Exception hierarchy in Python - Mini Project: Build & Apply

Exception hierarchy

📖 Scenario: Imagine you are building a simple program that handles different types of errors when processing user input. You want to organize these errors in a clear way using Python's exception hierarchy.

🎯 Goal: You will create a base exception class and two specific exception classes that inherit from it. Then you will write code to raise and catch these exceptions properly.

📋 What You'll Learn

Create a base exception class called InputError that inherits from Exception.

Create two exception classes called ValueTooSmallError and ValueTooLargeError that inherit from InputError.

Write code to raise ValueTooSmallError if a number is less than 10.

Write code to raise ValueTooLargeError if a number is greater than 100.

Use a try-except block to catch these exceptions and print a message.

💡 Why This Matters

🌍 Real World

Organizing errors in a program helps keep code clean and makes it easier to find and fix problems.

💼 Career

Understanding exception hierarchy is important for writing robust software that handles errors gracefully in professional development.

1

Create the base exception class

Create a class called InputError that inherits from Exception.

Python

# Your code here

Hint

Use class InputError(Exception): and add pass inside.

2

Create specific exception classes

Create two classes called ValueTooSmallError and ValueTooLargeError that both inherit from InputError.

Python

class InputError(Exception):
    pass

# Your code here

Hint

Define each class with class ClassName(InputError): and add pass inside.

3

Raise exceptions based on number value

Create a variable called number and set it to 5. Then write code to raise ValueTooSmallError if number is less than 10, and raise ValueTooLargeError if number is greater than 100.

Python

class InputError(Exception):
    pass

class ValueTooSmallError(InputError):
    pass

class ValueTooLargeError(InputError):
    pass

number = 5
# Your code here

Hint

Use if number < 10: and raise ValueTooSmallError("message"). Use elif number > 100: and raise ValueTooLargeError("message").

4

Catch and handle the exceptions

Use a try-except block to catch ValueTooSmallError and ValueTooLargeError. Print "Caught too small error" if ValueTooSmallError is caught, and print "Caught too large error" if ValueTooLargeError is caught.

Python

class InputError(Exception):
    pass

class ValueTooSmallError(InputError):
    pass

class ValueTooLargeError(InputError):
    pass

number = 5
try:
    if number  100:
        raise ValueTooLargeError("The value is too large")
except ValueTooSmallError:
    # Your code here
except ValueTooLargeError:
    # Your code here

Hint

Use try: block around the code that raises exceptions. Use except ValueTooSmallError: and except ValueTooLargeError: to catch and print messages.

Practice

1. Which of the following is the base class for all built-in exceptions in Python?

easy

A. Exception

B. BaseException

C. Error

D. RuntimeError

Solution

1. Step 1: Understand Python's exception hierarchy

   All exceptions in Python inherit from BaseException, which is the root of the hierarchy.

2. Step 2: Identify the base class

   Exception inherits from BaseException, but BaseException is the top-level base class.

3. Final Answer:

   BaseException -> Option B

4. Quick Check:

   BaseException is the root of all exceptions [OK]

Hint: Remember: BaseException is the root of all exceptions [OK]

Common Mistakes:

• Confusing Exception with BaseException
• Thinking Error is a built-in base class
• Choosing RuntimeError as base

2. Which of the following is the correct syntax to catch all exceptions except system-exiting ones?

easy

A. except SystemExit:

B. except BaseException:

C. except Exception:

D. except RuntimeError:

Solution

1. Step 1: Recall exception hierarchy for catching exceptions

   Catching Exception catches most errors but excludes system-exiting exceptions like SystemExit and KeyboardInterrupt.

2. Step 2: Identify correct syntax

   except Exception: is the standard way to catch all regular exceptions safely.

3. Final Answer:

   except Exception: -> Option C

4. Quick Check:

   Use Exception to catch all but system-exiting exceptions [OK]

Hint: Use except Exception to avoid catching system-exit errors [OK]

Common Mistakes:

• Using except BaseException catches system exit too
• Catching only RuntimeError misses many exceptions
• Using except SystemExit catches only exit exceptions

3. What will be the output of this code?

try:
    x = 1 / 0
except ArithmeticError:
    print('ArithmeticError caught')
except ZeroDivisionError:
    print('ZeroDivisionError caught')

medium

A. ArithmeticError caught

B. ZeroDivisionError caught

C. No output, program crashes

D. SyntaxError

Solution

1. Step 1: Understand exception hierarchy for ZeroDivisionError

   ZeroDivisionError is a subclass of ArithmeticError.

2. Step 2: Check which except block matches first

   Since ArithmeticError comes before ZeroDivisionError, the first except block catches the exception.

3. Final Answer:

   ArithmeticError caught -> Option A

4. Quick Check:

   Parent exception catches before child [OK]

Hint: Parent exceptions catch before child exceptions in order [OK]

Common Mistakes:

• Expecting ZeroDivisionError block to run first
• Thinking exception order does not matter
• Assuming program crashes without handling

4. Find the error in this code snippet:

try:
    open('file.txt')
except IOError:
    print('File error')
except FileNotFoundError:
    print('File not found')

medium

A. open() needs a mode argument

B. IOError should be replaced with Exception

C. No error, code is correct

D. FileNotFoundError should come before IOError

Solution

1. Step 1: Understand exception hierarchy between IOError and FileNotFoundError

   FileNotFoundError is a subclass of IOError.

2. Step 2: Check order of except blocks

   The more specific exception (FileNotFoundError) must come before the more general (IOError) to avoid unreachable code.

3. Final Answer:

   FileNotFoundError should come before IOError -> Option D

4. Quick Check:

   Specific exceptions must precede general ones [OK]

Hint: Place child exceptions before parent exceptions in except blocks [OK]

Common Mistakes:

• Putting general exceptions before specific ones
• Assuming IOError and FileNotFoundError are unrelated
• Thinking open() requires mode argument always

5. You want to catch all exceptions except KeyboardInterrupt and SystemExit. Which is the best way to write the except block?

hard

A. except Exception:

B. except BaseException:

C. except (KeyboardInterrupt, SystemExit):

D. except (Exception, KeyboardInterrupt, SystemExit):

Solution

1. Step 1: Recall exception hierarchy for KeyboardInterrupt and SystemExit

   Both KeyboardInterrupt and SystemExit inherit directly from BaseException, not Exception.

2. Step 2: Choose except block that excludes these exceptions

   except Exception: catches all exceptions except KeyboardInterrupt and SystemExit, which is the desired behavior.

3. Final Answer:

   except Exception: -> Option A

4. Quick Check:

   Exception excludes system-exiting exceptions [OK]

Hint: Use except Exception to exclude system-exiting exceptions [OK]

Common Mistakes:

• Using except BaseException catches everything including system exit
• Catching KeyboardInterrupt and SystemExit explicitly when not needed
• Combining Exception with KeyboardInterrupt in except tuple