[Python-porting] Strange behavior of eval() in Python 3.1

[Benjamin Peterson]

2010/8/29 Bo Peng <ben.bob at gmail.com>:
> Dear all,
>
> This might be the wrong mailinglist to ask, but I bumped into a
> strange problem when I ported my code from Python 2.7 to 3.1. More
> specifically, my code involves the evaluation of expressions in a lot
> of local namespaces (dictionaries). For example, the following code
> evaluates expressions in a dictionary dd under Python 2.7:
>
> Python 2.7 (r27:82525, Jul  4 2010, 09:01:59) [MSC v.1500 32 bit
> (Intel)] on win32
> Type "help", "copyright", "credits" or "license" for more information.
>>>> dd = {'a': {1: 0.1, 2: 0.2}}
>>>> print(eval('a[1]', {}, dd))
> 0.1
>>>> print(eval("[x for x in a.keys()]", {}, dd))
> [1, 2]
>>>> print(eval("[a[x] for x in a.keys()]", dd, dd))
> [0.1, 0.2]
>>>> print(eval("[a[x] for x in a.keys()]", {}, dd))
> [0.1, 0.2]
>
> However, the last statement fails under Python 3.1
>
> Python 3.1.2 (r312:79149, Mar 21 2010, 00:41:52) [MSC v.1500 32 bit
> (Intel)] on win32
> Type "help", "copyright", "credits" or "license" for more information.
>>>> dd = {'a': {1: 0.1, 2: 0.2}}
>>>> print(eval('a[1]', {}, dd))
> 0.1
>>>> print(eval("[x for x in a.keys()]", {}, dd))
> [1, 2]
>>>> print(eval("[a[x] for x in a.keys()]", dd, dd))
> [0.1, 0.2]
>>>> print(eval("[a[x] for x in a.keys()]", {}, dd))
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
>  File "<string>", line 1, in <module>
>  File "<string>", line 1, in <listcomp>
> NameError: global name 'a' is not defined
>>>>
>
> Because a is in clearly accessible from the passed dictionary, I have
> no idea why it has to be defined in the global dictionary. Does anyone
> know what is going on here? How can I change make it work with Python
> 3?

List comprehensions are now properly scoped, so having one in the
global scope, like you do in this case, will lookup the names in the
global namespace.



-- 
Regards,
Benjamin