raising (custom) exceptions in python

2020-06-25

~2 min read

281 words

If a Python program receives values it cannot handle, it can abort by raising an exception.

Most of errors in Python are built on the Exception class.

Here’s the beginning of the manual page for Exception:

>>> help(Exception)
class Exception(BaseException)
 |  Common base class for all non-exit exceptions.
 |
 |  Method resolution order:
 |      Exception
 |      BaseException
 |      object
 |
 |  Built-in subclasses:
 |      ArithmeticError
 |      AssertionError
 |      AttributeError
 |      BufferError
 |      ... and 15 other subclasses
 |
 |  Methods defined here:
 |
 |  __init__(self, /, *args, **kwargs)
 |      Initialize self.  See help(type(self)) for accurate signature.

The nice thing about this is it suggests the way forward for raising our own, custom exceptions!

We can create custom exceptions in Python by extending the Exception class.

For example, Goldilocks likes her porridge just right, not too hot and not too cold.

Let’s see how we might model that with some custom exceptions:

custom_exception.py

class TooHotError(Exception):
    """Raised when the temperature is too hot"""
    pass


class TooColdError(Exception):
    """Raised when the temperature is too cold"""
    pass


def porridge(temp):
  if temp < 90:
    raise TooColdError(f"Sorry, at {temp} degrees, the porridge is too cold for Goldilocks!")
  elif temp > 120:
    raise TooHotError(f"Sorry, at {temp} degrees, the porridge is too hot for Goldilocks!")
  return "Ahh, just right!"

porridge(80)
porridge(150)
porridge(110)

Refactoring porridge to a try catch block may be more idiomatic:

custom_exception.py

# ...

def porridge(temp):
    try:
        if temp < 90:
            raise TooColdError
        elif temp > 120:
            raise TooHotError
        return print("Ahh, just right!")
    except TooColdError:
        print(
            f"Sorry, at {temp} degrees, the porridge is too cold for Goldilocks!")
    except TooHotError:
        print(
            f"Sorry, at {temp} degrees, the porridge is too hot for Goldilocks!")


porridge(80)
porridge(110)
porridge(150)

Asserting Errors In Pytest

Hi there and thanks for reading! My name's Stephen. I live in Chicago with my wife, Kate, and dog, Finn. Want more? See about and get in touch!