This is expected and documented: http://docs.python.org/py3k/reference/executionmodel.html#interaction-with-dynamic-features
"Free variables are not resolved in the nearest enclosing namespace, but in the global namespace.", a free variable being a variable "used in a code block but not defined there".
And yes, a list comprehension defines a code block.

Try using exec(code, locals()). It's a good habit anyway to always pass a namespace to exec().