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Check out http://stackoverflow.com/questions/9327597/python-get-does-not-evaluate-to-true-even-though-there-is-an-object 
It turns out a cgi.FieldStorage object may consider itself False even when it has data. This happens when the initialization decided to use read_single() instead of one of the other ways of reading the field value (read_urlencoded() or read_multi()). Then self.list remains None, and __nonzero__ returns False no matter what the contents of self.file is.
To make things worse -- or better? -- the Python 3 version still defines __nonzero__() instead of __bool__().

PS. Simple repro:
import cgi; bool(cgi.FieldStorage('logo', u'tux.png'))
This reveals it's been fixed in Python 3 -- it raises TypeError there as it should. (But __nonzero__ should be deleted, and if someone adds __bool__ they should make sure it raises the same TypeError if self.list is None.)

FieldStorage("foo", "bar") is invalid because the first argument is supposed to
be file-like object and second one headers. Here we are sending invalid headers. By
default, if the headers is none, the content-type is urlencoded.
The right way of using FieldStorage is either using the keyword arguments in a tightly couple manner. Or use it as default instance (fs = cgi.FieldStorage())
So the expectation could be:
>>>fs = cgi.FieldStorage()
>>>bool(fs)
False
>>> # sending correct fs, env
>>> # http://hg.python.org/cpython/file/32de3923bb94/Lib/test/test_cgi.py#l259
>>> fs = cgi.FieldStorage(fs, environ=env)
>>> bool(fs)
True 
The TypeError failure in python3 for bool(fs) is, in the absence of __bool__, it is trying to do
a len() and which ultimately ends up calling keys():
http://hg.python.org/cpython/file/32de3923bb94/Lib/cgi.py#l579
The proper fix in Python3 IMO is the just define __bool__ instead of __nonzero__ and that will be return False instead of TypeError.

I believe you that the example invocation is wrong. But then shouldn't it raise an exception? I still think that if len() raises KeyError, bool() should raise KeyError too.

Hi Guido,
Agree with both your points.
Attaching a patch that fixes this issue.
1. Raises TypeError exception when header is not a Mapping or email.message.Message type.
2. Asserts for fp.read and fp.readline() to assert that fp, the first argument to FieldStorage is a file like object. I would have preferred to assert fp as a type of BytesIO object, but I saw some tests failings, so that could be taken as a separate 3.4 only backwards incompatible improvement (and not bug fix).
Finally, in the cases where read_single() is called for FieldStorage(), bool() raises valid TypeError for empty FieldStorage.
Please review and if it is OK, I can check this in.

Addressing storchaka's review comments. Here is the updated patch.

New changeset 2d6e7a5659f0 by Senthil Kumaran in branch '2.7':
Adding test coverage for cgi.FieldStorage based on the scenario mentioned in issue #19097
http://hg.python.org/cpython/rev/2d6e7a5659f0 

This is fixed in all active branches (2.7,3,3 and 3.4). I have addressed all review comments. Thanks.

Fixed in:
New changeset a3e49868cfd0 by Senthil Kumaran in branch '3.3':
Issue #19092 - Raise a correct exception when cgi.FieldStorage is given an
http://hg.python.org/cpython/rev/a3e49868cfd0
New changeset 1638360eea41 by Senthil Kumaran in branch 'default':
merge from 3.3
http://hg.python.org/cpython/rev/1638360eea41
(Thanks for correcting my bad merge Serhiy. I did fix :w and then did a hg resolve --mark, but still ended up with merge lines. I got to look more carefully next time).