How to Get the Most out of Your Python Exceptions

Sun 03 April 2016

var = input('Please enter a number not equal to 0: ')
try:
    var = 1/float(var)
except ZeroDivisionError as e:
    print('Error: ' + str(e))
    print('You had one job!')

Warning

This post does not cover what python exceptions are. If the above code doesn't make sense you should check out this Introduction to Python Exceptions from wiki.python.org before reading this post.

If you've fallen in love with Python you've no doubt discovered exceptions:

try:
    # do a thing
except:
    # thing did not work,
    # do something else

They are endlessly useful and help one to avoid writing checks upon checks upon checks before getting to the meat of your project. Just try a thing, catch the error, keep on rolling (or fail gracefully).

As powerful as they can be, I have found a lot of folks (past me included) who don't know how to find python exceptions and don't know how to write their own exceptions! So let's do that.

Finding Exceptions

When writing exception handling code it's kosher to explicitly state which error you expect.

# Bad:
try:
    # A thing that might not work
except:
    # Something else

# Good:
try:
    # A thing that might not work
except SpecificError as err:
    # Maybe print(err)
    # Something else

When I first found this out I thought Golly that does sound useful; I always try to be explicit in my error handling -- but how?? Thankfully future me is here to answer questions like this.

The exception you are looking for (for instance, SpecificError in the above pseudo-code) can be found in the python traceback:

$ echo "open('myfakefile.txt', 'r').close()" > my-unhandled-script.py
$ python my-unhandled-script.py
Traceback (most recent call last):
  File "/tmp/test.py", line 1, in <module>
    open('myfakefile.txt', 'r').close()
IOError: [Errno 2] No such file or directory: 'myfakefile.txt'

That bit on the last line IOError is the exception you're looking for. So when you write your code you'll say something like the following:

try:
    open('myfakefile.txt', 'r').close()
except IOError as e:
    print(e)
    print('File `myfakefile.txt` does not exist')

To recap, here is one way (and my preferred method for) 'doing' python exception handling:

Write breakable code.
Run breakable code, see what exceptions python spits out.
Wrap breakable code in explicit try/except blocks.
???
Profit.

Writing Your Own Exceptions

You (who me?), yes you can write custom python exceptions. It's dead easy too.

The long and short of it is you define an exception class which either inherits from the Exception class or another pre-existing exception.

class CusssstomError(Exception):
    '''Raise when snakes'''
    def __init__(self, message):
        self.message = message

def check_for_snakes(foo):
    if 'snake' in foo.lower():
        raise CusssstomError('Snakes! I hate snakes!')

try:
    foo = input("Just don't mention snakes: ")
    check_for_snakes(foo)
except CusssstomError as e:
    print(e.message)
else:
    print("Thanks. I appreciate it.")

The above code defines the CusssstomError exception class which you can raise. Very neat and pythonic

How to Get the Most out of Your Python Exceptions

Finding Exceptions

Writing Your Own Exceptions

Further Reading