Having too many custom exceptions on a project can be a pain, but a few choices are 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.
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("Function does not compute") # Assign DoesNotCompute exception to this_function this_function.DoesNotCompute = DoesNotCompute
Now I can import the function, and it won't just throw
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?
All we have to do is enhance our existing
logic.py file by adding
# logic.py class DoesNotCompute(Exception): """ Easy to understand naming conventions work best! """ pass # removed the function example for clarity class ThisClass: # Assign DoesNotCompute to this class DoesNotCompute = DoesNotCompute def this_method(self, x): """ This method only works on numbers.""" try: return x ** x except TypeError: raise self.DoesNotCompute("Class does not compute")
Now to demonstrate in the shell (Python REPL for the semantic purists):
>>> from logic import ThisClass >>> this_class = ThisClass() >>> this_class.this_method(3.3) 51.415729444066585 >>> this_class.this_method("Don't create too many 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 like ice cream') ... except ThisClass.DoesNotCompute: ... print('Milk, ice, and fruit in a blender is nice too') ... ... Milk, ice, and fruit in a blender is nice too
Rather than use this trick all over the place, considering using it in a few strategic places to powerful effect. For example, Django uses it only in a few places, and publicly only on
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.
- Updated 2020/04/20: Added missing
ThisClassthanks to Chiheb Nexus Lad.