🚫 Python Errors and Exception Handling – try, except, raise, finally, and Logging

Master Safe and Reliable Error Management in Python

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🧲 Introduction – Why Error Handling Matters?

In programming, things can go wrong — files may be missing, APIs may return bad data, or users may enter unexpected input. Python provides a robust exception-handling mechanism to gracefully handle such issues without crashing your application.

🎯 In this guide, you’ll learn:

• Difference between errors and exceptions
• How to use try…except and try…finally blocks
• How to raise, chain, and define custom exceptions
• Logging and asserting errors for better debugging

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📌 Topics Covered

⚙️ Topic	📝 Description
Python Errors vs. Exceptions	Understand the distinction between syntax errors and runtime exceptions
Python try…except Block	Catch and handle exceptions using try/except syntax
Python try…finally Block	Ensure important cleanup happens regardless of exceptions
Python Raising Exceptions	Use raise to trigger exceptions deliberately
Python Exception Chaining	Link multiple exceptions using raise ... from
Python Nested try Block	Use multiple try blocks inside one another safely
Python User-defined Exceptions	Define custom exceptions for your application logic
Python Built-in Exceptions	Use Python’s library of standard exceptions
Python Logging	Log exceptions to file or console using the logging module
Python Assertions	Use assert for quick checks in testing and debugging

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🧱 Python Errors vs. Exceptions

• Errors: Problems in the syntax of the code (e.g., missing colons, incorrect indentation).
• Exceptions: Runtime anomalies that occur during execution (e.g., division by zero, file not found).

# Syntax Error Example (compile-time)
if True
    print("Missing colon")

# Exception Example (runtime)
print(10 / 0)  # ZeroDivisionError

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🧪 Python try…except Block

Used to catch and handle exceptions.

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

✅ Explanation:

• The code inside try runs first.
• If an error occurs, control moves to except.

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🔐 Python try…finally Block

The finally block always executes, whether or not an exception occurred.

try:
    f = open("demo.txt")
finally:
    f.close()
    print("File closed.")

✅ Use Case: Closing resources like files or network connections.

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🚨 Python Raising Exceptions

Use raise to trigger an error manually.

age = -5
if age < 0:
    raise ValueError("Age cannot be negative.")

✅ Helpful for validating function arguments or enforcing rules.

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🔗 Python Exception Chaining

Use raise ... from ... to indicate the original cause of an exception.

try:
    int("abc")
except ValueError as e:
    raise RuntimeError("Conversion failed") from e

✅ Helps trace errors back to the root cause.

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🧩 Python Nested try Block

You can place one try block inside another.

try:
    try:
        x = int("a")
    except ValueError:
        print("Inner block handled ValueError")
except:
    print("Outer block")

✅ Useful when different types of exceptions need to be handled at different levels.

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🛠️ Python User-defined Exceptions

Create your own exception class by inheriting from Exception.

class NegativeAgeError(Exception):
    pass

def validate_age(age):
    if age < 0:
        raise NegativeAgeError("Age must be positive.")

✅ Helps enforce domain-specific rules.

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📚 Python Built-in Exceptions

Exception	Description
ZeroDivisionError	Division by zero
ValueError	Invalid value
TypeError	Mismatched data types
FileNotFoundError	File doesn’t exist
IndexError	Index out of bounds
KeyError	Dictionary key not found

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🧾 Python Logging

Use the logging module to record errors.

import logging

logging.basicConfig(level=logging.ERROR)
try:
    x = 1 / 0
except ZeroDivisionError as e:
    logging.error("An error occurred", exc_info=True)

✅ Preferred over print statements for production debugging.

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🧷 Python Assertions

Use assert for quick checks during development.

x = 10
assert x > 0, "x must be positive"

✅ Automatically raises an AssertionError if the condition fails. Useful for debugging and testing.

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📌 Summary – Recap & Next Steps

Python’s exception handling system is both powerful and flexible. It helps you build safe, fault-tolerant applications by catching, raising, and managing errors gracefully.

🔍 Key Takeaways:

• Use try…except to catch exceptions.
• Use finally to release resources like files.
• Raise custom errors using raise, and create your own with custom classes.
• Use the logging module for better error reporting.
• Use assert for validation and debugging.

⚙️ Real-World Relevance:
Exception handling is used in API integrations, file operations, user input validation, and system-level operations across nearly every Python application.

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❓ FAQ – Python Error Handling

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❓ What’s the difference between an error and an exception?
✅ Errors are syntax-level problems; exceptions happen during execution.

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❓ What is the benefit of using finally?
✅ It guarantees execution of cleanup code (e.g., closing files).

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❓ How do I define a custom exception?

class MyError(Exception):
    pass

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❓ Should I use logging instead of print()?
✅ Yes. Logging allows severity levels, formatting, and persistence.

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❓ Is assert used in production code?
❌ It’s typically used in testing and debugging — not for end-user error handling.

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