Python Packaging & Environments

Table of Contents

1. What is pip?
2. What is venv?
3. What is PyPI?
4. How They Work Together
5. Creating and Using Virtual Environments
6. Activation (Windows vs Linux/macOS)
7. Installing Packages into a venv
8. Deactivating and Deleting Environments
9. Requirements Files
10. Best Practices
11. Cheat Sheet

1. What is pip?

pip is Python’s package manager.

It allows you to:

• Install libraries
• Upgrade libraries
• Remove libraries
• Manage dependencies

Example:

pip install requests

2. What is venv?

venv (Virtual Environment) is a tool that creates isolated Python environments.

Why?

• Prevent dependency conflicts
• Use different versions of packages per project
• Keep your global Python clean

Each venv contains:

• Its own Python interpreter
• Its own pip
• Its own installed packages

3. What is PyPI?

PyPI (Python Package Index) is the central repository of Python packages.

Website: https://pypi.org Contains thousands of libraries pip downloads packages from PyPI by default

4. How They Work Together

Flow:

PyPI (online registry)
        ↓
     pip install
        ↓
   Virtual Environment (venv)
        ↓
   Your Python Project

Explanation:

You run pip install flask pip connects to PyPI Downloads the package Installs it inside your active venv

Important:

• a. If no venv is active → installs globally
• b. If venv is active → installs locally inside it

5. Creating a Virtual Environment

Create a venv

python -m venv myenv

This creates a folder:

myenv/ ├── bin/ (Linux/macOS) ├── Scripts/ (Windows) ├── lib/ ├── pyvenv.cfg

6. Activating the Virtual Environment

On Windows (CMD)

myenv\Scripts\activate

On Windows (PowerShell)

myenv\Scripts\Activate.ps1

On Linux/macOS

source myenv/bin/activate

7. What Happens When You Activate?

• Your shell PATH is modified
• python now points to the venv interpreter
• pip installs inside the venv

You will see:

(myenv) user@machine:~

8. Installing Packages into the venv

pip install flask
pip install requests

Check installed packages:

pip list

Show details:

pip show flask

9. Deactivating the Environment

deactivate

This returns you to the global Python environment.

10. Deleting a Virtual Environment

Just delete the folder:

rm -rf myenv      # Linux/macOS
rmdir /s myenv    # Windows

11. Requirements Files

Save dependencies:

pip freeze > requirements.txt

Install from file:

pip install -r requirements.txt

Example requirements.txt:

flask==2.3.2
requests==2.31.0

12. Best Practices

• Always use venv per project
• Never install packages globally unless necessary
• Use requirements.txt
• Name environments clearly (venv, .venv, env)
• Add venv to .gitignore

Example .gitignore:

venv/
.env/

13. Cheat Sheet

```bash

Create venv

python -m venv myenv #Activate #Windows CMD: myenv\Scripts\activate

#Windows PowerShell:
myenv\Scripts\Activate.ps1

#Linux/macOS:
source myenv/bin/activate

#Install package pip install package_name

#Upgrade package pip install –upgrade package_name

#Uninstall package pip uninstall package_name

#List packages pip list

#Freeze dependencies pip freeze > requirements.txt

#Install from requirements pip install -r requirements.txt

#Deactivate venv deactivate