Have a look at the following piece of code, which shows a list comprehension:
>>> i = 6
>>> s = [i * i for i in range(100)]
>>> print(i)
When you execute the code example in Python 2.6 it prints 99, but when you execute it in Python 3.x it prints 6.
What were the reasons for changing the behaviour and why is the output 6 in Python 3.x?
The old behaviour was a mistake but couldn't easily be fixed as some code relied on it.
The variable i inside the list comprehension should be a different i from the one at the top level. Logically it should have its own scope which does not extend outside the comprehension as its value only makes sense inside the comprehension. But in Python 2.x due to an implementation detail the scope was larger than necessary causing the variable to "leak" into the outer scope, causing the confusing results you see.
Python 3.0 was deliberately not intended to be backwards compatible with previous releases, so they used the opportunity to fix this undesirable behaviour.
In Python 2.3 and later releases, a list comprehension “leaks” the control variables of each for it contains into the containing scope. However, this behavior is deprecated, and relying on it will not work in Python 3.0