Types of Exceptions0%
Try-Except-Else-Finally

Types of Exceptions

Beginner12 min read•Updated: Jul 11, 2026
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Types of Exceptions in Python

Errors in Python fall into two broad categories: Syntax Errors (detected by the parser before code execution begins) and Exceptions (anomalies detected during execution at runtime). When an exception is not intercepted, Python halts program execution and displays a traceback.

1. Syntax Errors vs. Runtime Exceptions

Python
# 1. SYNTAX ERROR: Discovered before runtime (parsing phase)
# if True print("Hello") <-- Missing colon ':'
 
# 2. RUNTIME EXCEPTION: Syntactically valid, but fails during execution
denominator = 0
# result = 100 / denominator <-- ZeroDivisionError at runtime

2. The Python Built-in Exception Hierarchy

All built-in exceptions form an inheritance tree rooted at BaseException. Understanding this hierarchy prevents bugs where catching a broad exception inadvertently intercepts system-level termination signals.

Output
BaseException
├── SystemExit # Raised by sys.exit()
├── KeyboardInterrupt # Triggered when user presses Ctrl+C
├── GeneratorExit # Raised when generator close() is called
└── Exception # Root for all non-system-exiting exceptions
├── ArithmeticError
│ ├── ZeroDivisionError
│ ├── OverflowError
│ └── FloatingPointError
├── LookupError
│ ├── IndexError
│ └── KeyError
├── TypeError
├── ValueError
├── AttributeError
├── NameError
│ └── UnboundLocalError
├── ImportError
│ └── ModuleNotFoundError
└── OSError
├── FileNotFoundError
└── PermissionError
Critical Rule: Never write except BaseException:. Doing so catches KeyboardInterrupt and SystemExit, making it impossible to stop your program using Ctrl+C.

3. Deep Dive into Common Exceptions

1. TypeError vs. ValueError

  • TypeError: An operation or function is applied to an object of inappropriate type.
Python
# TypeError: can only concatenate str (not "int") to str
greeting = "Level: " + 5
  • ValueError: The type is correct, but the content / value is invalid.
Python
# ValueError: invalid literal for int() with base 10: 'abc'
num = int("abc") # Expects string of digits

2. LookupError: IndexError vs. KeyError

  • IndexError: Attempting to access an index outside the boundaries of a sequence (list, tuple).
Python
items = [10, 20, 30]
# IndexError: list index out of range
val = items[5]
  • KeyError: Looking up a dictionary key that does not exist.
Python
user = {"name": "Sita"}
# KeyError: 'email'
email = user["email"]

3. AttributeError

Occurs when attempting to access a method or attribute that does not exist on that object type:

Python
num = 42
# AttributeError: 'int' object has no attribute 'lower'
num.lower()

4. NameError vs. UnboundLocalError

  • NameError: Referencing a variable or function name that has not been defined in any local, enclosing, or global scope.
Python
# NameError: name 'undefined_variable' is not defined
print(undefined_variable)
  • UnboundLocalError: Referencing a local variable before it has been assigned inside a function.
Python
counter = 0
def increment():
# UnboundLocalError: local variable 'counter' referenced before assignment
counter += 1

5. ModuleNotFoundError

A specialized subclass of ImportError raised when Python cannot resolve an import target:

Python
# ModuleNotFoundError: No module named 'non_existent_engine'
import non_existent_engine

4. Multi-Catch and Inheritance Polymorphism

Because exceptions are standard Python classes, catching a parent class catches all derived subclasses:

Python
def safe_lookup(collection, key):
try:
return collection[key]
except LookupError as err:
# Catches BOTH IndexError (for lists) AND KeyError (for dicts)!
print(f"Lookup failure ({type(err).__name__}): {err}")
return None
 
print(safe_lookup([1, 2, 3], 9)) # Lookup failure (IndexError)
print(safe_lookup({"id": 101}, "age")) # Lookup failure (KeyError)

Multiple Choice Questions

1. Which base class sits at the very top of Python's exception hierarchy?

A. Exception B. RootError C. BaseException D. SystemError Answer: C Explanation: BaseException is the common base class for all built-in exceptions in Python.

2. What exception is raised by int("ninety-nine")?

A. TypeError B. ValueError C. CastError D. ParsingError Answer: B Explanation: Passing a string to int() satisfies the expected type, but the content cannot be parsed as an integer literal, raising a ValueError.

3. Both IndexError and KeyError inherit directly from which intermediate class?

A. CollectionError B. LookupError C. IndexableException D. KeyIndexError Answer: B Explanation: LookupError is the base class for exceptions raised when a key or index used on a mapping or sequence is invalid.

4. Why should developers avoid writing except BaseException: in normal application code?

A. It consumes too much CPU cache B. It intercepts user termination signals like KeyboardInterrupt (Ctrl+C) and SystemExit C. It only catches syntax errors D. Python 3 removed BaseException Answer: B Explanation: Catching BaseException intercepts KeyboardInterrupt and SystemExit, preventing normal script termination via Ctrl+C or sys.exit().

5. What exception is triggered when invoking (5).append(10)?

A. TypeError B. AttributeError C. ValueError D. MethodNotFoundError Answer: B Explanation: Integers do not have an append attribute or method, so Python raises an AttributeError.

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