Types of Exceptions in Python0%

Types of Exceptions in Python

Intermediate8 min read•Updated: 2026-10-01

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

1# 1. SYNTAX ERROR: Discovered before runtime (parsing phase)
2# if True print("Hello")  <-- Missing colon ':'
3 
4# 2. RUNTIME EXCEPTION: Syntactically valid, but fails during execution
5denominator = 0
6# 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.

1BaseException
├── 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.

1  # TypeError: can only concatenate str (not "int") to str
2  greeting = "Level: " + 5

ValueError: The type is correct, but the content / value is invalid.

1  # ValueError: invalid literal for int() with base 10: 'abc'
2  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).

items = [10, 20, 30]
# IndexError: list index out of range
val = items[5]

KeyError: Looking up a dictionary key that does not exist.

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:

1num = 42
2# AttributeError: 'int' object has no attribute 'lower'
3num.lower()

4. `NameError` vs. `UnboundLocalError`

NameError: Referencing a variable or function name that has not been defined in any local, enclosing, or global scope.

1  # NameError: name 'undefined_variable' is not defined
2  print(undefined_variable)

UnboundLocalError: Referencing a local variable before it has been assigned inside a function.

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:

1# ModuleNotFoundError: No module named 'non_existent_engine'
2import non_existent_engine

4. Multi-Catch and Inheritance Polymorphism

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

1def safe_lookup(collection, key):
2    try:
3        return collection[key]
4    except LookupError as err:
5        # Catches BOTH IndexError (for lists) AND KeyError (for dicts)!
6        print(f"Lookup failure ({type(err).__name__}): {err}")
7        return None
8 
9print(safe_lookup([1, 2, 3], 9))       # Lookup failure (IndexError)
10print(safe_lookup({"id": 101}, "age"))  # Lookup failure (KeyError)

⚠️ Common Intermediate Mistakes & Gotchas

1. Catching `BaseException` Blindly

Catching BaseException catches KeyboardInterrupt (Ctrl+C) and SystemExit, making your script unkillable from the terminal! Always inherit from and catch Exception.

2. Confusing `ValueError` with `TypeError`

TypeError: An argument is of the wrong type (e.g. passing a string to math.sqrt()).
ValueError: An argument is of the right type but inappropriate value (e.g. math.sqrt(-1)).

💻 Try It Yourself: Robust User Input Validator

Scenario

Write a script that prompts the user for their age. Handle both ValueError (if non-numeric string is entered) and custom range validation without crashing.

Complete Solution

1def parse_age(input_str: str) -> int:
2    try:
3        age = int(input_str)
4        if age < 0 or age > 120:
5            raise ValueError(f"Age {age} is out of realistic human range (0-120).")
6        return age
7    except ValueError as err:
8        print(f"Validation Error: {err}")
9        return -1
10 
11print("Test 1 ('25'):", parse_age("25"))
12print("Test 2 ('abc'):", parse_age("abc"))
13print("Test 3 ('150'):", parse_age("150"))

Expected Output

1Test 1 ('25'): 25
2Validation Error: invalid literal for int() with base 10: 'abc'
3Test 2 ('abc'): -1
4Validation Error: Age 150 is out of realistic human range (0-120).
5Test 3 ('150'): -1

Interactive Quiz• Knowledge Check

Practice Quiz

Question 1

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

Question 2

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

Question 3

Both IndexError and KeyError inherit directly from which intermediate class?

Question 4

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

Question 5

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

🚀 What's Next?

In the next lesson, we will continue your intermediate Python journey with Try-Except-Else-Finally (6: Exception Handling).

👉 Continue to Next Lesson: Try-Except-Else-Finally →

Next Lesson

Try-Except-Else-Finally

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Project: Bank Account Class System	Try-Except-Else-Finally

Practice Quiz

Test your understanding of this lesson with 5 questions. Each question has one correct answer.

Previous Next

PreviousPrevNext

MSK Institute • Shikohabad

Loading Page Content...

Preparing courses, live labs, and learning resources.सामग्री लोड हो रही है, कृपया प्रतीक्षा करें...

Connecting serverPlease wait...

💻 Practical Labs🚀 Free Demo📜 Verified Certificates

Try-Except-Else-Finally

Types of Exceptions in Python

Intermediate8 min read•Updated: 2026-10-01

Study Materials Cheatsheet

Types of Exceptions in Python

1. Syntax Errors vs. Runtime Exceptions

1# 1. SYNTAX ERROR: Discovered before runtime (parsing phase)
2# if True print("Hello")  <-- Missing colon ':'
3 
4# 2. RUNTIME EXCEPTION: Syntactically valid, but fails during execution
5denominator = 0
6# result = 100 / denominator  <-- ZeroDivisionError at runtime

2. The Python Built-in Exception Hierarchy

1BaseException
├── 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.

1  # TypeError: can only concatenate str (not "int") to str
2  greeting = "Level: " + 5

ValueError: The type is correct, but the content / value is invalid.

1  # ValueError: invalid literal for int() with base 10: 'abc'
2  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).

items = [10, 20, 30]
# IndexError: list index out of range
val = items[5]

KeyError: Looking up a dictionary key that does not exist.

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:

1num = 42
2# AttributeError: 'int' object has no attribute 'lower'
3num.lower()

4. `NameError` vs. `UnboundLocalError`

NameError: Referencing a variable or function name that has not been defined in any local, enclosing, or global scope.

1  # NameError: name 'undefined_variable' is not defined
2  print(undefined_variable)

UnboundLocalError: Referencing a local variable before it has been assigned inside a function.

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:

1# ModuleNotFoundError: No module named 'non_existent_engine'
2import non_existent_engine

4. Multi-Catch and Inheritance Polymorphism

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

1def safe_lookup(collection, key):
2    try:
3        return collection[key]
4    except LookupError as err:
5        # Catches BOTH IndexError (for lists) AND KeyError (for dicts)!
6        print(f"Lookup failure ({type(err).__name__}): {err}")
7        return None
8 
9print(safe_lookup([1, 2, 3], 9))       # Lookup failure (IndexError)
10print(safe_lookup({"id": 101}, "age"))  # Lookup failure (KeyError)

⚠️ Common Intermediate Mistakes & Gotchas

1. Catching `BaseException` Blindly

Catching BaseException catches KeyboardInterrupt (Ctrl+C) and SystemExit, making your script unkillable from the terminal! Always inherit from and catch Exception.

2. Confusing `ValueError` with `TypeError`

TypeError: An argument is of the wrong type (e.g. passing a string to math.sqrt()).
ValueError: An argument is of the right type but inappropriate value (e.g. math.sqrt(-1)).

💻 Try It Yourself: Robust User Input Validator

Scenario

Write a script that prompts the user for their age. Handle both ValueError (if non-numeric string is entered) and custom range validation without crashing.

Complete Solution

1def parse_age(input_str: str) -> int:
2    try:
3        age = int(input_str)
4        if age < 0 or age > 120:
5            raise ValueError(f"Age {age} is out of realistic human range (0-120).")
6        return age
7    except ValueError as err:
8        print(f"Validation Error: {err}")
9        return -1
10 
11print("Test 1 ('25'):", parse_age("25"))
12print("Test 2 ('abc'):", parse_age("abc"))
13print("Test 3 ('150'):", parse_age("150"))

Expected Output

1Test 1 ('25'): 25
2Validation Error: invalid literal for int() with base 10: 'abc'
3Test 2 ('abc'): -1
4Validation Error: Age 150 is out of realistic human range (0-120).
5Test 3 ('150'): -1

Interactive Quiz• Knowledge Check

Practice Quiz

Question 1

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

Question 2

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

Question 3

Both IndexError and KeyError inherit directly from which intermediate class?

Question 4

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

Question 5

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

🚀 What's Next?

In the next lesson, we will continue your intermediate Python journey with Try-Except-Else-Finally (6: Exception Handling).

👉 Continue to Next Lesson: Try-Except-Else-Finally →

Next Lesson

Try-Except-Else-Finally

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Project: Bank Account Class System	Try-Except-Else-Finally

Practice Quiz

Test your understanding of this lesson with 5 questions. Each question has one correct answer.

Previous Next

Types of Exceptions in Python

1. Syntax Errors vs. Runtime Exceptions

2. The Python Built-in Exception Hierarchy

3. Deep Dive into Common Exceptions

1. TypeError vs. ValueError

2. LookupError: IndexError vs. KeyError

3. AttributeError

4. NameError vs. UnboundLocalError

5. ModuleNotFoundError

4. Multi-Catch and Inheritance Polymorphism

⚠️ Common Intermediate Mistakes & Gotchas

1. Catching BaseException Blindly

2. Confusing ValueError with TypeError

💻 Try It Yourself: Robust User Input Validator

Scenario

Complete Solution

Expected Output

Practice Quiz

🚀 What's Next?

Try-Except-Else-Finally

Related Lessons

Practice Quiz

Loading Page Content...

Types of Exceptions in Python

1. Syntax Errors vs. Runtime Exceptions

2. The Python Built-in Exception Hierarchy

3. Deep Dive into Common Exceptions

1. TypeError vs. ValueError

2. LookupError: IndexError vs. KeyError

3. AttributeError

4. NameError vs. UnboundLocalError

5. ModuleNotFoundError

4. Multi-Catch and Inheritance Polymorphism

⚠️ Common Intermediate Mistakes & Gotchas

1. Catching BaseException Blindly

2. Confusing ValueError with TypeError

💻 Try It Yourself: Robust User Input Validator

Scenario

Complete Solution

Expected Output

Practice Quiz

🚀 What's Next?

Try-Except-Else-Finally

Related Lessons

Practice Quiz

1. `TypeError` vs. `ValueError`

2. `LookupError`: `IndexError` vs. `KeyError`

3. `AttributeError`

4. `NameError` vs. `UnboundLocalError`

5. `ModuleNotFoundError`

1. Catching `BaseException` Blindly

2. Confusing `ValueError` with `TypeError`

1. `TypeError` vs. `ValueError`

2. `LookupError`: `IndexError` vs. `KeyError`

3. `AttributeError`

4. `NameError` vs. `UnboundLocalError`

5. `ModuleNotFoundError`

1. Catching `BaseException` Blindly

2. Confusing `ValueError` with `TypeError`