Gracefully handle exception being "(None, None, None)"

Hi!

The built-in traceback module handles (None, None, None) tuple (returned if sys.exc_info() is called outside the context of an exception) without raising an error:

import traceback

traceback.print_exception(None, None, None)
# 'NoneType: None'

Using stackprinter, however:

import stackprinter

stackprinter.format((None, None, None))
# ValueError: Can't format (None, None, None). Expected an exception instance, sys.exc_info() tuple,a frame or a thread object.

You may argue that formatting such exception is pretty useless… You’re right. 😄

Still, in my situation it happens that the exception tuple looks more like (TypeError, None, None) (I’ll save you the details, but it happens if the error value can’t be serialized, then it is replaced by None by default). In such case, it would be nice to format the error like TypeError: None for example instead of raising a ValueError. That would be useful so that user could format any exception-tuple without having to check for edge cases like that.

[JulianOrteil]

Saw this, thank you. Already updated my installation 👍

[cknd]

cc @JulianOrteil ☝️