When using a directive that is not recognised by time.strptime, the following occurs in Python 3:

>>> time.strptime("", "%D")
Traceback (most recent call last):
  File "C:\Python33\lib\_strptime.py", line 320, in _strptime
    format_regex = _TimeRE_cache.compile(format)
  File "C:\Python33\lib\_strptime.py", line 268, in compile
    return re_compile(self.pattern(format), IGNORECASE)
  File "C:\Python33\lib\_strptime.py", line 262, in pattern
    self[format[directive_index]])
KeyError: 'D'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python33\lib\_strptime.py", line 494, in _strptime_time
    tt = _strptime(data_string, format)[0]
  File "C:\Python33\lib\_strptime.py", line 329, in _strptime
    (bad_directive, format))
ValueError: 'D' is a bad directive in format '%D'
>>>

Probably the KeyError should be supressed when raising the ValueError. The attached patch tries to fix this issue, by suppressing the context.

The fix LGTM.  The attached patch tweaks a bit your test and adds another one to check _strptime directly.

New changeset 3203e083aa7b by Ezio Melotti in branch '3.3':
#17572: Avoid chained exceptions while passing bad directives to time.strptime().  Initial patch by Claudiu Popa.
http://hg.python.org/cpython/rev/3203e083aa7b

New changeset d408c9bd4bf7 by Ezio Melotti in branch 'default':
#17572: merge with 3.3.
http://hg.python.org/cpython/rev/d408c9bd4bf7

Fixed, thanks for the report and the patch!