Hi, I'm Daniel Greenfeld, and welcome to my blog. I write about Python, Django, and much more.

Attaching custom exceptions to functions and classes

Thursday, August 02, 2012 (permalink)

Having too many custom exceptions on a project can be a pain, but a few choices ones are really nice. The problem is that in complex libraries having to import both functions and exceptions becomes a drag. To mitigate having to remember to import custom exceptions, this is a handy pattern you can use in a project and can be done on both functions and classes.

Attaching a custom exception to a function

This works because Python functions are first-class objects. They can be passed around as things, and in this case, have things assigned to them.

# logic.py
class DoesNotCompute(Exception):
    """ Easy to understand naming conventions work best! """
    pass

def this_function(x):
    """ This function only works on numbers."""
    try:
        return x ** x
    except TypeError:
        raise DoesNotCompute

# Assign DoesNotCompute exception to this_function
this_function.DoesNotCompute = DoesNotCompute

Now I can import the function, and it won't just through DoesNotCompute exceptions, it will also carry the function along with the import:

>>> from logic import this_function
>>> this_function(5)
3125
>>> this_function(4.5)
869.8739233809259
>>> this_function('will throw an error.')
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "logic.py", line 10, in this_function
    raise DoesNotCompute
DoesNotCompute

Alright, that doesn't seem like much, but let's add in some exception handling:

>>> try:
...     this_function('is an example')
... except this_function.DoesNotCompute:
...     print('See what attaching custom exceptions to functions can do?')
...
...
See what attaching custom exceptions to functions can do?

Attaching the custom exception to a class

All we have to do is enhance our existing logic.py file by adding ThisClass:

# logic.py
class DoesNotCompute(Exception):
    """ Easy to understand naming conventions work best! """
    pass

# removed the function example for clarity

class ThisClass(object):
    # Since the DoesNotCompute exception exists, let's just assign it
    # as an attribute of ThisClass
    DoesNotCompute = DoesNotCompute

    def this_method(self, x):
        """ This method only works on numbers."""
        try:
            return x ** x
        except TypeError:
            raise DoesNotCompute

Now to demonstrate in the shell (Python REPL for the semantic purists):

>>> from t import ThisClass
>>> this_class = ThisClass()
>>> this_class.this_method(3.3)
51.415729444066585
>>> this_class.this_method('Jack Diederich warned against custom exceptions')
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "logic.py", line 24, in this_method
    raise DoesNotCompute
DoesNotCompute
>>> try:
...     this_class.this_method('I need to write a follow-up on my OAuth post')
... except ThisClass.DoesNotCompute:
...     print('Waiting to see how the OAuth stuff pans out')
...
...
Waiting to see how the OAuth stuff pans out

Admonition: Don't go crazy

Rather than use this trick all over the place, considering using it in a few places to powerful effect. For example, Django uses it only in a few places, and publicly only on MyModelClass.DoesNotExist and MyModelClass.MultipleObjectsReturned. By limiting Django's use of this technique, Django libraries are that much easier to comprehend. In this case, less complexity means more.

I say this because this pattern lends itself to creating custom exceptions to the point of effectively replacing Python's stock exceptions with your own. This makes for harder-to-maintain and harder-to-learn projects.

Not that I've ever done that. Ahem.


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