How to print the Python Exception/Error Hierarchy?

We will learn about what an exception is before learning how to print Python exceptions.

An exception occurs when a program fails to execute in the direction it was intended to be. Python throws exceptions when unexpected errors or events occur during program execution.

Exceptions can be both recoverable and unrecoverable. The exception handling technique can be used when you suspect your code may create an error, preventing the software from crashing.

Following are some common exceptions in Python ?

• IOError (Input Output Error) ? When the file cannot be opened or accessed.
• ImportError ? When Python cannot find or load a module
• ValueError ? Occurs when a function receives an argument of correct type but inappropriate value
• EOFError (End of File Error) ? Caused when the input() encounters an end-of-file condition without reading any data.

Python Exception/Error Hierarchy

Python's exception hierarchy consists of various built-in exceptions organized in a tree structure. This hierarchy uses inheritance concepts where all built-in exceptions are instances of classes derived from BaseException.

This exception hierarchy can be printed using Python's inspect module, which allows you to examine classes, functions, and perform type checking.

Using inspect.getclasstree() Function

The inspect.getclasstree() function displays the hierarchy of exception classes, showing their inheritance relationships.

Syntax

inspect.getclasstree(classes, unique=False)

Example: Displaying BaseException Hierarchy

Here's how to print the complete hierarchy starting from BaseException ?

import inspect

def print_exception_tree(cls, indent=0):
    print('-' * indent, cls.__name__)
    for subclass in cls.__subclasses__():
        print_exception_tree(subclass, indent + 3)

print("Built-in exception hierarchy:")
print_exception_tree(BaseException)

Built-in exception hierarchy:
 BaseException
--- Exception
------ TypeError
------ StopAsyncIteration  
------ StopIteration
------ ImportError
--------- ModuleNotFoundError
------ OSError
--------- FileNotFoundError
------ ValueError
------ RuntimeError
------ NameError
------ AttributeError
------ SyntaxError
------ LookupError
--------- IndexError
--------- KeyError
--- GeneratorExit
--- SystemExit
--- KeyboardInterrupt

Example: Displaying Exception Hierarchy Only

To display only the Exception class hierarchy (excluding system-level exceptions) ?

import inspect

def print_class_hierarchy(cls, indent=0):
    print('.' * indent, cls.__name__)
    for subcls in cls.__subclasses__():
        print_class_hierarchy(subcls, indent + 3)

print("Exception class hierarchy:")
print_class_hierarchy(Exception)

Exception class hierarchy:
 Exception
... TypeError
... StopAsyncIteration
... StopIteration
... ImportError
...... ModuleNotFoundError
... OSError
...... FileNotFoundError
... ValueError
... RuntimeError
... NameError
... AttributeError
... SyntaxError
... LookupError
...... IndexError
...... KeyError

Understanding the Hierarchy Structure

Practical Exception Handling Example

Here's how understanding the hierarchy helps in exception handling ?

def safe_division(a, b):
    try:
        result = a / b
        return result
    except ZeroDivisionError:
        print("Cannot divide by zero!")
    except TypeError:
        print("Please provide numeric values!")
    except Exception as e:
        print(f"An unexpected error occurred: {e}")

# Test different scenarios
safe_division(10, 2)
safe_division(10, 0)
safe_division(10, "abc")

Cannot divide by zero!
Please provide numeric values!

Conclusion

Understanding Python's exception hierarchy helps you write better error handling code. Use inspect module to explore the hierarchy and catch exceptions at the appropriate level for your application needs.