Python not catching exception

Question

For some reason my code is not catching an exception when I throw it. I have

def trim_rad(rad): ... if not modrad.shape[0]: raise IndexError("Couldn't find main chunk") return modrad, thetas

Then later I call that function:

try: modrad, thetas = trim_rad(rad) except IndexError("Couldn't find main chunk"): return 0

Yet I still get a traceback with that exception. What am I doing wrong?

You're only supposed to put the type name there, not an instance of the type. If you want to further narrow it down to handle only a special IndexError that you will be throwing and leave regular ole IndexErrors alone, create a narrower exception type that inherits from IndexError and raise/catch that. — user1726343
– user1726343, Commented Aug 15, 2013 at 18:20
I'm surprised this doesn't TypeError. I thought I tried except 1: before and got a TypeError, but trying it now, it just passes through. — user2357112
– user2357112, Commented Aug 15, 2013 at 18:23
@user2357112 Evidently it does cause a TypeError in Python 3: ideone.com/yy4XAy. You're right though, it doesn't matter in 2.x — user1726343
– user1726343, Commented Aug 15, 2013 at 18:28
@Asad It's weird that there wasn't any warning that I was doing it wrong. So if I want to catch that specific error and not other IndexErrors, is it more standard to subclass IndexError, or parse the message and re-raise the error if it's not the specified message? I only use the error in this instance, would making a new subclass be a bit too heavy duty? — ari
– ari, Commented Aug 15, 2013 at 19:48
@ari It is better to subclass IndexErrors (IMO at least), but it is entirely up to you. Both approaches would work, although using string comparison for crucial decision making and control flow always feels gross to me. I'm no Python expert, so YMMV. — user1726343
– user1726343, Commented Aug 15, 2013 at 19:52

Sukrit Kalra · Accepted Answer · 2013-08-15 18:19:30Z

Catch just the IndexError.

try: raise IndexError('abc') except IndexError('abc'): print 'a' Traceback (most recent call last): File "<pyshell#22>", line 2, in <module> raise IndexError('abc') IndexError: abc try: raise IndexError('abc') except IndexError: print 'a' a # Output

So, reduce your code to

try: modrad, thetas = trim_rad(rad) except IndexError: return 0

If you want to catch the error message too, use the following syntax:

try: raise IndexError('abc') except IndexError as err: print err abc

score 11 · Accepted Answer · 2019-10-12 00:30:24Z

You gave except an instance of an IndexError. Do this instead:

try: modrad, thetas = trim_rad(rad) except IndexError: print "Couldn't find main chunk" return 0

Here is an example:

>>> try: ... [1][1] ... except IndexError('no'): ... pass ... Traceback (most recent call last): File "<stdin>", line 2, in <module> IndexError: list index out of range >>> try: ... [1][1] ... except IndexError: ... pass ... >>>

Joohwan · Accepted Answer · 2013-08-15 18:22:29Z

3

change

except IndexError("Couldn't find main chunk"):

to

except IndexError:

edited Aug 15, 2013 at 18:22

user2555451

answered Aug 15, 2013 at 18:20

Joohwan

2,5421 gold badge23 silver badges31 bronze badges

Comments

Alex · Accepted Answer · 2013-08-15 18:45:15Z

You seem to be catching the exception wrong. You catch exceptions of type, the notation below will assign the exception to e so you can read the description in your except handler.

try: modrad, thetas = trim_rad(rad) except IndexError as e: print e.message return 0

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