Python - Lists

Python Lists

List is one of the built-in data types in Python. A Python list is a sequence of comma separated items, enclosed in square brackets [ ]. The items in a Python list need not be of the same data type.

Following are some examples of Python lists −

list1 = ["Rohan", "Physics", 21, 69.75]
list2 = [1, 2, 3, 4, 5]
list3 = ["a", "b", "c", "d"]
list4 = [25.50, True, -55, 1+2j]

List is an ordered collection of items. Each item in a list has a unique position index, starting from 0.

A list in Python is similar to an array in C, C++ or Java. However, the major difference is that in C/C++/Java, the array elements must be of same type. On the other hand, Python lists may have objects of different data types.

A Python list is mutable. Any item from the list can be accessed using its index, and can be modified. One or more objects from the list can be removed or added. A list may have same item at more than one index positions.

Accessing Values in Lists

Toaccess values in lists, use the square brackets for slicing along with the index or indices to obtain value available at that index. For example −

list1 = ['physics', 'chemistry', 1997, 2000];
list2 = [1, 2, 3, 4, 5, 6, 7 ];
print ("list1[0]: ", list1[0])
print ("list2[1:5]: ", list2[1:5])

When the above code is executed, it produces the following result −

list1[0]:  physics
list2[1:5]:  [2, 3, 4, 5]

Updating Lists

You can update single or multiple elements of lists by giving the slice on the left-hand side of the assignment operator, and you can add to elements in a list with the append() method. For example −

list = ['physics', 'chemistry', 1997, 2000];
print ("Value available at index 2 : ")
print (list[2])
list[2] = 2001;
print ("New value available at index 2 : ")
print (list[2])

When the above code is executed, it produces the following result −

Value available at index 2 :
1997
New value available at index 2 :
2001

Delete List Elements

Toremove a list element, you can use either the del statement if you know exactly which element(s) you are deleting or the remove() method if you do not know. For example −

list1 = ['physics', 'chemistry', 1997, 2000];
print (list1)
del list1[2];
print ("After deleting value at index 2 : ")
print (list1)

When the above code is executed, it produces following result −

['physics', 'chemistry', 1997, 2000]
After deleting value at index 2 :
['physics', 'chemistry', 2000]

Note− remove() method is discussed in subsequent section.

Python List Operations

In Python, List is a sequence. Hence, we can concatenate two lists with ”+” operator and concatenate multiple copies of a list with ”*” operator. The membership operators “in” and “not in” work with list object.

Indexing, Slicing, and Matrixes

Because lists are sequences, indexing and slicing work the same way for lists as they do for strings.

Assuming following input −

L = ['spam', 'Spam', 'SPAM!']

Python ExpressionResultsDescriptionL[2]SPAM!Offsets start at zeroL[-2]SpamNegative: count from the rightL[1:][‘Spam’, ‘SPAM!‘]Slicing fetches sections

Python List Methods

Python includes following list methods −

Sr.No.Methods with Description1list.append(obj)Appends object obj to list.2list.clear()Clears the contents of list.3list.copy()Returns a copy of the list object.4list.count(obj)Returns count of how many times obj occurs in list5list.extend(seq)Appends the contents of seq to list6list.index(obj)Returns the lowest index in list that obj appears7list.insert(index, obj)Inserts object obj into list at offset index8list.pop(obj=list[-1])Removes and returns last object or obj from list9list.remove(obj)Removes object obj from list10list.reverse()Reverses objects of list in place11list.sort([func])Sorts objects of list, use compare func if given

Built-in Functions with Lists

Following are the built-in functions we can use with lists −

Sr.No.Function with Description1cmp(list1, list2)Compares elements of both lists.2len(list)Gives the total length of the list.3max(list)Returns item from the list with max value.4min(list)Returns item from the list with min value.5list(seq)Converts a tuple into list.