Exception hierarchy in Python - Step-by-Step Execution

Concept Flow - Exception hierarchy

BaseException

↓

Exception

↓

ValueError

↓

ZeroDivisionError

Exceptions in Python form a tree starting from BaseException. Most errors inherit from Exception, which branches into specific error types.

Execution Sample

Python

try:
    x = 1 / 0
except ZeroDivisionError:
    print("Cannot divide by zero")

This code tries to divide by zero and catches the ZeroDivisionError to print a message.

Execution Table

Step	Action	Evaluation	Result
1	Execute 'x = 1 / 0'	1 / 0	Raises ZeroDivisionError
2	Catch exception	Is exception ZeroDivisionError?	Yes
3	Run except block	print message	Outputs: Cannot divide by zero
4	End try-except	No more code	Program continues normally

💡 Exception caught by ZeroDivisionError handler, program does not crash.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
x	undefined	exception raised, no assignment	undefined	undefined

Key Moments - 3 Insights

Why is x not assigned after the division by zero?

Because the exception occurs during the calculation, the assignment never happens (see execution_table step 1).

Why does the program not crash after the error?

The ZeroDivisionError is caught by the except block (execution_table step 2), so the program handles it gracefully.

What if we catch Exception instead of ZeroDivisionError?

Catching Exception would also catch ZeroDivisionError because it is a subclass (see concept_flow showing hierarchy).

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 1?

A x is assigned the value 0

B A ZeroDivisionError is raised

C The except block runs

D The program ends

💡 Hint

Check the 'Evaluation' and 'Result' columns in execution_table row 1.

According to variable_tracker, what is the value of x after step 2?

A 0

B 1

C undefined

D None

💡 Hint

See variable_tracker row for x after step 2.

If we replaced 'except ZeroDivisionError' with 'except Exception', what would happen?

A The exception would be caught and handled

B The program would crash

C The exception would not be caught

D A different exception would be raised

💡 Hint

Refer to concept_flow showing Exception is a parent of ZeroDivisionError.

Concept Snapshot

Python exceptions form a hierarchy starting at BaseException.
Most user errors inherit from Exception.
You catch exceptions using try-except blocks.
Catching a parent exception catches all its children.
Example: catching Exception catches ZeroDivisionError.
Use specific exceptions to handle errors precisely.

Full Transcript

In Python, errors are organized in a hierarchy starting from BaseException. Most common errors inherit from Exception. For example, ZeroDivisionError is a child of Exception. When code runs, if an error happens, Python looks for a matching except block to handle it. If you try to divide by zero, Python raises ZeroDivisionError. If you have a try-except block catching ZeroDivisionError, it will run the except code and prevent the program from crashing. Variables are not assigned if an exception happens during their assignment. Catching a parent exception like Exception will also catch its child exceptions like ZeroDivisionError. This helps you handle errors broadly or specifically.