Python Lists – basics: define, access, assign, delete, print, concatenate

Python lists allow you to hold several objects in an order. Every element is assigned an index which indicates its position. The first element in a list is assigned an index value of zero (0), the second element is at position 1 and so on.

A list object can be created in a very simple manner. All values inside the list are written in square brackets and is separated by a comma. The list can be created as shown below:

lst1 = []
lst2 = [item1,item2,item3,…..]

Accessing elements of a list

Suppose you have a list of country names as shown below and you want to print the value of the second element in the list, you need to use index=1 since Python lists start with index=0. You just need to type: print (lst1[1])

list1=['India','America','Japan','Sweden','China','Poland']
#printing element of a list at position 2
print('We will now try to print the element at second position in the list');
print('element at position 2 is : '+ list1[1]);
print('***');

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Assign values to list elements

The elements of the list can be accessed and updated. You can assign a new value to any element of a list. Consider the below list :

lst1 = ['Ajay', 'Bobby','Ashok','Vijay','Anil','Rahul','Alex','christopher']
print(lst1[3]);
lst1[3]='Jackie';
print(lst1[3]);

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Delete Elements From the List

To delete an element we use del() command as shown in the following example.

lst1 = ['Ajay', 'Bobby','Ashok','Vijay','Anil','Rahul','Alex','Christopher']
print(lst1[3]);
del(lst1[3]);
print(lst1[3]);

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Length of a List

To find out the length of a list we use len() function

lst1 = ['Ajay', 'Bobby','Ashok','Vijay','Anil','Rahul','Alex','Christopher']
print(len(lst1));

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Printing all elements of a list

All elements of a list can be printed using ‘for’ command

lst1 = ['Ajay', 'Bobby','Ashok','Vijay','Anil','Rahul','Alex','Christopher']
For x in lst1: print(x)

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Concatenating two lists

There are multiple ways to concatenate two lists (or append one list to another).

The most basic way to concatenate two lists is using ‘+’ operator.
If you don’t want to use ‘+’ operator, you can call operator.add() function. Make sure you import operator module.
Lists can be concatenated by importing itertools module and then using the itertools.chain() method.
Lists can be also concatenated using extend() method.

lst1 = ['Ajay', 'Bobby']; lst2 = ['Rahul','Alex']
print("\nConcatenate using + operator")
lst3 = lst1+lst2
for x in lst3: print(x) # using + operator

print("\nConcatenate using operator.add() function")
import operator 
for item in operator.add(lst1, lst2): print (item) #using operator.add() method

print("\nConcatenate using itertools.chain() function")
import itertools 
for item in itertools.chain(lst1, lst2): print (item) # using itertools

print("\nConcatenate using extend() function")
lst4 = []
lst4.extend(lst1)
lst4.extend(lst2)
for item in lst4: print (item) # using extend() method

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This stackoverflow discussion has more information about concatenating lists in python.

Repetition of values in a list

You can use ‘*’ operator on the list to repeat all the values as many times you want

lst1 = ['Ajay', 'Bobby','Ashok','Vijay','Anil','Rahul','Alex','Christopher']
lst2 = lst1 * 2
for x in lst2: print(x)

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Verify if an object exists in a list

You can verify whether an object exists in a list or not by using an ‘if’ command. If yes, it returns true or else it will return false.

lst1 = ['Ajay', 'Bobby','Ashok','Vijay','Anil','Rahul','Alex','Christopher']
if 'Ajay' in lst1: print("true")

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