The docs say that "If a class defines __repr__() but not __str__(), then
__repr__() is also used when an “informal” string representation of
instances of that class is required."

but, repr is ignored:
>>> class E(Exception):
...     def __repr__(self):
...         return 'fancy!'
...
>>> raise E()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.E

only str is respected:
>>> class E(Exception):
...     def __str__(self):
...         return 'fancy!'
...
>>> raise E()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.E: fancy!