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')
```

New changeset dd6e4aa113c6db4fcf9d252b5063ab217e8330a2 by Mariatta (Subhendu Ghosh) in branch '2.7':
bpo-27470: Improve doc for commandline -3 option
https://github.com/python/cpython/commit/dd6e4aa113c6db4fcf9d252b5063ab217e8330a2