conversion to bool raises an exception, 'with' loses that exception.

Reviewed by Jeffrey Yasskin.
Already ported to 2.5, will port to 2.6 and 3.0

"raised" without setting x, err, or why to let the eval loop know.

string allocation.

It doesn't make sense for such a fundamental document to have
the most obscure function listed at the top.

Took Nick Coghlan's advice about importing warnings globally in logging, to avoid the possibility of race conditions: "This could deadlock if a thread spawned as a side effect of importing a module happens to trigger a warning. warnings is pulled into sys.modules as part of the interpreter startup - having a global 'import warnings' shouldn't have any real effect on logging's import time."

process on success or raise an exception on error.

support unusual filenames (such as those containing semi-colons) in
Content-Disposition headers.