Types of Errors in Python — Compile, Logical, and Runtime

Core idea: Every bug is not an exception. Python has 3 major error categories, but only runtime errors become exceptions that try/except can handle.

Learning objectives

By the end of this lesson, you will be able to:

• Identify compile-time, logical, and runtime errors
• Recognize when each error occurs
• Understand who fixes each error
• Know why try/except works only for runtime errors

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The big picture

Think of building a Python program like driving a car.

One-line memory trick

Error	Real life
Compile-time	Car will not start
Logical	Wrong destination
Runtime	Accident while driving

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1. Compile-time errors

What is a compile-time error?

A compile-time error happens before the program starts running. Python first checks whether your code follows its grammar rules (syntax). If the syntax is wrong, the program never starts.

Example

print("Hello"

Immediately Python says:

SyntaxError: '(' was never closed

Why? The closing parenthesis is missing.

Visual explanation

Characteristics

Property	Value
Happens	Before execution
Cause	Syntax mistake
Example	Missing ) or :
Fixed by	Programmer
try/except	❌ No

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2. Logical errors

What is a logical error?

The program runs successfully, but the answer is wrong.

No crash. No exception. Just incorrect logic.

Example

def discount_total(price, discount):
    return price + discount

print(discount_total(100, 10))

Output:

110

Is the program correct? No. Expected answer: 90. The formula is wrong.

Visual comparison

Characteristics

Property	Value
Happens	During execution
Cause	Wrong algorithm
Example	Wrong formula
Fixed by	Testing and debugging
try/except	❌ No

Important: try/except cannot detect wrong mathematics. It only catches exceptions.

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3. Runtime errors (Exceptions)

What is a runtime error?

A runtime error happens while the program is running because of an impossible or invalid operation.

Examples:

• User enters letters instead of numbers
• Divide by zero
• File does not exist
• Invalid list index

These become exceptions.

Example 1 — Invalid input

age = int("abc")

Python raises: ValueError

Example 2 — Division

10 / 0

Python raises: ZeroDivisionError

Without exception handling

result = 10 / 0
print(result)

Program terminates with a traceback.

With exception handling

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero")

Output:

Cannot divide by zero

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Why only runtime errors use try/except?

This is the most important concept in this lesson.

Why not compile-time?

Because the program never starts — syntax is checked before runtime.

Why not logical?

Because nothing technically goes wrong. Python thinks your logic is valid — you just get the wrong answer.

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Side-by-side comparison

Feature	Compile-time	Logical	Runtime
Program starts?	❌ No	✅ Yes	✅ Yes
Program crashes?	Yes (before run)	No	Yes (without handling)
Wrong result?	No	✅ Yes	Maybe
Exception created?	❌ No	❌ No	✅ Yes
Use try/except?	❌	❌	✅

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Real-world examples

Example 1 — Student marks system

Compile-time:

print("Marks"

Program never opens.

Logical:

average = total  # forgot to divide by count

Student receives wrong grade.

Runtime:

marks = int("ninety")

Result: ValueError — use exception handling.

Example 2 — ATM machine

Situation	Error type
Missing : in source code	Compile-time
Interest formula incorrect	Logical
Customer enters "abc" amount	Runtime

Only the third case uses exception handling.

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Interactive playground

Predict the output before running the playground:

def avgop(m, p):
    return m + p

print(avgop(10, 20))

try:
    print(10 / 0)
except ZeroDivisionError:
    print("Cannot divide by zero")

Step-by-step execution

Step	What happens
1	avgop(10, 20) returns 30
2	This is a logical error — average should be 1.5, not 30
3	10/0 raises ZeroDivisionError
4	except catches it and prints a friendly message

Final output

30
Cannot divide by zero

Notice: first line = wrong logic · second line = runtime exception handled successfully.

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Memory map

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Key vocabulary

Term	Meaning
Compile-time error	Syntax mistake detected before execution
SyntaxError	Invalid Python grammar
Logical error	Correct syntax but wrong algorithm
Runtime error	Error occurring during execution
Exception	Runtime error represented as an object
Exception handling	Using try and except to recover gracefully

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Interview questions

Q1. Which error is handled using try/except?

Answer: Runtime errors (exceptions).

Q2. Why cannot SyntaxError be caught by try/except?

Answer: Because the program never starts executing — syntax is checked before runtime.

Q3. What is the difference between logical and runtime errors?

Answer: A logical error produces an incorrect result without raising an exception; a runtime error raises an exception during execution.

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Key takeaways

• Compile-time → syntax mistakes before execution
• Logical → program runs but produces incorrect results
• Runtime → exceptions from invalid input or impossible operations
• Only runtime errors are handled using try + except
• Next: Built-in Exceptions — ValueError, TypeError, IndexError, KeyError, ZeroDivisionError, FileNotFoundError