Virtual Environments

# Create a virtual environment
# On Windows:
python -m venv venv

# On Mac/Linux:
python3 -m venv venv

# This creates a 'venv' folder with:
venv/
├── bin/          (or Scripts/ on Windows)
│   ├── python    # Python interpreter
│   ├── pip       # Package installer
│   └── activate  # Activation script
├── lib/          # Installed packages go here
└── include/      # C headers

# Activate the virtual environment
# On Windows:
venv\Scripts\activate

# On Mac/Linux:
source venv/bin/activate

# You'll see (venv) in your terminal prompt:
(venv) C:\project>

# Now any pip install goes into THIS environment only!

# Deactivate when done
deactivate

# Step 1: Create project directory
mkdir myproject
cd myproject

# Step 2: Create virtual environment
python -m venv venv

# Step 3: Activate it
source venv/bin/activate  # Mac/Linux
venv\Scripts\activate     # Windows

# Step 4: Install packages
pip install django
pip install djangorestframework
pip install psycopg2-binary

# Step 5: Work on your project
python manage.py runserver

# Step 6: When done, deactivate
deactivate

# Next time you work on this project:
cd myproject
source venv/bin/activate  # Start where you left off!

# Install a package
pip install django

# Install specific version
pip install django==4.2
pip install "django>=4.0,<5.0"  # Version range

# Install multiple packages
pip install django djangorestframework psycopg2

# Upgrade a package
pip install --upgrade django

# Uninstall a package
pip uninstall django

# List installed packages
pip list

# Show package details
pip show django

# Search for packages (deprecated, use PyPI.org)
# pip search django

# Install from requirements file (explained below)
pip install -r requirements.txt

# Check for outdated packages
pip list --outdated

# Install package from GitHub
pip install git+https://github.com/django/django.git

# Install in editable mode (for development)
pip install -e .

# Install from local directory
pip install ./mypackage/

# Generate requirements.txt (lists ALL installed packages)
pip freeze > requirements.txt

# Example requirements.txt
Django==4.2.7
djangorestframework==3.14.0
psycopg2-binary==2.9.9
python-dotenv==1.0.0
gunicorn==21.2.0

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

# Why this matters:
# 1. New teammate clones your repo
git clone https://github.com/yourrepo/project.git
cd project

# 2. Creates virtual environment
python -m venv venv
source venv/bin/activate

# 3. Installs exact same packages as you!
pip install -r requirements.txt

# 4. Now their environment matches yours perfectly!

# Pin versions for stability
Django==4.2.7              # Exact version (recommended)
djangorestframework~=3.14  # Compatible version (3.14.x)
psycopg2>=2.9             # Minimum version

# Multiple requirements files (common pattern)
requirements/
├── base.txt          # Common dependencies
├── dev.txt           # Development tools (debuggers, testing)
└── prod.txt          # Production only (gunicorn, logging)

# base.txt
Django==4.2.7
djangorestframework==3.14.0

# dev.txt
-r base.txt           # Include base
pytest==7.4.3
black==23.11.0
django-debug-toolbar==4.2.0

# prod.txt
-r base.txt           # Include base
gunicorn==21.2.0
sentry-sdk==1.38.0

# Install for development
pip install -r requirements/dev.txt

# virtualenv (older, more features than venv)
pip install virtualenv
virtualenv venv
source venv/bin/activate

# pipenv (combines pip + venv)
pip install pipenv
pipenv install django
pipenv shell  # Activates environment

# Uses Pipfile instead of requirements.txt
# Pipfile
[[source]]
url = "https://pypi.org/simple"
verify_ssl = true
name = "pypi"

[packages]
django = "==4.2.7"
djangorestframework = "*"

[dev-packages]
pytest = "*"

# poetry (modern dependency management)
pip install poetry
poetry init
poetry add django
poetry install
poetry shell

# Uses pyproject.toml
[tool.poetry.dependencies]
python = "^3.10"
django = "^4.2"

# conda (for data science)
conda create -n myenv python=3.10
conda activate myenv
conda install django

# Which to choose?
# - venv: Built-in, simple, sufficient for most projects
# - pipenv: Good for beginners, combines tools
# - poetry: Modern, best for libraries
# - conda: Data science projects (numpy, pandas, tensorflow)

myproject/
├── venv/                    # Virtual environment (don't commit!)
├── .gitignore               # Ignore venv, __pycache__, etc.
├── requirements.txt         # Dependencies
├── .env                     # Environment variables (don't commit!)
├── README.md                # Project documentation
├── manage.py                # Django management (if Django)
├── myapp/                   # Application code
│   ├── __init__.py
│   ├── models.py
│   ├── views.py
│   └── tests.py
└── config/                  # Configuration
    ├── settings.py
    └── urls.py

# .gitignore (MUST HAVE!)
# Virtual environment
venv/
env/
ENV/

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

# Environment variables
.env
.env.local

# Database
*.sqlite3
db.sqlite3

# IDE
.vscode/
.idea/
*.swp

# OS
.DS_Store
Thumbs.db

# Why ignore venv?
# - It's huge (100+ MB)
# - Anyone can recreate it with requirements.txt
# - Different OS have different files (Windows vs Mac)

# How to recreate environment on new machine:
git clone https://github.com/yourrepo/project.git
cd project
python -m venv venv
source venv/bin/activate
pip install -r requirements.txt
# Done! Same environment as original developer

# Starting a new Django project
mkdir mysite
cd mysite
python -m venv venv
source venv/bin/activate
pip install django
django-admin startproject config .
python manage.py runserver

# Adding a new package mid-project
source venv/bin/activate
pip install djangorestframework
# Add 'rest_framework' to INSTALLED_APPS
pip freeze > requirements.txt  # Update requirements

# Cloning teammate's project
git clone https://github.com/team/project.git
cd project
python -m venv venv
source venv/bin/activate
pip install -r requirements.txt
python manage.py migrate
python manage.py runserver

# Upgrading Python version
# 1. Install new Python version on your system
# 2. Delete old venv
rm -rf venv  # or manually delete folder
# 3. Create new venv with new Python
python3.11 -m venv venv
source venv/bin/activate
# 4. Reinstall packages
pip install -r requirements.txt

# Debugging "package not found" errors
# 1. Make sure venv is activated
which python  # Should show venv/bin/python

# 2. Install package
pip install package_name

# 3. Verify installation
pip show package_name

# 4. If still issues, restart Python interpreter

# Sharing code with non-Python users (Docker alternative)
# Use requirements.txt + README with setup instructions
# Or containerize with Docker (advanced)

# Install python-dotenv
pip install python-dotenv

# Create .env file (NEVER commit this to Git!)
# .env
SECRET_KEY=django-insecure-your-secret-key-here
DATABASE_URL=postgresql://user:password@localhost/dbname
API_KEY=your-api-key-here
DEBUG=True

# Load in Python
# settings.py
from dotenv import load_dotenv
import os

load_dotenv()  # Loads .env file

SECRET_KEY = os.getenv('SECRET_KEY')
DATABASE_URL = os.getenv('DATABASE_URL')
API_KEY = os.getenv('API_KEY')
DEBUG = os.getenv('DEBUG') == 'True'

# Provide defaults
DEBUG = os.getenv('DEBUG', 'False') == 'True'

# .env.example (commit this as template)
# .env.example
SECRET_KEY=your-secret-key
DATABASE_URL=postgresql://user:pass@localhost/db
API_KEY=your-api-key
DEBUG=True

# Team members copy .env.example to .env and fill in real values
cp .env.example .env
# Then edit .env with actual secrets

# "python: command not found"
# Use python3 instead
python3 -m venv venv

# "pip: command not found" after activation
# Deactivate and reactivate
deactivate
source venv/bin/activate

# "Permission denied" when creating venv on Windows
# Run PowerShell as Administrator
Set-ExecutionPolicy RemoteSigned

# Virtual environment not activating
# Check you're using correct activation script:
# Windows: venv\Scripts\activate
# Mac/Linux: source venv/bin/activate

# "Module not found" even after pip install
# 1. Verify venv is activated (should see (venv) in prompt)
# 2. Verify correct Python
which python  # Should be venv/bin/python

# If not, deactivate and reactivate

# Packages installed in wrong location
# Always activate venv BEFORE pip install!
source venv/bin/activate  # First!
pip install django        # Then install

# "No space left on device"
# Clean pip cache
pip cache purge

# Virtual environment too large
# Don't commit venv to Git
# Recreate when needed from requirements.txt

# Multiple Python versions confusion
# Use specific version
python3.10 -m venv venv
python3.11 -m venv venv

# VS Code
# 1. Install Python extension
# 2. Open Command Palette (Ctrl+Shift+P)
# 3. "Python: Select Interpreter"
# 4. Choose venv/bin/python

# .vscode/settings.json
{
    "python.defaultInterpreterPath": "${workspaceFolder}/venv/bin/python"
}

# PyCharm
# 1. File > Settings > Project > Python Interpreter
# 2. Add Interpreter > Existing Environment
# 3. Select venv/bin/python

# VS Code will now:
# - Auto-activate venv in terminal
# - Use correct Python for running/debugging
# - Provide autocomplete for installed packages