Python, distinguishing custom exceptions

Question

fairly new to Python here. Have this code:

def someFunction( num ):
    if num < 0:
        raise Exception("Negative Number!")
    elif num > 1000:
        raise Exception("Big Number!")
    else:
        print "Tests passed"


try:
    someFunction(10000)
except Exception:
    print "This was a negative number but we didn't crash"
except Exception:
    print "This was a big number but we didn't crash"
else:
    print "All tests passed and we didn't crash"

I originally used raise "Negative Number!" etc but quickly discovered that this was the old way of doing things and you have to call the Exception class. Now it's working fine but how do I distinguish between my two exceptions? For the code below it's printing "This was a negative number but we didn't crash". Any pointers on this would be great. Thanks!

See [the docs](https://docs.python.org/2/tutorial/errors.html). — erip, Sep 11 '15 at 12:07
You wouldn't normally raise `Exception`, but some subclass of it. — Peter Wood, Sep 11 '15 at 12:11

hiro protagonist · Accepted Answer · 2015-09-11T12:31:40.647

11

you need to create your own exception classes if you want to be able to distinguish the kind of exception that happened. example (i inherited from ValueError as i think this is the closest to what you want - it also allows you to just catch ValueError should the distinction not matter):

class NegativeError(ValueError):
    pass

class BigNumberError(ValueError):
    pass

def someFunction( num ):
    if num < 0:
        raise NegativeError("Negative Number!")
    elif num > 1000:
        raise BigNumberError("Big Number!")
    else:
        print "Tests passed"

try:
    someFunction(10000)
except NegativeError as e:
    print "This was a negative number but we didn't crash"
    print e
except BigNumberError as e:
    print "This was a big number but we didn't crash"
    print e
else:
    print "All tests passed and we didn't crash"

edited Sep 11 '15 at 12:31

answered Sep 11 '15 at 12:06

hiro protagonist

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4

[Use `except ... as ...` instead of `except ..., ...`](http://stackoverflow.com/a/2535770/2846140) – Vincent Sep 11 '15 at 12:11
Thanks for this, great solution. Is NegativeError as e equivalent to NegativeError, e ? – Kex Sep 11 '15 at 12:18
@Kex, `NegativeError as e` will work for python2 and 3 – Padraic Cunningham Sep 11 '15 at 12:24
in python 2.7 it is (i think...); python 3 will only accept the syntax with `as`. – hiro protagonist Sep 11 '15 at 12:26

Python, distinguishing custom exceptions

1 Answers1