Python’s Exception Group

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Well, let me tell ya, it’s like having a party for all your errors to hang out and share their stories. Except instead of snacks and drinks, they get to be part of a fancy traceback with notes attached to them!

Here’s how you can create an exception group:

# Importing the necessary module
from collections import abc
# Importing the sys module for system-specific parameters and functions
import sys

# Defining a class called ExceptionGroup that inherits from the Iterable class in the abc module
class ExceptionGroup(abc.Iterable):
    # Defining the constructor method with two parameters: message and exc_list
    def __init__(self, message, exc_list=None):
        # Assigning the message parameter to the instance variable 'message'
        self.message = message
        # Checking if the exc_list parameter is a list and has at least one element
        if not isinstance(exc_list, list) or len(exc_list) < 1:
            # If not, raise a TypeError with a message
            raise TypeError('exc_list must be a non-empty list')
        # Assigning the exc_list parameter to the instance variable '_excs'
        self._excs = exc_list
        
    # Defining the __str__ method to return a string representation of the ExceptionGroup object
    def __str__(self):
        # Using f-strings to format the message and the number of sub-exceptions in the ExceptionGroup
        return f'{self.message} ({len(self)} sub-exceptions)'
    
    # Defining the __repr__ method to return a string representation of the ExceptionGroup object
    def __repr__(self):
        # Using f-strings to format the message and the representation of the exc_list using the sys module's reprlib function
        return f"ExceptionGroup('{self.message}', {sys.reprlib.representation(self._excs)})"
    
    # Defining the __iter__ method to make the ExceptionGroup object iterable
    def __iter__(self):
        # Using a for loop to iterate through the exc_list and yield each exception
        for exc in self._excs:
            yield exc

This class is a subclass of `abc.Iterable`, which means it can be used as an iterable object and has the same methods like list, tuple or set.

To create an exception group, you need to pass in two arguments: message (the main error message) and exc_list (a list of sub-exceptions). The `__init__` method checks if the input is valid and raises a TypeError if it’s not.

Now let’s see how we can use this exception group in our code:

# This function is used to handle exceptions that may occur within the code
def my_function():
    try:
        # some code that might raise an error
        
    # The except block is used to catch any exceptions that may occur and assign it to the variable 'e'
    except Exception as e:
        # A list of sub-exceptions is created and assigned to the variable 'excs'
        excs = [OSError('error 1'), SystemError('error 2')]
        # An exception group is raised with a main error message and the list of sub-exceptions
        raise ExceptionGroup('there were problems', excs)

In this example, we’re raising a custom exception group instead of the regular `Exception`. The traceback will show us all the sub-exceptions and their notes.

You can also add notes to each individual error using the `add_note()` method:

# This function is used to handle potential errors that may occur within the code
def my_function():
    try:
        # some code that might raise an error
        
    # The "as" keyword is used to assign the exception object to the variable "e"
    except Exception as e:
        # A list of custom exceptions is created, each with a specific error message
        excs = [OSError('error 1'), SystemError('error 2')]
        
        # The enumerate() function is used to loop through the list of exceptions and assign an index to each one
        for i, exc in enumerate(excs):
            # The add_note() method is used to add a note to each individual exception, specifying the iteration number
            exc.add_note(f'Happened in Iteration {i+1}')
        
        # The ExceptionGroup class is used to group all the exceptions together and raise them as one
        raise ExceptionGroup('there were problems', excs)

This will add a note to each sub-exception with the iteration number where it occurred.

And if you want to handle only certain types of exceptions within an exception group, you can use `except*` instead of `except`. This allows you to selectively catch specific errors while letting others propagate:

# This script is used to handle exceptions in Python code and add annotations to each sub-exception.

# Define a function called my_function
def my_function():
    try:
        # some code that might raise an error
        
    # Use "except" to catch any exceptions that occur within the code
    except ExceptionGroup as e:
        # Use a for loop to iterate through each sub-exception in the ExceptionGroup
        for exc in e.excs:
            # Use "isinstance" to check if the sub-exception is of type OSError
            if isinstance(exc, OSError):
                # Print a message indicating a file-related issue has occurred
                print('Oops! We have a file-related issue')
    
    # Use "except*" to selectively catch specific types of exceptions within an exception group
    except* SystemError:
        # Handle system errors here
        pass # Placeholder for handling system errors

In this example, we’re catching only `ExceptionGroup` and its sub-exceptions that are instances of `OSError`. Any other type of error will be caught by the next clause with `except* SystemError`.

Python’s exception group is a powerful tool to handle complex errors in your code. With its ability to add notes and selectively catch specific types, you can create more informative tracebacks that help you debug issues faster.

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