Python异常类的__str__中的异常丢失

Question

Does anybody know how to catch exceptions in python Exception's derived classes's __str__ function? And how to debug that __str__ using pdb since I find it doesn't work when invoke pdb.set_trace() or setting breakpoint in pdb in function __str__ .

Example code:

class Ex(Exception):
    def __str__(self):
        raise KeyError('1')

raise Ex()

The output is

Traceback (most recent call last):
  File "./exex.py", line 10, in <module>
    raise Ex()
__main__.Ex

While I think the output should be something like

KeyError: '1'

And if I add pdb.set_trace() before raise KeyError('1') , python runtime does not interrupt at pdb.set_trace() . So does anyone know how to debug Exception.__str__ when it is a little complex?

---

Sorry for the pdb.set_trace() case. It works. However, if you forget to put import pdb , then python slightly ignore the exception which looks like pdb.set_trace() is ignored.

While setting breakpoint in pdb still does not work.

Answer 1

You don't need to override __str__ to get a message. Just do:

class Ex(Exception):
   pass

raise Ex('Key Error 1')

Prints:

Traceback (most recent call last):
  File "Untitled.py", line 4, in <module>
    raise Ex('Key Error 1')
__main__.Ex: Key Error 1

Answer 2

Python handles exceptions differently because it's in the process of printing an exception. Acknowledging another exception at that point would only to confuse the programmer, so Python swallows it and does the best it can.

Write your classes' __str__ to always return a string and you'll be happier.

Answer 3

I had the same problem the other day. I ended up wrapping my exception's __str__ method in a try/except block. Something like this:

import traceback

class ex(Exception):
    def do_str(self):
        return "ex: %s" % self.notexist

    def check_str(self):
        try:
            return self.do_str()
        except:
            traceback.print_exc()
            raise

    __str__ = check_str

raise ex()

Which when you run it prints:

Traceback (most recent call last):
  File "t100.py", line 16, in <module>
    raise ex()
__main__.exTraceback (most recent call last):
  File "t100.py", line 9, in check_str
    return self.do_str()
  File "t100.py", line 5, in do_str
    return "ex: %s" % self.notexist
AttributeError: 'ex' object has no attribute 'notexist'

Answer 4

FWIW, there's a Python bug I opened some while ago which at least makes the output a little bit less confusing: issue22836 .

There's also a patch, but unfortunately it's not merged so far.