Working with Python Libraries

Python's true power lies in its vast ecosystem of libraries and packages. With over 400,000 packages available on PyPI (Python Package Index), you can find tools for virtually any task - from web development to data science, machine learning to automation.

Learning to work with external libraries, manage dependencies, and create virtual environments is essential for professional Python development.

1. Understanding pip

pip is Python's package installer. It downloads and installs packages from PyPI (Python Package Index).

Basic pip Commands

# Check pip version
pip --version

# Install a package
pip install requests

# Install specific version
pip install requests==2.28.0

# Install minimum version
pip install requests>=2.25.0

# Upgrade a package
pip install --upgrade requests

# Uninstall a package
pip uninstall requests

# List installed packages
pip list

# Show package details
pip show requests

# Search for packages (deprecated, use pypi.org)
# pip search machine-learning

Installing Multiple Packages

# Install multiple packages at once
pip install requests pandas numpy

# Install from requirements file
pip install -r requirements.txt

2. Virtual Environments

Virtual environments create isolated Python installations for each project, preventing dependency conflicts.

Creating Virtual Environments

# Create a virtual environment
python -m venv myenv

# Or using a specific Python version
python3.11 -m venv myenv

# Activate the virtual environment

# On macOS/Linux:
source myenv/bin/activate

# On Windows:
myenv\Scripts\activate

# Your prompt changes to show the active environment:
(myenv) $

# Install packages (now only in this environment)
pip install requests pandas

# Deactivate when done
deactivate

Project Structure with Virtual Environment

my_project/
├── venv/                  # Virtual environment (don't commit to git)
├── src/                   # Source code
│   ├── __init__.py
│   └── main.py
├── tests/                 # Test files
├── requirements.txt       # Dependencies
├── .gitignore            # Ignore venv, __pycache__, etc.
└── README.md             # Project documentation

3. Managing Dependencies

requirements.txt

The requirements.txt file lists all project dependencies, making it easy to recreate the environment.

# Generate requirements.txt from current environment
pip freeze > requirements.txt

# Install from requirements.txt
pip install -r requirements.txt

Example requirements.txt

# Web framework
flask==2.3.0

# HTTP requests
requests>=2.28.0

# Data manipulation
pandas==2.0.0
numpy==1.24.0

# Database
sqlalchemy==2.0.0

# Environment variables
python-dotenv==1.0.0

4. Popular Python Libraries

By Category

5. Working with datetime Module

The datetime module (built-in) provides classes for working with dates and times.

from datetime import datetime, date, time, timedelta

# Current date and time
now = datetime.now()
print(now)  # 2025-10-29 14:30:45.123456

today = date.today()
print(today)  # 2025-10-29

# Create specific dates
birthday = date(1990, 5, 15)
meeting = datetime(2025, 10, 30, 14, 30)  # Oct 30, 2025 at 2:30 PM

# Format dates
formatted = now.strftime("%Y-%m-%d %H:%M:%S")
print(formatted)  # 2025-10-29 14:30:45

formatted2 = now.strftime("%B %d, %Y")
print(formatted2)  # October 29, 2025

# Parse strings to dates
date_string = "2025-10-29"
parsed = datetime.strptime(date_string, "%Y-%m-%d")

# Date arithmetic with timedelta
tomorrow = today + timedelta(days=1)
next_week = today + timedelta(weeks=1)
three_hours_ago = now - timedelta(hours=3)

# Calculate difference
age = today - birthday
print(f"Days old: {age.days}")

# Compare dates
if meeting > now:
    print("Meeting is in the future")

# Practical example: Calculate project deadline
def get_deadline(start_date, duration_days):
    """Calculate project deadline excluding weekends."""
    current_date = start_date
    days_added = 0
    
    while days_added < duration_days:
        current_date += timedelta(days=1)
        # Skip weekends (5=Saturday, 6=Sunday)
        if current_date.weekday() < 5:
            days_added += 1
    
    return current_date

project_start = date(2025, 10, 29)
deadline = get_deadline(project_start, 20)  # 20 business days
print(f"Deadline: {deadline}")

# Working with time zones (requires pytz)
# pip install pytz
import pytz

utc = pytz.UTC
eastern = pytz.timezone('US/Eastern')

utc_now = datetime.now(utc)
eastern_now = utc_now.astimezone(eastern)
print(f"UTC: {utc_now}")
print(f"Eastern: {eastern_now}")

6. Working with requests Library

The requests library makes HTTP requests simple and elegant.

# Install requests
pip install requests

import requests

# GET request
response = requests.get('https://api.github.com')
print(response.status_code)  # 200
print(response.json())       # Parse JSON response

# GET with parameters
params = {'q': 'python', 'sort': 'stars'}
response = requests.get('https://api.github.com/search/repositories', params=params)
data = response.json()

# POST request
payload = {'username': 'alice', 'email': 'alice@example.com'}
response = requests.post('https://httpbin.org/post', json=payload)

# Headers
headers = {'Authorization': 'Bearer YOUR_TOKEN'}
response = requests.get('https://api.example.com/data', headers=headers)

# Handling errors
try:
    response = requests.get('https://api.example.com/data', timeout=5)
    response.raise_for_status()  # Raises HTTPError for bad status
    data = response.json()
except requests.exceptions.Timeout:
    print("Request timed out")
except requests.exceptions.HTTPError as e:
    print(f"HTTP error: {e}")
except requests.exceptions.RequestException as e:
    print(f"Error: {e}")

# Practical example: Weather API client
class WeatherAPI:
    """Simple weather API client."""
    
    def __init__(self, api_key):
        self.api_key = api_key
        self.base_url = "https://api.openweathermap.org/data/2.5"
    
    def get_current_weather(self, city):
        """Get current weather for a city."""
        endpoint = f"{self.base_url}/weather"
        params = {
            'q': city,
            'appid': self.api_key,
            'units': 'metric'
        }
        
        try:
            response = requests.get(endpoint, params=params, timeout=10)
            response.raise_for_status()
            data = response.json()
            
            return {
                'city': data['name'],
                'temperature': data['main']['temp'],
                'description': data['weather'][0]['description'],
                'humidity': data['main']['humidity']
            }
        except requests.exceptions.RequestException as e:
            print(f"Error fetching weather: {e}")
            return None
    
    def get_forecast(self, city, days=5):
        """Get weather forecast."""
        endpoint = f"{self.base_url}/forecast"
        params = {
            'q': city,
            'appid': self.api_key,
            'units': 'metric',
            'cnt': days * 8  # 8 forecasts per day
        }
        
        response = requests.get(endpoint, params=params, timeout=10)
        response.raise_for_status()
        return response.json()

# Usage (requires API key from openweathermap.org)
# weather = WeatherAPI('YOUR_API_KEY')
# current = weather.get_current_weather('London')
# print(f"{current['city']}: {current['temperature']}°C, {current['description']}")

7. Practical Project: CLI Weather App

#!/usr/bin/env python3
"""
Simple weather CLI app using requests and datetime.

Install: pip install requests python-dotenv
Usage: python weather.py London
"""

import sys
import requests
from datetime import datetime
import os
from dotenv import load_dotenv

# Load environment variables from .env file
load_dotenv()

class WeatherApp:
    """Command-line weather application."""
    
    def __init__(self):
        self.api_key = os.getenv('WEATHER_API_KEY')
        if not self.api_key:
            print("Error: WEATHER_API_KEY not found in environment")
            sys.exit(1)
        
        self.base_url = "https://api.openweathermap.org/data/2.5/weather"
    
    def get_weather(self, city):
        """Fetch weather data for city."""
        params = {
            'q': city,
            'appid': self.api_key,
            'units': 'metric'
        }
        
        try:
            response = requests.get(self.base_url, params=params, timeout=10)
            response.raise_for_status()
            return response.json()
        except requests.exceptions.RequestException as e:
            print(f"Error: {e}")
            return None
    
    def display_weather(self, data):
        """Display weather information."""
        if not data:
            return
        
        # Parse data
        city = data['name']
        country = data['sys']['country']
        temp = data['main']['temp']
        feels_like = data['main']['feels_like']
        humidity = data['main']['humidity']
        description = data['weather'][0]['description'].title()
        wind_speed = data['wind']['speed']
        
        # Sunrise/sunset
        sunrise = datetime.fromtimestamp(data['sys']['sunrise'])
        sunset = datetime.fromtimestamp(data['sys']['sunset'])
        
        # Display
        print("\n" + "="*50)
        print(f"Weather in {city}, {country}")
        print("="*50)
        print(f"Temperature:    {temp}°C (feels like {feels_like}°C)")
        print(f"Conditions:     {description}")
        print(f"Humidity:       {humidity}%")
        print(f"Wind Speed:     {wind_speed} m/s")
        print(f"Sunrise:        {sunrise.strftime('%H:%M')}")
        print(f"Sunset:         {sunset.strftime('%H:%M')}")
        print("="*50 + "\n")
    
    def run(self, city):
        """Run the weather app."""
        print(f"Fetching weather for {city}...")
        data = self.get_weather(city)
        self.display_weather(data)

def main():
    """Main function."""
    if len(sys.argv) < 2:
        print("Usage: python weather.py CITY")
        print("Example: python weather.py London")
        sys.exit(1)
    
    city = ' '.join(sys.argv[1:])
    app = WeatherApp()
    app.run(city)

if __name__ == '__main__':
    main()

# Create .env file with:
# WEATHER_API_KEY=your_api_key_here

8. Best Practices

Example .gitignore

# Python
__pycache__/
*.py[cod]
*$py.class
*.so
.Python

# Virtual environments
venv/
env/
ENV/

# Environment variables
.env
.env.local

# IDE
.vscode/
.idea/
*.swp

# OS
.DS_Store
Thumbs.db

Key Takeaways