Python For Loop and While Loop (with SEO Examples)

Before we get into code, let's understand the problem loops solve.

Say you have 4 URLs:

https://example.com/page1
https://example.com/page2
https://example.com/page3
https://example.com/page4

The code works, but the problem is you wrote print() four times.

If you had 10,000 URLs, would you write print() ten thousand times?

Of course not.

This is where the idea of a loop comes in.

But repeating can happen in two different ways, and that's why we have both for and while.

First: for

The mindset

Use for when you have a group of items and want to do something with each one.

A simple example:

That's a list with 3 names.

I want to print each name.

Instead of writing:

Sarah
James
Ali

I can use:

Sarah
James
Ali

Now let's understand the code word by word.

What does for mean?

When you write:

you can read it in plain English as:

for here means Python goes through the items one by one.

What does in mean?

in here means: inside.

So:

means, more or less: for each name inside names.

Where did name come from?

We have the list:

And when we wrote:

we created a variable called name.

Each time round, Python puts one item from the list into it.

So the code:

can be pictured on the inside as:

Sarah
James
Ali

And the output is the same either way.

What does iteration mean?

You'll meet an important term: Iteration.

In the code:

there are 3 iterations.

So if you have 100 items, the for will usually do 100 iterations.

Do I have to call the variable name?

No.

This code is correct:

Sarah
James
Ali

And so is this:

Sarah
James
Ali

But it's better to pick a name that says what the item is.

If you have:

this is better:

/home
/about
/contact

than:

Indentation with for

Indentation is essential here.

Look at this example:

Sarah
James
Ali
Finished

Why did Finished appear only once?

Because print("Finished") isn't inside the for. Notice it starts at the beginning of the line.

But if we write:

Sarah
Finished
James
Finished
Ali
Finished

Why? Because print(name) and print("Finished") are both inside the for.

And that's a very important rule.

A first SEO use of for

We have some URLs:

/home
/about
/services
/contact

What happened:

So if you have a list of URLs, a for can go through them one by one.

Combining for with if

Now we start putting the lessons together.

We have some status codes:

We want to print the codes that aren't 200.

We write:

404
500

Let's go round by round.

Round one

status = 200. The condition status != 200 means 200 != 200, and the result is False.

So print(status) doesn't run.

Round two

status = 404. The condition becomes 404 != 200, which is True.

So print(status) prints 404.

Round three

status = 200. The condition is False, so nothing is printed.

Round four

status = 500. The condition 500 != 200 is True, so it prints 500.

And that's why the final output is 404 and 500.

Using a list of dictionaries

Now let's take data shaped more like real work.

Here we have a list. Inside the list, we have dictionaries.

Let's try this first:

{'url': '/home', 'status': 200}
{'url': '/about', 'status': 404}
{'url': '/services', 'status': 200}
{'url': '/contact', 'status': 500}

Why? Because in each iteration the variable page holds a whole dictionary.

Printing only the URL

If you want the url value inside each dictionary:

/home
/about
/services
/contact

Why did we write page["url"]? Because page itself is a dictionary.

For example, in the first iteration page equals {"url": "/home", "status": 200}, and when we write page["url"] we get /home.

Printing only the pages with a problem

Now we combine: list + dictionary + for + if.

The code:

/about
/contact

Why?

1. 1The first page is {"url": "/home", "status": 200}. The condition 200 != 200 is False. Nothing is printed.
2. 2The second page is {"url": "/about", "status": 404}. The condition 404 != 200 is True, so /about is printed.
3. 3The third page's status is 200, so it isn't printed.
4. 4The fourth page's status is 500, so /contact is printed.

Second: range()

Sometimes you don't have a list to go through; you just want to do something a set number of times.

That's where we use range().

An example:

0
1
2
3
4

Setting the start and the end

You can write:

1
2
3
4
5

Why? Because range(1, 6) means: start at 1 and stop before 6.

Adding a step

We can write:

0
2
4
6
8

Here the values mean: range(0, 10, 2) starts at 0, stops before 10, and moves by 2 each time.

Third: while

The mindset

for says: I have a group of items, go through them.

But while says: keep repeating as long as the condition is True.

So while doesn't really depend on a list; it depends on a condition.

A simple example:

1
2
3
4
5

Let's take this slowly.

The start

We wrote number = 1. So right now number equals 1.

Then while number <= 5: Python asks, is 1 less than or equal to 5? The answer is True, so it goes inside the while.

So the final output is 1, 2, 3, 4 and 5.

Why does number = number + 1 matter?

If we delete it:

1
1
1
1
1
1
1
1
...

What happens? At the start number = 1, and the condition 1 <= 5 is True, so it prints 1. But number never changed.

So the while comes back and asks again: 1 <= 5? Still True. It prints 1, then comes back again. And it goes on forever.

So with while, always ask yourself: what will happen inside the loop that eventually turns the condition from True to False?

In our example, number = number + 1 is what changed the value of number.

The basic difference between for and while

Remember the logic, not the code.

for

I have a group of items and want to do something with each one.

An SEO example: for url in urls: means go through every URL.

while

I want to repeat something as long as a condition holds.

An example: while attempts < 3: means keep going as long as the number of attempts is under 3.

Fourth: break

Before the code, the mindset.

An example:

1
2

Why didn't 3, 4 and 5 appear? Let's walk through it.

break means: leave the for at once. So it never reaches 4 and 5.

And 3 itself isn't printed, because break happened before print(number).

Fifth: continue

continue is different.

An example:

1
2
4
5

Notice that only 3 disappeared. 4 and 5 ran as normal.

Why? When number = 3, continue ran, and it means: that's it, skip the rest of this iteration's code and go to the next iteration.

So print() didn't run for 3. But the loop didn't stop: it moved on to number = 4, then number = 5.

The difference between break and continue

Look at the two examples side by side.

break

1
2

It means: at 3, stop the whole loop.

continue

1
2
4
5

It means: skip 3 only, and keep going.

An SEO example of continue

We have:

Say we don't want to process images. We write:

/home
/about
/contact

Note that we used endswith() here, which simply asks: does the text end with this?

True

So for the images, continue ran and they were skipped.

A quick mindset before the final example

We now have four ideas.

Now we'll put all four in one example. It's the most important example in the lesson.

The final example: for + while + continue + break

Here's the code:

/home
/about
/home
/about
/home
/about

Now let's take the example apart completely.

First: the data

That's a list with 5 items. Then:

So the number of attempts starts at zero.

Where does attempts = attempts + 1 belong?

Look at the indentation:

The line attempts = attempts + 1 is inside the while, so it belongs to the while. But it isn't inside the for.

Notice the levels: while attempts < 3:, and inside it for url in urls:, and everything inside the for gets extra indentation, like print(url). But attempts = attempts + 1 steps back one level.

Running it

At the start, attempts = 0. The while tests 0 < 3, and the result is True. So we go into the first attempt.

The first attempt

The for starts from the first item in the list. That matters a lot.

So we never reach print(url) for /image.jpg, which is why it doesn't show in the result.

And break means: stop the current for completely. So the for never reaches the next item, /contact. That's the key reason /contact never appears at all.

We leave the for, but we're still inside the while, so we reach attempts = attempts + 1. It was attempts = 0, so now it's attempts = 1.

The second attempt

The while comes back and tests 1 < 3, which is True. So a new round starts.

And here's a very important point: the for inside it starts again from /home, not from /contact. Why? Because it's a new for, starting from the top of the list.

The third attempt

The while asks 2 < 3, which is True. So it starts a third time, and the for starts again from the first item.

Is there a fourth attempt?

The while tests the condition again: 3 < 3, and the result is False. So it stops.

That's why we got only 3 attempts.

Why didn't /contact appear even once?

Because the list is in this order: /home, then /image.jpg, then /about, then /stop, then /contact. And in every attempt, when the for reaches /stop, break runs and the for ends.

So it never got to /contact at all. Not in the first attempt, not in the second, and not in the third.

Why didn't /image.jpg appear?

Because continue made Python skip that iteration only. So /image.jpg was reached, but skipped.

/contact is a different story: Python never reached it, because of break.

The difference between continue and break in this example

At /image.jpg, continue ran, meaning: leave this item and go to the next one. So we moved on to /about.

But at /stop, break ran, meaning: stop the whole for. So we never moved on to /contact.

The whole lesson's mindset

When you hit a problem and want to know what to use, think like this.

I have a set of data

Like URLs, keywords, status codes or pages, and I want to go through them one by one? Use for.

I want the repeating to continue based on a condition?

Like: keep going while attempts is under 3. Keep going while there's a next page. Keep going while the API is still returning data. Use while.

I'm inside a loop and want to skip the current item only?

Like: I found an image and I don't want to analyze images. Use continue.

I'm inside a loop and want to stop it completely?

Like: I hit the maximum. I found the item I was looking for. I reached a certain value and want to end the process. Use break.

The quiz

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12, the most important one

Question 13, the lesson challenge

Without running the code, predict the full output:

If you answered all these questions and understood why /contact never appeared even once, then you really understand loops, not just what they look like.