Why Do Python's `for` Loop Control Variables Remain in Scope After the Loop Completes?

Why Do Control Variables in Python 'for' Loops Remain in Scope Afterwards?

While Python's scoping rules are generally understood, the design choice of maintaining the control variable in scope after exiting the loop remains a mystery to some. Consider the following code:

for foo in xrange(10):
    bar = 2
print(foo, bar)

This prints (9, 2), which may seem counterintuitive as 'foo' controls the loop and 'bar' is defined within it. It's understandable why 'bar' would be accessible outside the loop, but the persistence of 'foo' raises concerns.

The likely explanation lies in grammatical simplicity. Assigning to variables within loops doesn't require scope disambiguation, as assignments imply scope. Declaring variables within specific scopes is not a practice in Python. The 'global' keyword exists to explicitly denote global scope assignments.

This design has not significantly hindered adoption, and users have embraced the ability to access both control and defined variables outside the loop. It avoids the complexities of scoping and disambiguation, simplifying code readability and development.

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