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Exception hierarchy

Learn ValueError, TypeError, KeyError and others.

Exception hierarchy

In Python all errors form a tree. The root is BaseException; Exception is its subclass. Common errors like ValueError, TypeError and KeyError are subclasses of Exception:

BaseException
├── Exception
│   ├── ValueError
│   ├── TypeError
│   └── KeyError
└── KeyboardInterrupt

except Exception catches all program errors but not system errors like KeyboardInterrupt. This is safer than a bare except:.

Knowing the hierarchy helps you write the right except block.

Multiple except blocks

After a single try block you can write multiple except blocks. Python reads top to bottom and runs the first match:

try:
    value = int("abc")
except ValueError:
    print("değer hatası")
except TypeError:
    print("tür hatası")
except Exception as e:
    print(f"diğer: {e}")

With as e you bind the error object to a variable and access its message. Always put the most general one (Exception) last.

This lets you react differently to different error types.

Practice Catch value error with multiple except

Inside a try block write value = int("abc"). Inside an except ValueError: block write print("değer hatası"). Inside an except Exception as e: block write print(f"diğer: {e}").

Starter code

main.py
# open a try block
# catch ValueError and Exception in separate except blocks

Sample solution

main.py
try:
    value = int("abc")
except ValueError:
    print("değer hatası")
except Exception as e:
    print(f"diğer: {e}")

Expected output

değer hatası

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which is NOT a subclass of Exception?

  • KeyboardInterrupt Correct answer
  • TypeError
  • KeyError
  • ValueError

Explanation: KeyboardInterrupt is a direct subclass of BaseException, not Exception.

2 Match the hierarchy

Term Meaning
BaseException Root class
Exception Program errors
ValueError Subclass
as e Binds the error object

3 Complete the except as e pattern

Question 
try:
    value = int("abc")
____ as e:
    print(f"hata: {e}")
Answer 
try:
    value = int("abc")
except Exception as e:
    print(f"hata: {e}")

Explanation: except Exception as e catches all program errors and binds them to e.

All lessons in Beginner

1 Python basics

  1. What is Python?
  2. REPL and print
  3. Comments
  4. Indentation blocks
  5. python -c and scripts
  6. Summary - Python basics

2 Variables

  1. Variable assignment
  2. Multiple assignment
  3. Naming variables
  4. id and type
  5. Swapping variables
  6. Summary - Variables

3 Data types

  1. int and float
  2. bool
  3. str basics
  4. None
  5. Type conversion
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add/subtract
  3. Multiply/divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic
  2. Comparison
  3. Logical
  4. Bitwise
  5. Membership and identity
  6. Summary - Operators

6 String basics

  1. f-string
  2. Concatenation
  3. str.format
  4. Raw strings
  5. Triple-quote
  6. Summary - String basics

7 Conditionals

  1. The if statement
  2. elif and else
  3. Nested if
  4. Truthiness
  5. Conditional expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. range
  3. The while loop
  4. break and continue
  5. Loop else
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Updating quantity
  5. Putting it together

10 Lists

  1. Declaring lists
  2. Index and slice
  3. List methods
  4. in and count
  5. Sorting lists
  6. Summary - Lists

11 List comprehension

  1. Comprehension basics
  2. Filtered comprehension
  3. Dict and set comprehension
  4. Nested comprehension
  5. Generator expression
  6. Summary - List comprehension

12 Tuple, set and dict

  1. Tuple
  2. Set
  3. Dict basics
  4. Dict methods
  5. Dict comprehension
  6. Summary - Tuple, set and dict

13 Functions

  1. def
  2. return
  3. Docstring
  4. Default parameters
  5. *args and **kwargs
  6. Summary - Functions

14 Function parameters

  1. Positional parameters
  2. Keyword parameters
  3. Keyword-only
  4. Positional-only
  5. Type hints basics
  6. Summary - Function parameters

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string methods

  1. split and join
  2. strip, lstrip, rstrip
  3. replace
  4. find and index
  5. startswith, endswith
  6. Summary - Advanced strings

17 Input and output

  1. input()
  2. int/float conversion
  3. print parameters
  4. f-string formatting
  5. sys.argv
  6. Summary - Input and output

18 Error handling

  1. try/except
  2. else and finally
  3. raise
  4. Exception hierarchy
  5. Custom exceptions
  6. Summary - Error handling

19 Modules

  1. import
  2. from import
  3. as alias
  4. __name__ == '__main__'
  5. Standard library
  6. Summary - Modules

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. Error handling
  5. Putting it together