A For-loop and List Comprehension Are Fundamentally Different at Scope Level

A lesser-talked difference between for-loop and list comprehension.

“For-loops and list comprehensions work the same way and do the same thing.”

This is a statement that I see in almost every Python introductory course/video/blog (or a differently phrased statement…I hope you get the idea).

But this is not entirely true.

Of course, purpose-wise, they do the exact same thing, and I am not denying it.

But what most Python programmers don’t know is that there’s a pretty fine difference between how a list comprehension and a for-loop operate internally at a scope level, and it’s something that every Python programmer must know.

Today, I want to share that with you.

Let’s begin!

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Consider this simple for-loop code:

After running this for-loop, if we try to access the loop variable (loop_var), we notice that the variable is still accessible, as depicted below:

All good so far.

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Now, let’s do this with a list comprehension instead. The following code is the list comprehension equivalent of the above for-loop:

After running this list comprehension, if we try to access the loop variable (loop_var), we notice that the variable is NOT accessible:

Strange, right?

Why is the loop variable accessible when we use a for-loop but not when we use a list comprehension?

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This brings us to an essential difference between how for-loop and list comprehension fundamentally work, and it’s something I don’t find most Python tutorials talking about.

When we use a for-loop, Python leaks the loop variable into the surrounding scope. In other words, we can still access the loop variable once the loop is over.

But a list comprehension does not work this way. Thus, the loop variable always remains local to the list comprehension, and it is never leaked outside.

Also, if we use a pre-existing variable as our loop variable, for-loop will update its value while list comprehension will leave it unchanged.

This can be verified from the image below:

The value of the existing variable (loop_var):

• gets updated when used in a for-loop.

• remains unchanged when used in a list comprehension.

List comprehension does not update the value because, as discussed above, it creates a local copy of the loop variable.

This discussion is closely linked to the concept of ‘scope’ in Python, which can be of four types and is a critical topic in Python. I intend to cover that in an upcoming issue.

👉 Until then, it’s over to you: What are some other Python technical concepts that most programmers misinterpret?

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