python-doc/Docs/Basic/07-error-handeling.md

06 – Error Handling, Linting, Formatting, and Testing in Python

This document explains how Python handles runtime errors, how to write safer code using try / except, and how to improve code quality using linting, formatting, and unit testing tools.

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1. Error Handling with try / except

Python uses try / except blocks to handle runtime errors gracefully without crashing the program.

Example

def abbas(a, b):
    try:
        res = a / b
        print(res)
    except ZeroDivisionError:
        print("Zero Number Detected")
    except Exception as e:
        print(f"Error Detected {e}")

abbas(1, 0)

Explanation

• The code inside try is executed first.
• If no error occurs, the result is printed.
• If b is 0, a ZeroDivisionError is raised.
• The specific ZeroDivisionError block runs first.
• Any other error is caught by the generic Exception block.

Key Rule

• Always catch specific exceptions first.
• Use Exception only as a fallback.

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2. Full try / except Structure

Python supports additional blocks for more control.

Syntax

try:
    # code that may raise an error
except:
    # runs if an error occurs
else:
    # runs if no error occurs
finally:
    # always runs

Explanation

• try: code that may fail
• except: handles errors
• else: runs only if no exception occurred
• finally: runs no matter what (used for cleanup)

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3. Linting with pylint

Linting analyzes code for:

• Syntax errors
• Style problems
• Bad practices

Basic Command

pylint main.py

Detailed Report

pylint --report y main.py

Explanation

• pylint gives a score and suggestions.
• Helps maintain readable and maintainable code.
• Commonly used in professional Python projects.

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4. Code Formatting with black

black is an automatic code formatter that enforces a consistent style.

Command

black main.py

Explanation

• Reformats code automatically.
• Removes style debates.
• Safe and widely used.

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5. Unit Testing with unittest

Unit tests verify that individual parts of code behave as expected.

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Application Code

abbas.py

def bemola(a, b):
    try:
        res = a / b
        print(res)
    except ZeroDivisionError:
        print("Zero Number Detected")
    except Exception as e:
        print(f"Error Detected {e}")

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Test Code

abbas_test.py

import unittest
from abbas import bemola

class TestAbbas(unittest.TestCase):

    def test_bemola(self):
        a = 10
        b = 2
        self.assertEqual(bemola(a, b), 5)

if __name__ == "__main__":
    unittest.main()

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Explanation

unittest.TestCase

• Base class for writing test cases.

Test Method

def test_bemola(self):

• Test methods must start with test_.

Assertion

self.assertEqual(bemola(a, b), 5)

• Checks if the function returns the expected result.

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Important Note (Design Issue)

The function bemola prints the result but does not return it.

print(res)

This causes the test to fail because the function returns None.

Correct Implementation

def bemola(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        return "Zero Number Detected"
    except Exception as e:
        return f"Error Detected {e}"

This version:

• Returns values instead of printing
• Is testable
• Follows best practices

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Summary

• try / except prevents program crashes
• else runs only when no error occurs
• finally always runs
• pylint improves code quality
• black enforces formatting
• unittest verifies correctness
• Functions should return values, not print them, when tested