# What are loops in python?

*By Waqas Hussain (GlitchWH) Computer System Engineering student specializing in Python and Cyber Security*

### Introduction

Programs often need to repeat work: print 100 lines, check every item in a list, keep asking until the user types the right password. Writing the same line 100 times is not an option. A **loop** tells Python to repeat a block of code automatically, either a fixed number of times or until a condition changes. Python has two loops: `for` and `while`.

![](https://cdn.hashnode.com/uploads/covers/6a045b1b5418a55a522a6d57/f308ce28-c7fd-4283-9a3c-d4d08d5f5182.png align="center")

![](https://cdn.hashnode.com/uploads/covers/6a045b1b5418a55a522a6d57/5646b03f-7307-4ed5-b766-5e4b131a59d0.png align="center")

Two loops, two different ways of deciding when to stop. Here is how each works.

### Step 1: Why Loops Exist

A loop runs the same indented block again and again. Without loops, repeating a task 1,000 times would need 1,000 lines of code. With a loop, it takes two. Loops also work on data whose size you don't know in advance, like the lines of a file or the items in a list.

### Step 2: The `for` Loop

A `for` loop takes an **iterable** (anything Python can go through one item at a time, such as a list, string, tuple, dictionary, or `range`) and runs the block once per item.

![](https://cdn.hashnode.com/uploads/covers/6a045b1b5418a55a522a6d57/dd7db71c-818d-4a63-b90d-451725dd8294.png align="center")

On each pass, the loop variable (`letter`) is assigned the next item. When the items run out, the loop ends. Use `for` when you know what you are looping over.

### Step 3: `range()` Generates Numbers

`range()` produces a sequence of integers and has three forms:

![](https://cdn.hashnode.com/uploads/covers/6a045b1b5418a55a522a6d57/1574d6f3-a40f-4118-b8d8-33d878951aa0.png align="center")

![](https://cdn.hashnode.com/uploads/covers/6a045b1b5418a55a522a6d57/ba47992d-90f5-4756-bd05-d476936a5076.png align="center")

### Step 4: The `while` Loop

A `while` loop checks a condition **before every iteration**. If the condition is `True`, the block runs; if it is `False`, the loop ends.

![](https://cdn.hashnode.com/uploads/covers/6a045b1b5418a55a522a6d57/9613bd26-6551-4295-8132-48fe6c03123f.png align="center")

Use `while` when you don't know in advance how many repetitions you need, only when to stop. The danger is that if the condition never becomes `False`, the loop runs forever (an **infinite loop**). Always make sure something inside the loop moves it toward stopping.

### Step 5: What Happens Inside a `for` Loop

Python does not simply "count through" your data. Internally it:

1.  Calls `iter()` on the iterable to get an **iterator**.
    
2.  Calls `next()` on that iterator to get each item.
    
3.  Stops when the iterator raises `StopIteration`, which Python handles silently.
    

This is why `for` works on so many different types: anything that supports this protocol can be looped over. We'll cover this fully in the next article.

### Step 6: Controlling a Loop

Three tools change a loop's normal flow:

![](https://cdn.hashnode.com/uploads/covers/6a045b1b5418a55a522a6d57/94a6a713-f118-4de8-ad69-3ce754c25d22.png align="center")

![](https://cdn.hashnode.com/uploads/covers/6a045b1b5418a55a522a6d57/4f2d825a-03f3-4f87-bc62-5fcb3af4da4a.png align="center")

*   `break` exits the loop immediately.
    
*   `continue` skips the rest of the current iteration.
    
*   `else` on a loop runs only if the loop finished **without** hitting `break`. Here `break` fired, so the `else` block did not run.
    

### Step 7: Nested Loops

A loop can sit inside another loop. The inner loop runs completely for every single pass of the outer one:

![](https://cdn.hashnode.com/uploads/covers/6a045b1b5418a55a522a6d57/3c615f51-402b-4092-bad8-c9491ef2b3a7.png align="center")

![](https://cdn.hashnode.com/uploads/covers/6a045b1b5418a55a522a6d57/acaa4815-5614-464d-84e6-84327104c158.png align="center")

1st loop starting from 1, stop at 3  
2nd loop starting from 1, stop at 4

This runs 2 × 3 = 6 times. Nested loops are powerful but multiply work quickly, so use them carefully on large data.

### Why This Matters

Almost every real program depends on loops: processing files, scanning data, handling user input, running games. Choosing correctly between `for` (known collection or count) and `while` (repeat until a condition changes) makes your code shorter, safer, and easier to read.

### Conclusion

A loop repeats a block of code. `for` goes through the items of an iterable one by one, `while` repeats as long as a condition stays true, and `break`, `continue`, and `else` give you control over the flow. Under the hood, `for` works by calling `iter()` and `next()` until the data runs out.