Python Basics — Virtual Environments & pip

Virtual Environments & pip

Different projects need different packages — and sometimes different versions of the same package. Virtual environments solve this by giving each project its own isolated Python installation.

Why Virtual Environments Matter

Without virtual environments:

Project A needs requests 2.28
Project B needs requests 2.31
→ They conflict — one will break!

With virtual environments:

Project A → venv_a/ → requests 2.28 ✅
Project B → venv_b/ → requests 2.31 ✅
→ Each project has its own packages, no conflicts

Creating a Virtual Environment

# Navigate to your project directory
cd my-project

# Create a virtual environment
python -m venv venv

# This creates:
# venv/
# ├── bin/         (or Scripts/ on Windows)
# ├── include/
# ├── lib/
# └── pyvenv.cfg

Activating / Deactivating

# macOS / Linux
source venv/bin/activate

# Windows (Command Prompt)
venv\Scripts\activate

# Windows (PowerShell)
venv\Scripts\Activate.ps1

When activated, your prompt changes:

(venv) $ python --version
Python 3.12.0

(venv) $ which python
/Users/you/my-project/venv/bin/python

To deactivate:

(venv) $ deactivate
$   # back to normal prompt

pip — Installing Packages

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

Install a Package

(venv) $ pip install requests
# Downloading requests-2.31.0...
# Successfully installed requests-2.31.0

(venv) $ pip install flask==2.3.0   # specific version
(venv) $ pip install "django>=4.0"  # minimum version

Use the Package

import requests

response = requests.get("https://api.github.com")
print(response.status_code)  # 200
print(response.json()["current_user_url"])

Uninstall a Package

(venv) $ pip uninstall requests

List Installed Packages

(venv) $ pip list
Package    Version
---------- -------
pip        23.0
requests   2.31.0
certifi    2023.7.22
...

(venv) $ pip show requests
Name: requests
Version: 2.31.0
Summary: Python HTTP for Humans.
Location: /Users/you/venv/lib/python3.12/site-packages
Requires: certifi, charset-normalizer, idna, urllib3

pip freeze and requirements.txt

Saving Dependencies

# Save all installed packages and versions
(venv) $ pip freeze > requirements.txt

# requirements.txt
certifi==2023.7.22
charset-normalizer==3.3.0
idna==3.4
requests==2.31.0
urllib3==2.0.7

Installing from requirements.txt

When someone else clones your project:

# Create and activate a new virtualenv
python -m venv venv
source venv/bin/activate

# Install all dependencies
pip install -r requirements.txt

Common pip Commands

Command	Purpose
pip install package	Install a package
pip install package==1.0	Install specific version
pip install -r requirements.txt	Install from requirements file
pip uninstall package	Remove a package
pip list	List installed packages
pip show package	Show package details
pip freeze	Output installed packages in requirements format
pip install --upgrade package	Upgrade a package
pip install --upgrade pip	Upgrade pip itself

pip vs pipenv vs poetry

Tool	What It Does	Best For
pip + venv	Basic package management	Learning, simple projects
pipenv	pip + venv combined, with Pipfile	Medium projects
poetry	Full dependency management + packaging	Professional projects
conda	Package + environment manager	Data science

pipenv Quick Look

pip install pipenv

cd my-project
pipenv install requests    # creates Pipfile + virtualenv
pipenv shell               # activate
pipenv install --dev pytest  # dev dependency

poetry Quick Look

pip install poetry

poetry new my-project      # scaffold a project
cd my-project
poetry add requests        # install + add to pyproject.toml
poetry install             # install all dependencies
poetry shell               # activate

Best Practices

1. Always use a virtual environment — never install packages globally
2. Keep requirements.txt updated — run pip freeze > requirements.txt after installing packages
3. Add venv/ to .gitignore — don’t commit the virtual environment
4. Pin versions in production — requests==2.31.0, not just requests
5. Separate dev dependencies — use requirements-dev.txt for testing tools

.gitignore Entry

# Virtual environment
venv/
.venv/
env/

# Python
__pycache__/
*.pyc
*.pyo

Summary

• Virtual environments isolate project dependencies — one venv per project
• Create with python -m venv venv, activate with source venv/bin/activate
• pip installs packages from PyPI: pip install package
• Freeze dependencies: pip freeze > requirements.txt
• Restore dependencies: pip install -r requirements.txt
• Never install packages globally — always use a virtual environment
• For professional projects, consider poetry or pipenv

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Next up: Modules & Packages — Practice Exercises →