220 lines
3.8 KiB
Markdown
220 lines
3.8 KiB
Markdown
# 06 – Error Handling, Linting, Formatting, and Testing in Python
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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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---
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## 1. Error Handling with `try / except`
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Python uses `try / except` blocks to handle runtime errors gracefully without crashing the program.
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### Example
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```python
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def abbas(a, b):
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try:
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res = a / b
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print(res)
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except ZeroDivisionError:
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print("Zero Number Detected")
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except Exception as e:
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print(f"Error Detected {e}")
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abbas(1, 0)
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```
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### Explanation
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* The code inside `try` is executed first.
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* If no error occurs, the result is printed.
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* If `b` is `0`, a `ZeroDivisionError` is raised.
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* The specific `ZeroDivisionError` block runs first.
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* Any other error is caught by the generic `Exception` block.
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### Key Rule
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* Always catch **specific exceptions first**.
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* Use `Exception` only as a fallback.
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---
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## 2. Full `try / except` Structure
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Python supports additional blocks for more control.
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### Syntax
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```python
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try:
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# code that may raise an error
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except:
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# runs if an error occurs
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else:
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# runs if no error occurs
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finally:
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# always runs
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```
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### Explanation
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* `try`: code that may fail
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* `except`: handles errors
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* `else`: runs only if no exception occurred
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* `finally`: runs no matter what (used for cleanup)
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---
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## 3. Linting with `pylint`
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Linting analyzes code for:
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* Syntax errors
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* Style problems
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* Bad practices
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### Basic Command
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```bash
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pylint main.py
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```
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### Detailed Report
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```bash
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pylint --report y main.py
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```
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### Explanation
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* `pylint` gives a score and suggestions.
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* Helps maintain readable and maintainable code.
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* Commonly used in professional Python projects.
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---
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## 4. Code Formatting with `black`
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`black` is an automatic code formatter that enforces a consistent style.
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### Command
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```bash
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black main.py
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```
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### Explanation
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* Reformats code automatically.
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* Removes style debates.
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* Safe and widely used.
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---
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## 5. Unit Testing with `unittest`
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Unit tests verify that individual parts of code behave as expected.
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---
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### Application Code
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#### `abbas.py`
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```python
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def bemola(a, b):
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try:
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res = a / b
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print(res)
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except ZeroDivisionError:
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print("Zero Number Detected")
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except Exception as e:
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print(f"Error Detected {e}")
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```
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---
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### Test Code
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#### `abbas_test.py`
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```python
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import unittest
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from abbas import bemola
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class TestAbbas(unittest.TestCase):
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def test_bemola(self):
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a = 10
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b = 2
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self.assertEqual(bemola(a, b), 5)
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if __name__ == "__main__":
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unittest.main()
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```
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---
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### Explanation
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#### `unittest.TestCase`
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* Base class for writing test cases.
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#### Test Method
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```python
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def test_bemola(self):
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```
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* Test methods must start with `test_`.
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#### Assertion
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```python
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self.assertEqual(bemola(a, b), 5)
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```
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* Checks if the function returns the expected result.
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---
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### Important Note (Design Issue)
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The function `bemola` **prints** the result but does not return it.
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```python
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print(res)
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```
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This causes the test to fail because the function returns `None`.
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#### Correct Implementation
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```python
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def bemola(a, b):
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try:
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return a / b
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except ZeroDivisionError:
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return "Zero Number Detected"
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except Exception as e:
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return f"Error Detected {e}"
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```
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This version:
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* Returns values instead of printing
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* Is testable
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* Follows best practices
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---
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## Summary
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* `try / except` prevents program crashes
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* `else` runs only when no error occurs
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* `finally` always runs
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* `pylint` improves code quality
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* `black` enforces formatting
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* `unittest` verifies correctness
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* Functions should **return values**, not print them, when tested
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