Loops in Python

Python programming language provides the following types of loops to handle looping requirements. Python provides three ways for executing the loops. While all the ways provide similar basic functionality, they differ in their syntax and condition-checking time.

While Loop in Python

In python, a while loop is used to execute a block of statements repeatedly until a given condition is satisfied. And when the condition becomes false, the line immediately after the loop in the program is executed.

Syntax:

while expression:
    statement(s)

All the statements indented by the same number of character spaces after a programming construct are considered to be part of a single block of code. Python uses indentation as its method of grouping statements. 

Example of Python While Loop 

Let’s see a simple example of while loop in Python.

Hello Geek
Hello Geek
Hello Geek

Using else statement with While Loop in Python

The else clause is only executed when your while condition becomes false. If you break out of the loop, or if an exception is raised, it won’t be executed. 

Syntax of While Loop with else statement:

while condition:
     # execute these statements
else:
     # execute these statements

Examples of While Loop with else statement

Here is an example of while loop with else statement in Python:

Hello Geek
Hello Geek
Hello Geek
In Else Block

Infinite While Loop in Python

If we want a block of code to execute infinite number of time, we can use the while loop in Python to do so.

Note: It is suggested not to use this type of loop as it is a never-ending infinite loop where the condition is always true and you have to forcefully terminate the compiler.

For Loop in Python

For loops are used for sequential traversal. For example: traversing a list or string or array etc. In Python, there is “for in” loop which is similar to for each loop in other languages. Let us learn how to use for in loop for sequential traversals.

Syntax:

for iterator_var in sequence:
    statements(s)

It can be used to iterate over a range and iterators.

0
1
2
3

Example with List, Tuple, string, and dictionary iteration using For Loops in Python

We can use for loop to iterate lists, tuples, strings and dictionaries in Python.

List Iteration
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for
Lazyroar

Tuple Iteration
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for
Lazyroar

String Iteration
G
e
e
k
s

Dictionary Iteration
xyz  123
abc  345

Set Iteration
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2
3
4
5
6

Iterating by the index of sequences

We can also use the index of elements in the sequence to iterate. The key idea is to first calculate the length of the list and in iterate over the sequence within the range of this length. See the below example: 

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for
Lazyroar

Using else statement with for loop in Python

We can also combine else statement with for loop like in while loop. But as there is no condition in for loop based on which the execution will terminate so the else block will be executed immediately after for block finishes execution. 

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for
Lazyroar
Inside Else Block

Nested Loops

Python programming language allows to use one loop inside another loop. Following section shows few examples to illustrate the concept. 

Syntax:

for iterator_var in sequence:
   for iterator_var in sequence:
       statements(s)
   statements(s)

The syntax for a nested while loop statement in the Python programming language is as follows: 

while expression:
   while expression: 
       statement(s)
   statement(s)

A final note on loop nesting is that we can put any type of loop inside of any other type of loop. For example, a for loop can be inside a while loop or vice versa.

1 
2 2 
3 3 3 
4 4 4 4 

Loop Control Statements

Loop control statements change execution from their normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. Python supports the following control statements. 

Continue Statement

the continue statement in Python returns the control to the beginning of the loop.

Current Letter : g
Current Letter : k
Current Letter : f
Current Letter : o
Current Letter : r
Current Letter : g
Current Letter : k

Break Statement

The break statement in Python brings control out of the loop.

Current Letter : e

Pass Statement

We use pass statement in Python to write empty loops. Pass is also used for empty control statements, functions and classes.

Last Letter : s

How for loop in Python works internally?

Before proceeding to this section, you should have a prior understanding of Python Iterators.

Firstly, lets see how a simple for loop looks like.

apple
orange
kiwi

Here we can see the for loops iterates over iterable object fruit which is a list. Lists, sets, dictionaries are few iterable objects while an integer object is not an iterable object. For loops can iterate over any of these iterable objects.

Now with the help of the above example, let’s dive deep and see what happens internally here.

1. Make the list (iterable) an iterable object with help of the iter() function.
2. Run an infinite while loop and break only if the StopIteration is raised.
3. In the try block, we fetch the next element of fruits with the next() function.
4. After fetching the element we did the operation to be performed with the element. (i.e print(fruit))

apple
orange
kiwi

We can see that under the hood we are calling iter() and next() method.