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.

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