Python exception weirdness26 Feb 2011

This may have been obvious to others, but I have just found out that in Python, when Exceptions get raised, they are converted to strings. That is:

>>> class C(object):
...     __str__ = lambda s: '<C: str>'
...     __repr__ = lambda s: '<C: repr>'

>>> c = C()
>>> exc = Exception(c)

>>> exc
Exception(<C: repr>,)

>>> repr(exc)
'Exception(<C: repr>,)'

>>> str(exc)
'<C: str>'

And now comes the interesting part:

>>> raise exc:
Traceback (most recent call last):
...
Exception: <C: str>

The traceback displays the Exception class’s name (__class__.__name__), and the parameters passed when instantiating the exception, converted to string (str())! The same thing happens in both Python 2 and Python 3.

The thing is, I’d expect repr() instead of str(). But good to know how it works. If I want to see consistent output in both pdb and tracebacks, I just need to use classes where str(obj) == repr(obj), that is, obj.__str__ is obj.__repr__.