The assumption behind string.format and friends is that people who override __getattr__ and __getitem__ know what they're doing. Which is probably a reasonably safe tradeoff in order to allow attribute access.

I'm not worried about executing arbitrary code, because it's not user-supplied and it's normal for a templating library to access attributes of objects you pass to it (if it was unsafe you shouldn't have done that). This is more of a snafu with the number of sensitive and powerful attributes available by default on Python objects.