95

I want to catch a specific ValueError, not just any ValueError.
I tried something like this:

try: maquina['WPF'] = macdat(ibus, id, 'WPF')
except: ValueError, 'For STRING = ’WPF’, this machine is not a wind machine.':
    pass

But it raises a SyntaxError: can't assign to literal.
Then I tried:

try: maquina['WPF'] = macdat(ibus, id, 'WPF')
except ValueError, e:
    if e != 'For STRING = ’WPF’, this machine is not a wind machine.':
        raise ValueError, e

But it raises the exception, even if it is the one I want to avoid.

Paul P
  • 3,346
  • 2
  • 12
  • 26
José
  • 1,582
  • 5
  • 15
  • 22
  • 8
    You got your answer below, but when you re-raise the exception just use `raise` on its own: that way if the exception isn't handled you still get the original stack backtrace instead of one that starts where it was re-raised. – Duncan Nov 23 '12 at 14:48

3 Answers3

119

in except ValueError,e, e is an instance of the exception, not a string. So when you test if e is not equal to a particular string, that test is always False. Try:

if str(e) != "..."

instead.

Example:

def catch(msg):
    try:
        raise ValueError(msg)
    except ValueError as e:  # as e syntax added in ~python2.5
        if str(e) != "foo":
            raise
        else:
            print("caught!")

catch("foo")
catch("bar")

Typically, you don't really want to rely on the error message if you can help it -- It's a little too fragile. If you have control over the callable macdat, instead of raising a ValueError in macdat, you could raise a custom exception which inherits from ValueError:

class MyValueError(ValueError): pass

Then you can only catch MyValueError and let other ValueErrors continue on their way to be caught by something else (or not). Simple except ValueError will still catch this type of exception as well so it should behave the same in other code which might also be catching ValueErrors from this function.

mgilson
  • 300,191
  • 65
  • 633
  • 696
  • 1
    It's what I want. Thanks for your help and suggestions. – José Nov 23 '12 at 15:39
  • 3
    The `Exception.message` attribute is gone in Python 3, could you update your answer to reflect that and suggest an alternative (for example, `str(e)`)? This question is a good canonical duplicate for this sort of questions – vaultah Oct 04 '17 at 13:22
  • 1
    Is there some canonical way of handling specific built-in exceptions, like wrong number of positional arguments TypeError, without relying on the message? – Todor Markov Feb 27 '19 at 10:16
  • @TodorMarkov -- Not really. I think that the argument for that one is that it is clearly a programming bug and probably _shouldn't_ be handled in *most* cases. – mgilson Feb 28 '19 at 16:18
5

The method for the last one is correct (but print repr(e) to see why it doesn't work).

However, if you want the exception information to be correct, you should not raise a new exception (as you do now), but raise the same one. Otherwise more code catching it, or the error message if it isn't caught, will show your code as the source, while it should be the original source.

To do this, use raise without an argument (within the except block, of course, otherwise there is no "current" exception).

Bas Wijnen
  • 1,288
  • 1
  • 8
  • 17
3

You can use: type(e) and e.args for this. It returns a tuple, match the tuple with your own.

Siddharth Mishra
  • 124
  • 5