nonobie — Real dev skills, explained like a friend would

Warming up the neural circuits...

What you'll learn

By the end of this chapter you will:

Create and activate virtual environments on any OS
Install, upgrade, and uninstall packages with pip — and read version specifiers
Freeze and recreate environments with requirements.txt
Explain why global pip installs are a time bomb

The why

Two projects, two incompatible versions, one Python. Project A needs requests==2.28, Project B needs requests==2.31. Without isolation, installing one breaks the other. Virtual environments solve this by giving each project its own Python installation.

Core concepts: Virtual environments

Creating Virtual Environments

bash

# Create virtual environment
python -m venv venv
 
# Activate (macOS/Linux)
source venv/bin/activate
 
# Activate (Windows)
venv\Scripts\activate
 
# Deactivate
deactivate
 
# Verify
which python  # should point to venv/bin/python

Command	Effect
`python -m venv venv`	Create isolated Python
`source venv/bin/activate`	Activate (macOS/Linux)
`venv\Scripts\activate`	Activate (Windows)
`deactivate`	Return to system Python

pip: Installing Packages

bash

# Install
pip install requests
 
# Specific version
pip install requests==2.28.0
 
# Version specifiers
pip install "requests>=2.28,<3.0"
 
# Upgrade
pip install --upgrade requests
 
# Uninstall
pip uninstall requests

Command	Effect
`pip install pkg`	Install latest
`pip install pkg==1.0`	Install exact version
`pip install "pkg>=1.0,<2.0"`	Install range
`pip install --upgrade pkg`	Upgrade
`pip uninstall pkg`	Remove
`pip list`	List all
`pip show pkg`	Package details

requirements.txt

bash

# Save current environment
pip freeze > requirements.txt
 
# Recreate environment
pip install -r requirements.txt
 
# requirements.txt format
requests==2.28.0
numpy==1.24.0
pandas==2.0.0

Best practices:

Pin exact versions (==) for reproducibility
Use >= for minimum versions in libraries
Commit requirements.txt to version control
Don't commit the venv/ folder

Why Virtual Environments?

bash

# BAD: Global install
pip install requests  # installs globally
# Project A: requests==2.28
# Project B: requests==2.31
# Installing one breaks the other!
 
# GOOD: Virtual environment
python -m venv venv
source venv/bin/activate
pip install requests  # installs in venv only

Why global is bad:

Different projects need different versions
System Python may be needed by OS tools
No way to reproduce the environment on another machine
No way to know which project uses which packages

Mental model: Virtual environments

Think of virtual environments as separate Python installations:

Each project gets its own python and pip
Packages install in venv/lib/python3.x/site-packages/
Activating changes PATH to use the venv Python
Deactivating restores the system Python

Common mistakes

Mistake	Why it's wrong	Fix
`pip install` without venv	Installs globally	Always activate venv first
Committing `venv/` to git	Huge, machine-specific	Add `venv/` to `.gitignore`
Forgetting to activate	Installs in wrong environment	Check `which python`
`pip freeze` without venv	Captures system packages	Always run in activated venv

Practice: Prove your Mastery

beginner Challenge

3 min

Create Virtual Environment

Create a virtual environment named "myenv" and activate it.

Input— given data

data

Terminal commands

Expected Output— your code should produce this

expected

Virtual environment created and activated

Write your solutionCtrl+Enter to run

Ctrl+Enter

beginner Challenge

3 min

Install Package

Install requests version 2.28.0 in a virtual environment.

Input— given data

data

pip install requests==2.28.0

Expected Output— your code should produce this

expected

requests installed

Write your solutionCtrl+Enter to run

Ctrl+Enter

intermediate Challenge

5 min

Requirements File

Save current environment to requirements.txt and show how to recreate it.

Input— given data

data

pip freeze commands

Expected Output— your code should produce this

expected

requirements.txt created

Write your solutionCtrl+Enter to run

Ctrl+Enter

intermediate Challenge

5 min

Version Specifiers

What does "requests>=2.28,<3.0" mean?

Input— given data

data

Version specifier

Expected Output— your code should produce this

expected

Install requests 2.28 or newer, but less than 3.0

Write your solutionCtrl+Enter to run

Ctrl+Enter

intermediate Challenge

5 min

Global vs Local

Why is pip install requests without a virtual environment bad?

Input— given data

data

Explain global install problem

Expected Output— your code should produce this

expected

Breaks project isolation

Write your solutionCtrl+Enter to run

Ctrl+Enter

Interview Questions

Beginner:

"What is a virtual environment?"

An isolated Python installation for a specific project. Each project gets its own set of packages, so Project A can use requests==2.28 while Project B uses requests==2.31 without conflicts.
"How do you create and activate a virtual environment?"

Create: python -m venv venv. Activate: source venv/bin/activate (macOS/Linux) or venv\Scripts\activate (Windows). Deactivate: deactivate.

Senior:

"What's the difference between pip install and pip install -r requirements.txt?"

pip install package installs one package. installs all packages listed in the file. The file is created with and ensures reproducible environments.

Summary

Virtual environments isolate project dependencies. Create with python -m venv venv.
Activate before installing packages. Deactivate when done.
pip install package installs. pip freeze > requirements.txt saves.
pip install -r requirements.txt recreates the environment.
Never install globally — always use virtual environments.

Quick recall

text

I want to…
├── Create venv → python -m venv venv
├── Activate → source venv/bin/activate
├── Install → pip install package
├── Save deps → pip freeze > requirements.txt
├── Recreate → pip install -r requirements.txt
└── Deactivate → deactivate

Self-Check Quiz

Q1 / 3

MCQ

How do you create a virtual environment?

Chapter 7 — Virtual Environments & pip

The why

Core concepts: Virtual environments

Creating Virtual Environments

pip: Installing Packages

requirements.txt

Why Virtual Environments?

Common mistakes

Practice: Prove your Mastery

Create Virtual Environment

Install Package

Requirements File

Version Specifiers

Global vs Local

Interview Questions

Summary

Quick recall

Self-Check Quiz