Exception hierarchy in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the base class for all exceptions in Python?

The base class for all exceptions in Python is <code>BaseException</code>. All other exceptions inherit from it.

beginner

Which class do most user-defined exceptions inherit from?

Most user-defined exceptions inherit from the <code>Exception</code> class, which is a subclass of <code>BaseException</code>.

intermediate

What is the difference between Exception and BaseException?

<code>BaseException</code> is the root of the exception hierarchy. <code>Exception</code> is a subclass used for most errors that programs should catch. Some exceptions like <code>SystemExit</code> and <code>KeyboardInterrupt</code> inherit directly from <code>BaseException</code> and are usually not caught by normal code.

intermediate

Name two exceptions that inherit directly from BaseException and are usually not caught by normal exception handlers.

Two exceptions that inherit directly from BaseException are SystemExit and KeyboardInterrupt. They are used to exit programs or interrupt execution and are usually not caught by normal except Exception blocks.

beginner

How does the exception hierarchy help in writing exception handling code?

The exception hierarchy allows you to catch broad categories of errors by catching a parent class like <code>Exception</code>, or catch specific errors by catching subclasses. This helps write flexible and clear error handling.

Which class is the root of all exceptions in Python?

A Error

B Exception

C StandardError

D BaseException

BaseException is the root class for all exceptions in Python.

Which exception is NOT a subclass of Exception?

A ValueError

B IndexError

C KeyboardInterrupt

D TypeError

KeyboardInterrupt inherits directly from BaseException, not Exception.

If you want to catch most errors in your program, which class should you catch?

A Exception

B KeyboardInterrupt

C SystemExit

D BaseException

Catching Exception will catch most program errors but not system-exiting exceptions.

Which exception is raised when a program is asked to exit?

A SystemExit

B RuntimeError

C ValueError

D Exception

SystemExit is raised when the program is asked to exit.

Why should you avoid catching BaseException directly?

A It does not catch any exceptions.

B It catches all exceptions including system-exiting ones, which can cause problems.

C It only catches syntax errors.

D It is deprecated.

Catching BaseException catches all exceptions including SystemExit and KeyboardInterrupt, which can interfere with program exit or interruption.

Explain the Python exception hierarchy starting from BaseException.

Think about which exceptions are caught normally and which are special.

Why is it important to understand the exception hierarchy when writing try-except blocks?

Consider how catching a parent class affects what errors you handle.

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