'c # '] i= 0 while i < len(languages): print(languages[i]) i= i+3
The above codes will print java ruby
Generating Large Lists of numbers using range()
The range() works with a set of numbers efficiently. The range function starts at 0 by default and stops one number below the number passed.
To understand the range() better, let's generate and print a numbers from 0-10
numbers=  for number in range(11): print(number)
we can se the range() with list() to make a list of numbers between 1 and 10
# Make a list of numbers from 1-10 numbers = list(range(1, 10)) # output [1, 2, 3, 4, 5, 6, 7, 8, 9]
Provide a more elegant way of creating lists with way lesser code than using the for loops.
To write a comprehension, define an expression, the
values you want to store in the list. Then write a for loop to
generate input values needed to make the list.
list_variable= [x for x in iterable]
we are going to create a
for loop that print the letters of a string
# print the letters in the word 'python' name_letters = [ ] for name in 'Python': name_letters.append(name) print(name_letters)
we could simply write the above using list comprehensions as
name_letters = [name for name in 'python'] print(name_letters)
Now you know what lists are and how to manipulate them. This is simply an introduction.Practice more to master the concepts .Leave a comment below for any queries and Follow me on twitter