Exceptions In Python

This post will look at home exceptions are rescued and will demonstrate how to handle different exceptions raised.

Prerequisites

Getting started

Let's create the hello-python-exceptions directory and install Pillow.

# Make the `hello-python-exceptions` directory
$ mkdir hello-python-exceptions
$ cd hello-python-exceptions

# Init the virtual environment
$ pipenv --three
$ pipenv install --dev jupyterlab
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Now we can start up the notebook server.

# Startup the notebook server
$ pipenv run jupyter-lab
# ... Server is now running on http://localhost:8888/lab
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The server will now be up and running.

Creating the notebook

Once on http://localhost:8888/lab, select to create a new Python 3 notebook from the launcher.

Ensure that this notebook is saved in hello-python-exceptions/docs/<your-file-name>.

We will create four cells to handle four parts of this mini project:

Try/Except

We can use the try/except statement to handle errors. You raise an Exception whenever you want to throw an error from a code block.

The Exception is self can then be delegated higher up the code chain to be handled.

For example, if we execute this in our first JupyterLab code block:

try:
  raise Exception('You shall not pass')
  print('Success')
except:
  print('There was an issue')
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We notice the output is There was an issue. We do not make it passed the raised exception.

We can also capture exceptions by their type. For example, if we execute this in our second JupyterLab code block:

try:
  raise Exception('You shall not pass')
  print('Success')
except Exception:
  print('Did not make it to final catch-all block')
except:
  print('There was an issue')
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You will notice that 'Did not make it to final catch-all block' was captured and printed, where as the final except code block is used a capture all.

Raising an error from a function

Defining a function that raises an error will traverse up the code block to the top level.

def example_exception():
  raise Exception('You shall not pass')

def example_fn():
  example_exception()

try:
  example_fn()
except:
  print('There was an issue')
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This also prints out 'There was an issue'.

Creating and using custom errors

We can create custom errors simply by defining a class that extends Exception.

class CustomError(Exception):
    """Custom error"""
    pass
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We can see this in action working randomly when we use the randrange function as a helper to determine which error to raise:

from random import randrange

# define Python user-defined exceptions
class CustomError(Exception):
    """Raised when using a custom error"""
    pass

def example_exception():
  val = randrange(10)
  if val <= 5:
    raise CustomError('You shall not pass')
  else:
    raise Exception("There was an error")

try:
  example_exception()
except CustomError as e:
  print(e)
except Exception as e:
  print(e)
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Running this code in a block will print out 'You shall not pass' or 'There was an error' based on the random value.

That means that our CustomError is being handled in the except CustomError as e block.

Summary

Today's post demonstrated how to create custom errors by extending Exception and demonstrating how errors are raised in a simple manner.

Managing errors in Python is a necessity when working with more complex code that requires more fine-grained control over the possible error outcomes.

Resources and further reading

Photo credit: freestocks