When using list comprehensions,the scope of temp variable doesn't express as you expect.

For example

Python 2.7.5 (default, May 15 2013, 22:43:36) [MSC v.1500 32 bit (Intel)] on win32
Type "copyright", "credits" or "license()" for more information.
>>> L=[i for i in range(10)]
>>> print i
9
>>>

and the output is 9,But maybe you shouldn't want this.

If you have variable i in the toper scope.

>>> i=2
>>> L=[i for i in range(10)]
>>> print i
9
>>>

the  value of i was overwritten。

Happily,at Python 3 . the temp i in list comprehensions is local variable.When the list comprehensions finished,the variables were disappear.

Python 3.3.2 (v3.3.2:d047928ae3f6, May 16 2013, 00:03:43) [MSC v.1600 32 bit (Intel)] on win32
Type "copyright", "credits" or "license()" for more information.
>>> L=[i for i in range(10)]
>>> print (i)
Traceback (most recent call last):
File "\<pyshell#1>", line 1, in \<module>
print (i)
NameError: name 'i' is not defined
>>> i=2
>>> L=[i for i in range(10)]
>>> print (i)
2