Built-in Exceptions in Python — ValueError, TypeError, and More

Lesson goal: Python already provides dozens of ready-made exception classes. Instead of creating every error yourself, you use these built-in exceptions for universal programming problems.

What you will learn

• Built-in vs custom exceptions
• The 7 most common Python exception classes
• What triggers each exception
• How exceptions are objects
• How to catch specific exception types

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Before we begin — Think like a doctor

Imagine you are a doctor diagnosing patients. Every patient has different symptoms. Likewise, every programming error has a different exception class.

Key idea: The exception class tells Python exactly what kind of problem occurred.

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1. Two types of exceptions

Python exceptions are divided into two categories.

Easy comparison

Feature	Built-in	Custom
Created by	Python	Developer
Purpose	Universal problems	Business rules
Examples	ValueError	InvalidPinError
Available immediately	✅ Yes	❌ You create it

Today's lesson focuses only on built-in exceptions. Custom exceptions come in a later lesson.

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2. The built-in exception family

Every exception is actually a class.

Think of it like a family tree:

• BaseException → parent of all exceptions
• Exception → main family for program errors
• ValueError, KeyError, etc. → children you catch in except

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3. The 7 most common built-in exceptions

These are the exceptions you will use almost every day.

ValueError — most common

Raised when the value is invalid, even though the type is correct.

age = int("abc")  # ValueError

Real life: User types "twenty" into an age field.

TypeError

Raised when you perform an operation on incompatible data types.

"Hello" + 5  # TypeError

Real life: Adding text and numbers together.

ZeroDivisionError

Raised when dividing by zero.

10 / 0  # ZeroDivisionError

Real life: Average calculation where total students = 0.

IndexError

Raised when accessing a list or tuple position that does not exist.

nums = (10, 20)
print(nums[5])  # IndexError

Real life: Opening the 10th photo when only 2 exist.

KeyError

Raised when a dictionary key does not exist.

student = {"name": "Sam"}
print(student["age"])  # KeyError

Real life: Looking for "salary" in employee data that only contains "name".

NameError

Raised when using a variable that has never been created.

print(total)  # NameError — total was never assigned

Real life: Using a variable before assigning it.

FileNotFoundError

Raised when opening a file that does not exist.

open("marks.txt")  # FileNotFoundError

Real life: User selects a deleted document.

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4. Exception explorer — IndexError

colors = ["red", "blue"]
print(colors[9])

Raised because: Index 9 is outside the list. Valid indices are 0 and 1 only.

Try changing 9 to 0 or 1 in the playground — no error. Change it back to 9 — IndexError.

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5. Exceptions are objects (very important)

Many beginners think exceptions are just messages. They are not. They are objects created by the Python Virtual Machine (PVM).

Example

try:
    int("xyz")
except ValueError as err:
    print(type(err))
    print(err)

Output:

<class 'ValueError'>
invalid literal for int() with base 10: 'xyz'

Code	Meaning
err	Exception object
type(err)	Returns the class
print(err)	Returns the technical message

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6. How Python chooses the correct exception

Imagine a security guard checking ID cards. The exception class must match.

Python compares:

• Raised exception → ValueError
• except ValueError → Match ✅ — handler runs
• except KeyError → No match ❌ — skipped

Always catch the specific exception type when you can.

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7. Real-world case study — Student portal

Imagine a university marks portal.

User action	Exception
Types "A+" as age	ValueError
Adds "Math" + 90	TypeError
Divides by zero students	ZeroDivisionError
Opens 100th student	IndexError
Looks for "chemistry" key	KeyError

The exception tells you exactly what went wrong.

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8. Playground challenge

Predict the output before running the playground:

demos = [
    ("ValueError", lambda: int("abc")),
    ("ZeroDivisionError", lambda: 10 / 0),
    ("TypeError", lambda: "hi" + 5),
    ("IndexError", lambda: [1, 2][9]),
    ("KeyError", lambda: {"a": 1}["z"]),
]

for name, fn in demos:
    try:
        fn()
    except Exception as err:
        print(name, type(err).__name__)

Expected output

ValueError ValueError
ZeroDivisionError ZeroDivisionError
TypeError TypeError
IndexError IndexError
KeyError KeyError

Why does every line print? Because each exception is caught inside the loop, allowing the program to continue instead of crashing.

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Quick reference sheet

Exception	Cause	Example
ValueError	Invalid value	int("abc")
TypeError	Wrong type	"a" + 5
ZeroDivisionError	Divide by zero	10 / 0
IndexError	Invalid list index	lst[99]
KeyError	Missing dictionary key	d["x"]
NameError	Undefined variable	print(x)
FileNotFoundError	Missing file	open("a.txt")

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Interview questions

Q1. What is the difference between ValueError and TypeError?

• ValueError: Correct type, wrong value (e.g. int("abc")).
• TypeError: Wrong data type used in an operation (e.g. "a" + 5).

Q2. Why is KeyError different from IndexError?

• IndexError → lists and tuples (position)
• KeyError → dictionaries (key)

Q3. What does except ValueError as err mean?

err is the exception object created by the Python Virtual Machine when the error occurs.

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Key takeaways

• Built-in exceptions are predefined classes provided by Python
• Every runtime error becomes an exception object
• except must match the correct exception class
• Custom exceptions are for business rules; built-in exceptions handle universal programming problems
• Next: try and except Basics — syntax, multiple except blocks, and your first robust programs