Lists are one of the most useful and versatile data types available in Python. Lists are a collection of arbitrary objects, just like arrays in other programming languages.
In this tutorial you will learn:
- An overview of lists and how they are defined.
- Methods to insert data in a list using:
list.append(),list.extendandlist.insert(). - Syntax, code examples, and output for each data insertion method.
- How to implement a stack using list insertion and deletion methods.
Prerequisites
For this tutorial, you need:
- Python 3.
- A code editor of your choice.
Lists in Python
Lists have the following properties that make them powerful and flexible:
- Lists are ordered.
- Lists are accessed using the index. The first index starts at
0. - Lists are mutable and dynamic which means they can be modified after their creation.
How to create a list in Python
You create a list using square brackets in Python.
We can leave them empty and supply values later in the program:
# Create an empty list
programming_lang = []We can also provide values while creating a list:
# Create a filled list
programming_lang = ['P','Y','T','H','O','N']This would create a list as shown in the image below:
How to access items in a list
As list items are ordered, you can access them using their index.
Syntax: list[index].
In the image below, "P" is at index "0" whereas "H" is at index "3".
Let's write a short program to define a list and access its items:
programming_lang = ['P','Y','T','H','O','N']
print(programming_lang)
print("At index 0:", programming_lang[0])
print("At index 3:",programming_lang[3])Output:
You can also access items using a negative index, where -1 represents the last list item. If we wanted to access the last item from the list above, we could also use index -1:
programming_lang = ['P','Y','T','H','O','N']
print(programming_lang)
print("At index -1:", programming_lang[-1])
print("At index -5:",programming_lang[-5])Output:
How to find the length of a list
We can easily find the length of a list using the len() method.
programming_lang = ['P','Y','T','H','O','N']
print("Length of List: ",len(programming_lang))Output:
Methods to Add Items to a List
We can extend a list using any of the below methods:
list.insert()– inserts a single element anywhere in the list.list.append()– always adds items (strings, numbers, lists) at the end of the list.list.extend()– adds iterable items (lists, tuples, strings) to the end of the list.
How to insert items in a list with insert()
You can insert items in a list at any index using the insert() method. There are more insertion methods and we will look at them later in this post.
Syntax of insert: insert(index, element).
Example of insert():
# create a list of odd numbers
odd_n = [1,3,5,7,9]
# '21' is inserted at index 3 (4th position)
odd_n.insert(3, 21)
print('List of odd #:', odd_n)
Before insertion:
After insertion:
Output:
How to append an item to a list using list.append()
We can add a single item at the end of the list using list.append().
Syntax: list.append(item).
Example:
# crops list
crops = ['corn', 'wheat', 'cotton']
# Add 'cane' to the list
crops.append('cane')
print('Updated crops list: ', crops)Output:
⚠️Note that trying to append more than one item gives an exception, as list.append() takes only a single argument.
list.append().How to add multiple items in a list using list.extend()
We can add multiple items to a list using the extend() method.
The below example combines two lists into a single list.
# create a list
even_numbers = [2, 4, 8]
# create another list
more_even_numers = [100, 400]
# add all elements of even_numbers to more_even_numbers
even_numbers.extend(more_even_numers)
print('List after extend():', even_numbers)
Output:
extend().Other ways to extend lists in Python:
List slicing
Slicing allows us to select a range of values in a list.
The syntax is shown below:
list[starting index:upto index]
For example,
- list[1:3] would return items starting from index 1 up to (not including) index 3.
- Missing left index implies starting from index 0.
list[:len(list)]means start from index 0 and continue until the end.
- Missing right index implies until the last index.
list[0:]implies to start from index 0 until the last item.
Let's see how we can add lists using slicing.
Example:
A = [99, 100, 101]
B = [103, 104, 105]
# starting from last index +1, add items from list B
A[len(A):] = B
print('A =', A)Output:
Combining arrays using the + operator
Let's combine two arrays odd and even into a single list using the + operator.
Example:
odd = [1, 3, 5, 7]
even = [2, 4, 6, 8]
odd += even # odd = odd + even
# Output: [1, 2, 3, 4]
print('odd and even combined =', odd)Output:
How to populate an empty list using for loop and append()
There are two ways to populate empty lists: using a for loop with append() and using list comprehension.
Let's first use for loop with append().
Example:
In this example, we are calculating area of a square and appending the result in an array.
# Return area of square
# Area of square = length x length
def square_area(side_length):
result = []
for length in side_length:
result.append(length*length)
return result
lengths = [1, 4, 9, 20]
print(square_area(lengths))
Output:
We can make the above code efficient by completely skipping the for loop - append() combination and using list comprehension instead. Let's see how in the next section.
How to populate an empty list using list comprehension
List comprehension makes the code simple and readable by combining the for loop and append() into a single line.
We can modify our previous example to achieve list comprehension. Notice the commented out lines here:
# Return area of square
# Area of square = length x length
def square_area(side_length):
#result = []
#for length in side_length:
# result.append(length*length)
return [length*length for length in side_length]
lengths = [1, 4, 9, 20]
print(square_area(lengths))
return [length*length for length in side_length] is list comprehension.Output:
Both of the methods for filling an empty list are valid and suitable in different scenarios.
Append() vs Insert() vs Extend()
Append() always adds a single item at the end of a list. It is useful when only a single item needs to be inserted.
But if you need to make multiple additions, extend() is a better option as it adds iterable items in one batch.
You should use Insert() when insertion is required at a specific index or range of indexes.
How to Implement a Stack (LIFO)
What is a stack (LIFO)?
Stack is an arrangement of items that follows a last-in-first-out order. The item that goes last is the one that comes out first. An example of a stack would be the undo/redo stack in photo editing apps.
The diagram below visually explains a stack.
You can add an item by using append().
You can remove an item by using pop(). See details of the pop() method here.
Coding stacks
Let's create a stack class where we first declare an empty list in the init method.
The push() method appends an item to the list.
The pop() method deletes the last item of the list using pop(). If there is no item in the list, an exception will be thrown.
The __len__ method determines the length of the stack.
Lastly, __repr__ returns the output in a readable format.
Class definition:
class Stack:
def __init__(stack_t):
stack_t._items = []
def push(stack_t, item):
stack_t._items.append(item)
def pop(stack_t):
try:
return stack_t._items.pop()
except IndexError:
print("Stack is empty, all items deleted")
def __len__(stack_t):
return len(stack_t._items)
def __repr__(stack_t):
return f"stack ({stack_t._items})"
Code body:
Let's call the class functions and see the output in action.
stack = Stack()
# Push items onto the top of the stack
stack.push(3)
stack.push(5)
stack.push(8)
stack.push(99)
# Print stack
print(stack)
# Find the length of the stack
print("Length of stack is:" ,len(stack))
# Pop items from the stack
print("popping last item")
stack.pop()
print(stack)
print("popping last item again")
stack.pop()
print(stack)
print("finally the stack is")
print(stack)Output:
We have added 3, 5, 8, 99 to the stack. Next we printed the stack and its length. Afterwards, we popped two items and printed the stack each time.
Wrapping up
In this tutorial, we learned list creation methods. We also looked at some examples along with a practical implementation of stacks to see how it all works.
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You can read my other posts here.