How a For-loop and List Comprehension Differ 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!
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.
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?
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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Enlightening
Nice