Exception hierarchy in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of exception handling with hierarchy

What is the output of this code snippet?

Python

class MyError(Exception):
    pass

class MySubError(MyError):
    pass

try:
    raise MySubError("Oops")
except MyError:
    print("Caught MyError")
except Exception:
    print("Caught Exception")

A No output

B Caught Exception

C RuntimeError

D Caught MyError

💡 Hint

Remember that catching a base exception also catches its subclasses.

MySubError is a subclass of MyError, so the first except block catches it and prints 'Caught MyError'.

❓ Predict Output

intermediate

Exception hierarchy and multiple except blocks

What will be printed when this code runs?

Python

try:
    raise KeyError('key')
except LookupError:
    print('Caught LookupError')
except KeyError:
    print('Caught KeyError')

A Caught LookupError

B Caught KeyError

C No output

D KeyError not caught

💡 Hint

Check the order of except blocks and exception inheritance.

KeyError is a subclass of LookupError. The first except block matches and runs, so 'Caught LookupError' is printed.

❓ Predict Output

advanced

Output of exception chaining with hierarchy

What is the output of this code?

Python

class BaseError(Exception):
    pass

class DerivedError(BaseError):
    pass

try:
    try:
        raise DerivedError('inner')
    except DerivedError as e:
        raise BaseError('outer') from e
except BaseError as e:
    print(type(e).__name__)
    print(type(e.__cause__).__name__)

A DerivedError\nDerivedError

B BaseError\nBaseError

C BaseError\nDerivedError

D DerivedError\nBaseError

💡 Hint

Look at the exception chaining with 'from' keyword.

The outer exception is BaseError, and its cause is the inner DerivedError exception.

❓ Predict Output

advanced

Exception hierarchy and catching order

What will this code print?

Python

class A(Exception):
    pass

class B(A):
    pass

class C(B):
    pass

try:
    raise C()
except B:
    print('Caught B')
except A:
    print('Caught A')

A Caught B

B Caught A

C No output

D RuntimeError

💡 Hint

The first matching except block is executed.

C is subclass of B, so except B catches it first and prints 'Caught B'.

🧠 Conceptual

expert

Identify the base class of all built-in exceptions

Which class is the base class for all built-in exceptions in Python?

A Exception

B BaseException

C StandardError

D Error

💡 Hint

This class is the root of the exception hierarchy and includes system-exiting exceptions.

BaseException is the root class for all exceptions, including system-exiting ones like SystemExit and KeyboardInterrupt.

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