Virtual Environments¶

A virtual environment is an isolated Python installation. Each project gets its own set of packages, so they don't conflict with each other.

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Visualize It¶

See how Python finds packages via sys.path — the key to understanding environments: Open in Python Tutor

Why virtual environments matter¶

Without virtual environments:

Project A needs requests 2.28
Project B needs requests 2.31
→ Installing one breaks the other!

With virtual environments:

Project A → .venv/ → requests 2.28 (isolated)
Project B → .venv/ → requests 2.31 (isolated)
→ Both work fine!

Creating a virtual environment¶

# Navigate to your project folder.
cd my_project

# Create a virtual environment named ".venv".
python -m venv .venv

This creates a .venv/ folder containing a private copy of the Python interpreter and pip.

Activating it¶

You must activate the environment before using it:

# macOS / Linux
source .venv/bin/activate

# Windows (Command Prompt)
.venv\Scripts\activate

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

# Windows (Git Bash)
source .venv/Scripts/activate

When activated, your terminal prompt changes:

(.venv) $ python --version

Installing packages¶

With the environment activated, pip installs packages into .venv/ only:

pip install requests
pip install flask
pip install pandas

requirements.txt¶

Save your project's dependencies:

# Save current packages.
pip freeze > requirements.txt

# Install from requirements file (on another machine or in CI).
pip install -r requirements.txt

Example requirements.txt:

requests>=2.31
flask>=3.0
pandas>=2.2

Deactivating¶

deactivate

The workflow¶

Every time you start a new project:

# 1. Create the project folder.
mkdir my_project && cd my_project

# 2. Create a virtual environment.
python -m venv .venv

# 3. Activate it.
source .venv/bin/activate    # or .venv\Scripts\activate on Windows

# 4. Install packages.
pip install requests flask

# 5. Save dependencies.
pip freeze > requirements.txt

# 6. Work on your project...

# 7. When done, deactivate.
deactivate

.gitignore¶

Never commit the .venv/ folder to git. Add it to .gitignore:

.venv/
venv/

Other developers recreate it from requirements.txt.

Common mistakes¶

Installing globally:

pip install requests    # Without activating venv → installs globally!

Always check your prompt for (.venv) before installing.

Committing .venv to git: The .venv/ folder is large and machine-specific. Only commit requirements.txt.

Wrong Python version:

python3 -m venv .venv    # Use python3 explicitly if needed

Practice¶

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