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Lecture 13: Pip & Virtual Envs

When building Python applications, you will often need to use third-party packages that are not part of the standard library. To manage these libraries, we use Pip and Virtual Environments.

1. Third-Party Packages and PyPI

The Python Package Index (PyPI) is a repository of software for the Python programming language. Developers publish libraries here, and you can download them using pip (Python's package installer).

2. Why Virtual Environments?

A virtual environment is a self-contained directory tree that contains a Python installation for a particular version of Python, plus a number of additional packages.

The Problem:

If you install all packages globally, different projects might require different versions of the same package (e.g., Project A needs Django 3.2, Project B needs Django 4.0). This leads to dependency conflicts.

The Solution:

Use a virtual environment for each project to isolate its dependencies.

3. Managing Virtual Environments

Creating a Virtual Environment

Run this command in your project directory:

python -m venv .venv

This creates a folder named .venv containing a local copy of Python and pip.

Activating the Environment

Before installing packages, you must activate the virtual environment:

  • Windows (Command Prompt): 
    .venv\Scripts\activate.bat
  • Windows (PowerShell): 
    .venv\Scripts\activate.ps1
  • macOS / Linux: 
    source .venv/bin/activate

Once activated, your terminal prompt will be prefixed with (.venv).

Deactivating

When you're done working, deactivate it:

deactivate

4. Using Pip to Manage Packages

With your virtual environment activated, you can install packages:

# Install a package
pip install requests

# Install a specific version
pip install requests==2.31.0

# Upgrade a package
pip install --upgrade requests

# Uninstall a package
pip install -y requests

5. Requirements Files

To share projects, you should document the dependencies. We do this by creating a requirements.txt file.

# Generate requirements.txt
pip freeze > requirements.txt

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

🧠 Try It Yourself

  1. Create a virtual environment named .venv_test in a temporary folder.
  2. Activate it, install the requests library, and check its version using pip show requests.
  3. Deactivate the environment and delete the folder.