Mastering Python Lists: Your Roadmap to Data Organization - w9school

Python Lists

In Python, a list is a built-in data structure used to store a collection of items. It is a mutable, ordered sequence of elements enclosed in square brackets `[ ]`, where each element is separated by a comma. Lists can contain elements of different data types, including numbers, strings, booleans, or even other lists.

Here's an example of creating a list:

```python
fruits = ['apple', 'banana', 'orange', 'grape']
```​

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Key features and operations associated with Python lists include:

1. Accessing Elements: Elements in a list can be accessed using their index, starting from 0. You can retrieve an element by specifying its index in square brackets `[]`.

Example:

```python
fruits = ['apple', 'banana', 'orange']
print(fruits[0])  
print(fruits[2])  
```​

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2. Modifying Elements: Lists are mutable, meaning you can change or update their elements by assigning new values to specific indices.

Example:

```python
fruits = ['apple', 'banana', 'orange']
fruits[1] = 'grape'
print(fruits)  
```​

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3. List Length: You can determine the number of elements in a list using the `len()` function.

Example:

```python
fruits = ['apple', 'banana', 'orange']
print(len(fruits))  ```

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4. List Operations: Lists support various operations such as concatenation using the `+` operator and repetition using the `*` operator.

Example:

```python
fruits1 = ['apple', 'banana']
fruits2 = ['orange', 'grape']
combined_fruits = fruits1 + fruits2
print(combined_fruits)  

repeated_fruits = fruits1 * 3
print(repeated_fruits)  
```

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5. List Methods: Python provides several built-in methods for manipulating lists. Some common methods include `append()`, `insert()`, `remove()`, `pop()`, `sort()`, and `reverse()`, among others.

Example:

```python
fruits = ['apple', 'banana', 'orange']
fruits.append('grape')
print(fruits)  

fruits.insert(1, 'kiwi')
print(fruits)

fruits.remove('banana')
print(fruits)  

popped_fruit = fruits.pop(2)
print(popped_fruit)  
print(fruits)  

fruits.sort()
print(fruits)  

fruits.reverse()
print(fruits)  
```

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Python lists offer flexibility and versatility for storing and manipulating collections of data. They are widely used due to their dynamic nature and support for a range of operations and methods.

In Python, Lists are used to store multiple items in a single variable. Lists are one of 4 built-in data types in Python used to store collections of data, the other 3 are Tuple, Set, and Dictionary, all with different qualities and usage.

Lists are created using square brackets:

Example

Create a List:

thislist = ["apple", "banana", "cherry"]
print(thislist)​

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List Items

List items are ordered, changeable, and allow duplicate values.

List items are indexed, the first item has index [0], the second item has index [1] etc.

Ordered

When we say that lists are ordered, it means that the items have a defined order, and that order will not change.

If you add new items to a list, the new items will be placed at the end of the list.

Changeable

The list is changeable, meaning that we can change, add, and remove items in a list after it has been created.

Allow Duplicates

Since lists are indexed, lists can have items with the same value:

thislist = ["apple", "banana", "cherry", "apple", "cherry"]
print(thislist)​

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List Length

As above mentioned, To determine how many items a list has, use the len() function:

thislist = ["apple", "banana", "cherry"]
print(len(thislist))

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List Items - Data Types

List items can be of any data type:

list1 = ["apple", "banana", "cherry"]
list2 = [1, 5, 7, 9, 3]
list3 = [True, False, False]

list1 = ["abc", 34, True, 40, "male"]

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type()

From Python's perspective, lists are defined as objects with the data type 'list':

mylist = ["apple", "banana", "cherry"]
print(type(mylist))

The list() Constructor

It is also possible to use the list() constructor when creating a new list.

thislist = list(("apple", "banana", "cherry")) # note the double round-brackets
print(thislist)

Python Collections (Arrays)

There are four collection data types in the Python programming language:

• List is a collection which is ordered and changeable. Allows duplicate members.
• Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
• Set is a collection which is unordered, unchangeable*, and unindexed. No duplicate members.
• Dictionary is a collection which is ordered** and changeable. No duplicate members.

When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.

Python - Access List Items

Access Items

List items are indexed and you can access them by referring to the index number:

thislist = ["apple", "banana", "cherry"]
print(thislist[1])​

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[:4])

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[2:5])

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[:4])

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[2:])

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[-4:-1])

Check if Item Exists

To determine if a specified item is present in a list use the in keyword:

thislist = ["apple", "banana", "cherry"]
if "apple" in thislist:
  print("Yes, 'apple' is in the fruits list")

Python - Change List Items

Change Item Value

To change the value of a specific item, refer to the index number:

thislist = ["apple", "banana", "cherry"]
thislist[1] = "blackcurrant"
print(thislist)

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "mango"]
thislist[1:3] = ["blackcurrant", "watermelon"]
print(thislist)

thislist = ["apple", "banana", "cherry"]
thislist[1:2] = ["blackcurrant", "watermelon"]
print(thislist)

thislist = ["apple", "banana", "cherry"]
thislist[1:3] = ["watermelon"]
print(thislist)

Python - Add List Items

Append Items

To add an item to the end of the list, use the append() method:

thislist = ["apple", "banana", "cherry"]
thislist.append("orange")
print(thislist)

thislist = ["apple", "banana", "cherry"]
thislist.insert(2, "watermelon")
print(thislist)

thislist = ["apple", "banana", "cherry"]
tropical = ["mango", "pineapple", "papaya"]
thislist.extend(tropical)
print(thislist)

Add Any Iterable

The extend() method does not have to append lists, you can add any iterable object (tuples, sets, dictionaries etc.).

thislist = ["apple", "banana", "cherry"]
thistuple = ("kiwi", "orange")
thislist.extend(thistuple)
print(thislist)

Python - Remove List Items

Remove Specified Item

The remove() method removes the specified item.

thislist = ["apple", "banana", "cherry"]
thislist.remove("banana")
print(thislist)

thislist = ["apple", "banana", "cherry"]
thislist.pop(1)
print(thislist)

thislist = ["apple", "banana", "cherry"]
thislist.pop()
print(thislist)

thislist = ["apple", "banana", "cherry"]
del thislist[0]
print(thislist)

thislist = ["apple", "banana", "cherry"]
del thislist

thislist = ["apple", "banana", "cherry"]
thislist.clear()
print(thislist)

Python - Loop Lists

Loop Through a List

You can loop through the list items by using a for loop:

thislist = ["apple", "banana", "cherry"]
for x in thislist:
  print(x)

thislist = ["apple", "banana", "cherry"]
for i in range(len(thislist)):
  print(thislist[i])

thislist = ["apple", "banana", "cherry"]
i = 0
while i < len(thislist):
  print(thislist[i])
  i = i + 1

thislist = ["apple", "banana", "cherry"]
[print(x) for x in thislist]

Python - List Comprehension

List Comprehension

List comprehension offers a shorter syntax when you want to create a new list based on the values of an existing list.

Example:

Based on a list of fruits, you want a new list, containing only the fruits with the letter "a" in the name.

Without list comprehension you will have to write a for statement with a conditional test inside:

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = []

for x in fruits:
  if "a" in x:
    newlist.append(x)

print(newlist)

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = [x for x in fruits if "a" in x]

print(newlist)

newlist = [x for x in fruits if x != "apple"]

newlist = [x for x in fruits]

newlist = [x for x in range(10)]

newlist = [x for x in range(10) if x < 5]

newlist = [x.upper() for x in fruits]

newlist = ['hello' for x in fruits]

Return "orange" instead of "banana":

newlist = [x if x != "banana" else "orange" for x in fruits]

Python - Sort Lists

Sort List Alphanumerically

List objects have a sort() method that will sort the list alphanumerically, ascending, by default:

thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort()
print(thislist)

thislist = [100, 50, 65, 82, 23]
thislist.sort()
print(thislist)

Sort Descending

To sort descending, use the keyword argument reverse = True:

thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort(reverse = True)
print(thislist)

thislist = [100, 50, 65, 82, 23]
thislist.sort(reverse = True)
print(thislist)

def myfunc(n):
  return abs(n - 50)

thislist = [100, 50, 65, 82, 23]
thislist.sort(key = myfunc)
print(thislist)

thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort()
print(thislist)

thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort(key = str.lower)
print(thislist)

thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.reverse()
print(thislist)

Python - Copy Lists

Copy a List

You cannot copy a list simply by typing list2 = list1, because: list2 will only be a reference to list1, and changes made in list1 will automatically also be made in list2.

There are ways to make a copy, one way is to use the built-in List method copy().

thislist = ["apple", "banana", "cherry"]
mylist = thislist.copy()
print(mylist)

Another way to make a copy is to use the built-in method list().

thislist = ["apple", "banana", "cherry"]
mylist = list(thislist)
print(mylist)

Python - Join Lists

Join Two Lists

There are several ways to join, or concatenate, two or more lists in Python.

One of the easiest ways are by using the + operator.

list1 = ["a", "b", "c"]
list2 = [1, 2, 3]

list3 = list1 + list2
print(list3)

Another way to join two lists is by appending all the items from list2 into list1, one by one:

list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]

for x in list2:
  list1.append(x)

print(list1)

Or you can use the extend() method, where the purpose is to add elements from one list to another list:

list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]

list1.extend(list2)
print(list1)

Python - List Methods

List Methods

Python has a set of built-in methods that you can use on lists.

Test Yourself With Exercises

Now you have learned a lot about lists, and how to use them in Python.

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