Exceptions In Python
This is Day 15 of the #100DaysOfPython challenge.
This post will look at home exceptions are rescued and will demonstrate how to handle different exceptions raised.
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
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
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
We will create four cells to handle four parts of this mini project:
- A demonstration on how
- Looking at raising an error from a function.
- Creating and using custom errors.
We can use the
try/except statement to handle errors. You
Exception whenever you want to throw an error from a code block.
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')
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')
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')
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
class CustomError(Exception): """Custom error""" pass
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)
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.
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
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Exceptions In Python