The man page for python says:

> Warn about Python 3.x incompatibilities that 2to3 cannot trivially fix.

The official documentation (https://docs.python.org/2/using/cmdline.html#cmdoption-3) does not mention 2to3 at all:

> Warn about Python 3.x possible incompatibilities by emitting a DeprecationWarning for features that are removed or significantly changed in Python 3.

This seems like a pretty big oversight when the following code issues no warnings (presumably because 2to3 can trivially handle this change):

```
from __future__ import print_function

class test(object):
    def __nonzero__(self):
        return False

t = test()
if t:
    print('Hello')
```