A list in python is a sequence of values. values can be any type. The values in the list are called elements or items.
Table of Contents
What is the list in Python? | A Definition
How to define a list in python?
list = [“item1”, “item2”, “item3”]
The second method is using the list() Constructor. for example,
list = list( (“element1”, “element2”, “element3”) )
Note: enclose all the elements in round brackets and pass it as one argument otherwise you will get the following error.
TypeError: list expected at most 1 arguments
A list can be of the same type or it can contain different data types. Here are some examples for listing in python.
## # Examples #list in python #list of four integers int_list = [10, 20, 30, 40] #list of fruits fruits = ["apple", "banana", "pineapple"] #list (mixed data types) myData = ["male", 11, True, 40, "USA"] ## Empty list list = [] ##
How lists are mutable in python?
lists are mutable because you can change the order of items in a list or reassign an item in a list.
The following example explains, how lists are mutable in python.
## #Example numbers = [10, 20] # change the second element numbers[1] = 5 print(numbers) # output: [10, 5] ##
In the above example, the second element is now 5 instead of 20.
list as a relationship between indices and elements. This relationship is called ” mapping “. Each index “maps to” one of the elements in a list.
Here are some rules related to indices and elements.
- The index should be an integer or an integer expression can be used as an index.
- Reading or writing an element that does not exist in the list results in an
IndexError
. - If an index has a negative value, it counts backward from the end of the list.
How to traverse the list in python?
## Example for item in list: print(item) ##
This will display the elements of the list.
If you want to write or update the elements, you need the indices.
For example, in the following code, we will update the list.
## Example for i in range(len(list)): list[i] = list[i] * 2 ##
This loop traverses the list and updates each element. len()
returns the length of the number of elements in the list. range()
returns a list of indices from 0 to n − 1, where n is the length of the list.
Each time through the loop, i
gets the index of the next element.
The assignment statement ( = ) in the body uses i
to read the old value of the element and to assign the new value.
How to slice lists in Python?
## Example ## slice in python list[start:end] # items start through end-1 list[start:] # items start through the rest of the array list[: end] # items from the beginning through end-1 list[:] # a copy of the whole array ##
Here is a simple example of slicing in Python
## Example t = ['a', 'e', 'i', 'o', 'u'] t[1:3] # output: ['e', 'o'] t[:4] # output: ['a', 'e', 'i', 'o'] t[3:] #output: ['o', 'u'] ##
You can also specify steps as a third value.
Note: end value is not included in the output. the output will be at end-1
## Example # list[start:end:step] numbers = [10, 20, 10, 20, 30 , 3, 4, 7, 8, 8, 7, 9, 0, 7] print(numbers[1:8:2]) #this code will print every 2nd element from index 1 to 8 #output: [20, 20, 3, 7] ##
What are the list methods in Python?
method. Here are examples for each method for list in python.Method | Description |
---|---|
append() | Adds an element at the end of the list |
clear() | Removes all the elements from the list |
copy() | Returns a copy of the list |
count() | Returns the number of elements with the specified value |
extend() | Add lists and combine all the elements of specified lists into a single list. |
index() | Returns the index of the specified element of the list |
insert() | Adds an element at the specified position |
pop() | Removes the element at the specified position |
remove() | Removes the first item with the specified value |
reverse() | Reverses the order of the list |
sort() | Sorts the list |
##Example #Append Method list = ['a', 'b', 'c', 'd'] list.append("e") print(list) #output: ['a', 'b', 'c', 'd', 'e'] #Clear Method list = ['a', 'b', 'c', 'd'] list.clear() print(list) #output: [] #Copy Method list = ['a', 'b', 'c', 'd'] list2 = list.copy() print(list2) #output: ['a', 'b', 'c', 'd'] #Count Method list = ['a', 'b', 'b', 'c', 'd'] print( list.count('b') ) #output: 2 #Extend Method list = ['a', 'b', 'c', 'd'] list2 = ['e', 'f', 'g', 'h'] print( list.extend(list2) ) #output: ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'] #Index Method list = ['a', 'b', 'b', 'c', 'd'] print( list.index('b') ) #output: 2 #Insert Method list = ['a', 'b', 'c', 'd'] list.insert(1, "e") print(list) #output: ['a', 'e', 'b', 'c', 'd'] #Pop Method list = ['a', 'b', 'c', 'd'] list.pop(1) print(list) #output: ['a', 'c', 'd'] #Remove Method list = ['a', 'b', 'c', 'd'] list.remove('a') print(list) #output: ['b', 'c', 'd'] #Reverse Method list = ['a', 'b', 'c', 'd'] list.reverse() print(list) #output: ['d', 'c', 'b', 'a'] #Sort Method list = ['b', 'a', 'c', 'v', 'd'] list.sort() print(list) #output: ['a', 'b', 'c', 'd', 'v'] ##
Youtube video to learn list in python
References
https://www.programiz.com/python-programming/list
https://stackoverflow.com/questions/509211/understanding-slice-notation