pip, Virtual Environments, and Project Structure - Python Mastery

pip: Python's Package Manager

pip installs packages from PyPI (Python Package Index) — over 450,000 packages. It's the primary way to add third-party libraries to your projects.

# Install a package
pip install requests

# Install specific version
pip install requests==2.31.0

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

# Upgrade a package
pip install --upgrade requests

# Uninstall
pip uninstall requests

# List installed packages
pip list
pip freeze                      # With exact versions (good for requirements.txt)

# Generate requirements file
pip freeze > requirements.txt

# Show package info
pip show requests

# Search (limited without PyPI web)
pip index versions requests     # All available versions

pip vs pip3

# On some systems, pip points to Python 2's pip
# Always use pip3 if unsure, or use:
python3 -m pip install requests   # Guaranteed to use Python 3's pip

Virtual Environments

Virtual environments isolate project dependencies. Each project gets its own Python installation and packages — no version conflicts between projects.

# Create a virtual environment
python3 -m venv venv     # Creates venv/ directory

# Activate
source venv/bin/activate         # macOS/Linux
.\venv\Scripts\activate         # Windows PowerShell
source venv/Scripts/activate     # Windows Git Bash

# Your prompt changes to: (venv) $
# Now you're working in the isolated environment

# Install packages (isolated to this project)
pip install flask requests pandas

# Deactivate
deactivate

# Delete environment (just delete the folder)
rm -rf venv/

pyproject.toml: Modern Project Configuration

# pyproject.toml — modern way to configure Python projects
[build-system]
requires = ["setuptools>=61.0"]
build-backend = "setuptools.backends.legacy:build"

[project]
name = "my-project"
version = "1.0.0"
description = "A cool Python project"
requires-python = ">=3.9"
dependencies = [
    "requests>=2.28.0",
    "flask>=2.3.0",
]

[project.optional-dependencies]
dev = [
    "pytest>=7.0",
    "black>=23.0",
    "mypy>=1.0",
]

[tool.black]
line-length = 88

[tool.mypy]
python_version = "3.11"
strict = true

Project Structure Best Practices

# Professional Python project structure
my-project/
├── src/
│   └── mypackage/
│       ├── __init__.py
│       ├── main.py
│       └── utils.py
├── tests/
│   ├── __init__.py
│   ├── test_main.py
│   └── test_utils.py
├── docs/
├── .gitignore
├── pyproject.toml
├── README.md
└── requirements.txt

# .gitignore for Python projects
venv/
__pycache__/
*.pyc
*.pyo
.env
.pytest_cache/
*.egg-info/
dist/
build/

Key Takeaways

One virtual environment per project: activate before working, every time
requirements.txt = reproducible installs: commit this to git
pip freeze > requirements.txt: capture exact versions
pyproject.toml: modern way to configure builds, linters, type checkers
python3 -m pip: use this form when multiple Python versions are installed

Practice Exercises

Create a new project directory, set up a virtual environment, and install 3 packages. Generate requirements.txt.
Create a minimal pyproject.toml for a hypothetical project with 2 dependencies.
Install a package, use it in a script, then create a fresh virtual environment and verify the script fails, then install from requirements.txt and verify it works.

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