In C/C++, the scope of a variable declared in a for or while loop (or any other bracketed block, for that matter) is from the open bracket to the close bracket.
You will learn about the four different scopes with the help of examples: local, enclosing, global, and built-in. These scopes together form the basis for the LEGB rule used by the Python interpreter when working with variables.
In Python, on the other hand, variables declared in if-statements, for-loop blocks, and while-loop blocks are not local variables, and stay in scope outside of the block. Thus we say that C++ has “block-level” scoping, while Python uses only “function-level” scoping.
Python doesn't have a temporary namespace tool like let in Lisp or Scheme.
The likeliest answer is that it just keeps the grammar simple, hasn't been a stumbling block for adoption, and many have been happy with not having to disambiguate the scope to which a name belongs when assigning to it within a loop construct. Variables are not declared within a scope, it is implied by the location of assignment statements. The global
keyword exists just for this reason (to signify that assignment is done at a global scope).
Update
Here's a good discussion on the topic: http://mail.python.org/pipermail/python-ideas/2008-October/002109.html
Previous proposals to make for-loop variables local to the loop have stumbled on the problem of existing code that relies on the loop variable keeping its value after exiting the loop, and it seems that this is regarded as a desirable feature.
In short, you can probably blame it on the Python community :P
Python does not have blocks, as do some other languages (such as C/C++ or Java). Therefore, scoping unit in Python is a function.
A really useful case for this is when using enumerate
and you want the total count in the end:
for count, x in enumerate(someiterator, start=1):
dosomething(count, x)
print "I did something {0} times".format(count)
Is this necessary? No. But, it sure is convenient.
Another thing to be aware of: in Python 2, variables in list comprehensions are leaked as well:
>>> [x**2 for x in range(10)]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> x
9
But, the same does not apply to Python 3.
One of the primary influences for Python is ABC, a language developed in the Netherlands for teaching programming concepts to beginners. Python's creator, Guido van Rossum, worked on ABC for several years in the 1980s. I know almost nothing about ABC, but as it is intended for beginners, I suppose it must have a limited number of scopes, much like early BASICs.
If you have a break statement in the loop (and want to use the iteration value later, perhaps to pick back up, index something, or give status), it saves you one line of code and one assignment, so there's a convenience.
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