A Lesser-Known Difference Between For-Loops and List Comprehensions

In the code above, the for-loop updated the existing variable (a), but list comprehension didn't. Can you guess why? Read more to know.

A loop variable is handled differently in for-loops and list comprehensions.

A for-loop leaks the loop variable into the surrounding scope. In other words, once the loop is over, you can still access the loop variable.

We can verify this below:

In the main snippet above, as the loop variable (a) already existed, it was overwritten in each iteration.

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

We can verify this below:

That is why the existing variable (a), which was also used inside the list comprehension, remained unchanged. The list comprehension defined the loop variable (a) local to its scope.

Over to you: What are some other differences that you know of between for-loops and list comprehension?

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