Python Loop Index Variable Scope

While crash-studying python for a new job, I found out that this code is actually not an error!

for i in [1, 2, 3]:
pass # Do nothing.
print(i)

It blew my mind that this code actually prints 3. For some crazy reason, python keeps the index variables around after the loop exits; they are not in the scope of the loop.

I found this in the python documentation; but it is described much better here in this blog post: https://eli.thegreenplace.net/2015/the-scope-of-index-variables-in-pythons-for-loops/.

I heavily recommend reading that link as it has lots of good info (thanks to Eli Bendersky).  But in case you’re lazy, here’s a historical anecdote quoted from it that I particularly liked:

“Why this is so

I actually asked Guido van Rossum about this behavior and he was gracious enough to reply with some historical background (thanks Guido!). The motivation is keeping Python’s simple approach to names and scopes without resorting to hacks (such as deleting all the values defined in the loop after it’s done – think about the complications with exceptions, etc.) or more complex scoping rules.

In Python, the scoping rules are fairly simple and elegant: a block is either a module, a function body or a class body. Within a function body, names are visible from the point of their definition to the end of the block (including nested blocks such as nested functions). That’s for local names, of course; global names (and other nonlocal names) have slightly different rules, but that’s not pertinent to our discussion.

The important point here is: the innermost possible scope is a function body. Not a for loop body. Not a with block body. Python does not have nested lexical scopes below the level of a function, unlike some other languages (C and its progeny, for example).

So if you just go about implementing Python, this behavior is what you’ll likely to end with. Here’s another enlightening snippet:

for i in range(4):
    d = i * 2
print(d)

Would it surprise you to find out that d is visible and accessible after the for loop is finished? No, this is just the way Python works. So why would the index variable be treated any differently?

By the way, the index variables of list comprehensions are also leaked to the enclosing scope. Or, to be precise, were leaked, before Python 3 came along.”

And for those like me who didn’t know, Guido van Rossum is the author of the Python programming language.

Oh, and by the way, you can avoid this variable leak with a lambda according to the python documentation here: https://docs.python.org/3.6/tutorial/datastructures.html.

For example:

squares = list(map(lambda x: x**2, range(10)))

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