Python Libraries (Modules)

Introduction to Python Modules

Python modules are reusable pieces of code that extend Python's functionality. They can be standard library modules (built-in) or third-party modules (installed via pip).

Module Organization Strategy

• Core/Important Modules - Essential for system interaction, file handling, testing
• Utility/Other Modules - Specific functionality, optional features
• Import Order - Standard library → Third-party → Local imports

Import Best Practices

# Standard library imports
import os
import sys

# Third-party imports
import requests
import numpy as np

# Local imports
from mypackage import utils

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Core/Important Modules

Command Line & System Interaction

sys - System-specific Parameters

import sys

sys.argv           # Command line arguments list
sys.argv[0]        # Script name
sys.argv[1]        # First argument
sys.exit(0)        # Exit with status code (0=success)
sys.exit(1)        # Exit with error
sys.version        # Python version string
sys.platform       # Operating system platform
sys.path           # Python module search paths
sys.modules        # Loaded modules dictionary
sys.executable     # Python interpreter path
sys.maxsize        # Largest integer value

os - Operating System Interface

import os

# Directory operations
os.getcwd()                    # Get current working directory
os.chdir('/path/to/dir')       # Change directory
os.listdir('.')               # List directory contents
os.mkdir('dirname')           # Create directory
os.makedirs('path/to/dirs')   # Create nested directories
os.rmdir('dirname')           # Remove directory
os.remove('filename')         # Remove file
os.rename('old', 'new')       # Rename file/directory

# Path operations
os.path.join('path', 'file')   # Join path components
os.path.exists('path')         # Check if path exists
os.path.isfile('path')         # Check if path is file
os.path.isdir('path')          # Check if path is directory
os.path.abspath('path')        # Get absolute path
os.path.getsize('file')        # Get file size in bytes
os.path.getmtime('file')       # Get modification time

# Environment variables
os.environ.get('VAR_NAME')     # Get environment variable

subprocess - External Process Management

import subprocess

# Run command and wait
result = subprocess.run(['ls', '-l'],
                       capture_output=True,
                       text=True,
                       check=True)

# Access results
result.returncode              # Exit code
result.stdout                  # Standard output
result.stderr                  # Standard error

# Quick command execution
subprocess.call(['ls', '-l'])         # Run and return exit code
subprocess.check_output(['ls'])       # Run and return output

# Arguments
# capture_output=True    - Capture stdout/stderr
# text=True             - Return as string
# check=True            - Raise error if command fails
# timeout=30            - Set maximum execution time

argparse - Command Line Argument Parsing

import argparse

parser = argparse.ArgumentParser(description='Program description')

# Add arguments
parser.add_argument('filename')                    # Positional argument
parser.add_argument('-n', '--count',              # Optional argument
                   type=int,
                   default=1,
                   help='Number of iterations')
parser.add_argument('--verbose',
                   action='store_true',            # Boolean flag
                   help='Enable verbose output')

args = parser.parse_args()

# Access arguments
print(args.filename)
print(args.count)
print(args.verbose)

---

Testing Modules

unittest - Built-in Testing Framework

import unittest

class TestMath(unittest.TestCase):

    def test_addition(self):
        # Arrange
        a, b = 2, 3

        # Act
        result = a + b

        # Assert
        self.assertEqual(result, 5)

    def test_division_by_zero(self):
        with self.assertRaises(ZeroDivisionError):
            10 / 0

# Common assertions
self.assertEqual(a, b)         # a == b
self.assertNotEqual(a, b)      # a != b
self.assertTrue(x)             # bool(x) is True
self.assertFalse(x)            # bool(x) is False
self.assertIsNone(x)           # x is None
self.assertIn(a, b)            # a in b
self.assertIsInstance(a, str)  # isinstance(a, str)

# Run tests
if __name__ == '__main__':
    unittest.main()

pytest - Third-party Testing Framework

# pip install pytest
import pytest

def test_addition():
    assert 2 + 3 == 5

def test_string_operations():
    text = "hello"
    assert text.upper() == "HELLO"
    assert len(text) == 5

def test_exception_handling():
    with pytest.raises(ValueError):
        int("not_a_number")

    with pytest.raises(ZeroDivisionError):
        1 / 0

# Run with: pytest test_file.py
# Key advantages:
# - Simple assert statements
# - Better error messages
# - Fixtures for test setup
# - Plugin ecosystem

Testing Concepts

Type	Description	Use Case
Unit Testing	Test individual components	Functions, classes
Integration Testing	Test component interactions	API calls, database
Regression Testing	Ensure fixes don't break existing code	After bug fixes
Load Testing	Test performance under load	High traffic scenarios
TDD	Write tests before code	Red-Green-Refactor cycle

Test-Driven Development (TDD) Cycle:

1. Red - Write failing test
2. Green - Write minimal code to pass
3. Refactor - Improve code while keeping tests green

---

Utility/Other Modules

Data & Math

random - Random Number Generation

import random

random.randint(1, 10)          # Random integer between 1-10
random.randrange(0, 10, 2)     # Random even number 0-8
random.random()                # Random float 0.0-1.0
random.choice(['a', 'b', 'c']) # Random choice from list
random.shuffle(my_list)        # Shuffle list in-place

statistics - Statistical Functions

import statistics

data = [1, 2, 3, 4, 5, 6, 7, 8, 9]

statistics.mean(data)          # Average
statistics.median(data)        # Middle value
statistics.mode(data)          # Most common value
statistics.stdev(data)         # Sample standard deviation
statistics.pstdev(data)        # Population standard deviation

collections.Counter - Count Occurrences

from collections import Counter

text = "hello world"
counter = Counter(text)
# Counter({'l': 3, 'o': 2, 'h': 1, 'e': 1, ' ': 1, 'w': 1, 'r': 1, 'd': 1})

counter.most_common(3)         # Top 3 most common
counter['l']                   # Count of 'l'

Web & Network

requests - HTTP Library

# pip install requests
import requests

response = requests.get('<https://api.example.com/data>')
response.status_code           # HTTP status code
response.json()                # Parse JSON response
response.text                  # Response as text
response.headers               # Response headers

# POST request
requests.post('<https://api.example.com>',
             json={'key': 'value'})

File & Data Processing

datetime - Date and Time

import datetime

now = datetime.datetime.now()   # Current date and time
today = datetime.date.today()   # Current date

shutil - High-level File Operations

import shutil

shutil.copy2('src.txt', 'dst.txt')     # Copy file with metadata
shutil.copytree('src_dir', 'dst_dir')  # Copy entire directory tree
shutil.move('src', 'dst')              # Move file or directory
shutil.rmtree('directory')             # Remove directory tree

time - Time-related Functions

import time

time.sleep(5)                  # Pause execution for 5 seconds
time.time()                    # Current Unix timestamp
time.strftime('%Y-%m-%d')      # Format current time

Text & String Processing

string - String Constants and Templates

import string

string.ascii_letters           # 'abcdefghijklmnopqrstuvwxyzABC...'
string.digits                  # '0123456789'
string.punctuation             # '!"#$%&\\'()*+,-./:;<=>?@[\\\\]^_`{|}~'

io.StringIO - In-memory String Streams

from io import StringIO

string_buffer = StringIO()
string_buffer.write("Hello ")
string_buffer.write("World")
content = string_buffer.getvalue()  # "Hello World"

Color & Image Processing

colorsys - Color Space Conversions

import colorsys

# Convert RGB to HSV
h, s, v = colorsys.rgb_to_hsv(r, g, b)

# Convert HSV to RGB
r, g, b = colorsys.hsv_to_rgb(h, s, v)

# Other conversions available:
# rgb_to_hls, hls_to_rgb
# rgb_to_yiq, yiq_to_rgb

PIL (Pillow) - Image Processing

# pip install pillow
from PIL import Image

img = Image.open('photo.jpg')
img.resize((800, 600))         # Resize image
img.rotate(90)                 # Rotate image
img.save('output.png')         # Save in different format

Utility Functions

copy - Object Copying

import copy

# Shallow copy
new_list = copy.copy(original_list)

# Deep copy (nested objects)
new_dict = copy.deepcopy(original_dict)

---

Module Import Organization Template

#!/usr/bin/env python3
"""
Module docstring describing the script purpose.
"""

# Standard library imports (alphabetical)
import argparse
import os
import sys
import subprocess
from collections import Counter
from datetime import datetime

# Third-party imports (alphabetical)
import requests  # pip install requests
import pytest   # pip install pytest

# Local application imports
from mypackage.utils import helper_function
from mypackage.models import DataProcessor

# Optional imports with error handling
try:
    from PIL import Image
    HAS_PIL = True
except ImportError:
    HAS_PIL = False
    print("PIL not available, image processing disabled")

def main():
    """Main function."""
    pass

if __name__ == "__main__":
    main()

Installation Commands

# Core modules are built-in, no installation needed

# Third-party modules
pip install requests
pip install pytest
pip install pillow

Documentation Quick Links

• Python Standard Library
• PyPI - Python Package Index
• pip Documentation

Remember: Always check if a standard library module exists before installing third-party alternatives! 🐍✨