Have a look at this simple Python code:
class A:
x = 123
y = [(x,x*x) for x in [1,2,3]]
print A().x
A.x is 123, but when I do A().x, it prints 3. Why?
$ python a.py
3
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Dog
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In my Python 3.3, it prints 123. – mayaa Jul 27 '13 at 14:44
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@mayaa: see the dupe question, this is a Python 2.x problem where variable scope for list comprehensions is handled differently. – Martijn Pieters Jul 27 '13 at 14:46
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@MartijnPieters: That is not the same question. Perhaps the answer covers this question, but that's a lot of bytecode to read through just to find out... – Dog Jul 27 '13 at 14:48
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It is *the same basic problem*. – Martijn Pieters Jul 27 '13 at 15:05
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And [Python list comprehension rebind names even after scope of comprehension. Is this right?](http://stackoverflow.com/q/4198906) is a dupe too, minus the class scope. – Martijn Pieters Jul 27 '13 at 15:06
2 Answers
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It's a behaviour related to py2.x. In py2.x the list comprehensions don't have their own scope. So, the list comprehension in your case actually modified the variable x and as x was assigned the value 3 at the end of list comprehension so you'll get 3 for both A().x and A.x.
...and in particular the loop control variables are no longer leaked into the surrounding scope.
Ashwini Chaudhary
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But there's no scope in class definitions? Why would there be one? The only reason I could think of is if you were allowed to do something like `class A: if True: z = 4`. – Dog Jul 27 '13 at 14:57
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@Dog [Classes are namespaces themselves and yes they have scope.](http://docs.python.org/2/tutorial/classes.html#python-scopes-and-namespaces) – Ashwini Chaudhary Jul 27 '13 at 15:01
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You have overriden the x in the list comprehension. Should use another name.
zhangyangyu
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