Lists

In Python, lists are used to store multiple items in a single variable. Lists are one of the most versatile data types available in Python.

Key Concepts of Lists in Python

List Definition: A list can be defined by placing all the items (elements) inside square brackets [], separated by commas. Lists can contain items of different data types, including other lists.


my_empty_list = []
my_empty_list = list()  # same as above
my_list = [1, 2, 3]
my_list = list([1, 2, 3])  # same as above
mixed_list = [1, "hello", 3.14, True]
nested_list = [1, [2, 3], [4, 5, 6]]

Ordered: Lists are ordered, meaning that the elements have a defined order, and this order will not change unless you explicitly modify the list.
```
fruits = ["apple", "banana", "cherry"]
print(fruits[0])  # Output: apple
print(fruits[2])  # Output: cherry
```

Mutable: Lists are mutable, meaning that you can change their content without changing their identity. You can add, remove, or modify elements in a list.


my_list = [1, 2, 3]
my_list[0] = 10  # my_list is now [10, 2, 3]
my_list.append(4)  # my_list is now [10, 2, 3, 4]
my_list.remove(2)  # my_list is now [10, 3, 4]

Allow Duplicate Values: Lists can have duplicate values, meaning the same value can appear multiple times in a list.
```
my_list = [1, 2, 2, 3, 3, 3]
print(my_list)  # Output: [1, 2, 2, 3, 3, 3]
```
Accessing List Elements: Elements in a list can be accessed by their index. Python uses zero-based indexing, so the first element has an index of 0. Negative indexing can be used to access elements from the end of the list.
```
my_list = ["a", "b", "c", "d"]
first_item = my_list[0]  # Output: 'a'
last_item = my_list[-1]  # Output: 'd'
```
Slicing: You can access a range of elements in a list by using slicing. The syntax is list[start:stop:step], where start is the index to begin slicing, stop is the index to end slicing (exclusive), and step is the interval between elements.
```
my_list = [0, 1, 2, 3, 4, 5]
slice1 = my_list[1:4]  # Output: [1, 2, 3]
slice2 = my_list[:3]   # Output: [0, 1, 2]
slice3 = my_list[3:]   # Output: [3, 4, 5]
slice4 = my_list[::2]  # Output: [0, 2, 4]
```
List Methods: Lists come with several built-in methods that make them easy to work with:
- append(item): Adds an item to the end of the list.
- extend(iterable): Extends the list by appending elements from an iterable.
- insert(index, item): Inserts an item at a specified index.
- remove(item): Removes the first occurrence of an item.
- pop(index=-1): Removes and returns the item at the specified index (default is the last item).
- clear(): Removes all items from the list.
- index(item, start=0, end=len(list)): Returns the index of the first occurrence of an item.
- count(item): Returns the number of occurrences of an item.
- sort(key=None, reverse=False): Sorts the list in ascending order.
- reverse(): Reverses the elements of the list.
- copy(): Returns a shallow copy of the list.
```
my_list = [3, 1, 4, 1, 5, 9]
my_list.append(2)  # [3, 1, 4, 1, 5, 9, 2]
my_list.sort()     # [1, 1, 2, 3, 4, 5, 9]
my_list.reverse()  # [9, 5, 4, 3, 2, 1, 1]
my_list.pop()      # [9, 5, 4, 3, 2, 1]
```
Nested Lists: Lists can contain other lists as elements, creating a nested list. This allows for the creation of more complex data structures like matrices or tables.
```
matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
print(matrix[1][2])  # Output: 6 (second row, third column)
```

List Iteration: You can iterate over the elements of a list using a for loop.


my_list = ["apple", "banana", "cherry"]
for fruit in my_list:
    print(fruit)
# Output:
# apple
# banana
# cherry

List Length: The len() function returns the number of elements in a list.
```
my_list = [1, 2, 3, 4]
length = len(my_list)  # Output: 4
```

Notes about Python

Lists

Key Concepts of Lists in Python