Virtual environments

Isolate each project's dependencies with venv and pip, and record them in requirements.txt.

Lesson 12 of 120% of the track complete

What you'll learn

What a virtual environment is, how to create and activate one, install packages with pip, and share dependencies with requirements.txt.

Why it matters

Project A needs version 1 of a library and project B needs version 2. If everything installs into one place they clash. A virtual environment gives each project its own private set of packages, so nothing breaks when you start another project.

Explanation

A virtual environment (venv) is a folder that contains its own copy of the Python interpreter and its own installed packages. You switch it on (activate) for a work session.

Commands are typed in a terminal on your own computer.

Add .venv/ to .gitignore: the environment is rebuilt from requirements.txt, not committed.

Example

A requirements.txt looks like this. This script reads it and prints what would be installed:

Step-by-step walkthrough

Each line of requirements.txt names a package, optionally with a version rule.
== pins an exact version, >= sets a minimum, and a bare name means any version.
The script skips blank lines and comments, then splits each line at the operator.
It prints requests: ==2.31.0, flask: >=3.0 and numpy: any version, then 3 packages.
pip install -r requirements.txt does the real work of installing these on your machine.

Try it yourself

Add a line pandas<=2.2 and confirm it is read. Then add a comment line starting with #.

Read a requirements filePython

Ctrl+Enter to run

Common mistakes

Installing packages globally and creating version clashes. Activate the venv first.
Committing .venv to git.
Forgetting to update requirements.txt after installing something.
Running pip from a different Python than the one in your environment. Use python -m pip.

Quick quiz

Question 1 of 3

What does a virtual environment do?

Gives a project its own isolated packagesMakes Python fasterRuns code in the cloud

Question 2 of 3

How do you create one?

python -m venv .venvpip create venvpython venv.py

Question 3 of 3

What does pip install -r requirements.txt do?

Installs every package listed in the fileCreates the fileUninstalls all packages

Summary

A venv gives each project its own packages.
Create with python -m venv, activate, then pip install.
Share dependencies through requirements.txt and keep .venv out of git.

You finished the Python track. Try the practice exercises to lock it in.

Terminal

# 1. create (once per project)
python -m venv .venv

# 2. activate
#    Windows PowerShell:  .venv\Scripts\Activate.ps1
#    macOS / Linux:       source .venv/bin/activate

# 3. install packages
pip install requests

# 4. record them
pip freeze > requirements.txt

# 5. someone else recreates the same setup
pip install -r requirements.txt

# 6. leave the environment
deactivate

Python

requirements = """
requests==2.31.0
flask>=3.0
numpy
"""

packages = {}
for line in requirements.strip().splitlines():
    line = line.strip()
    if not line or line.startswith("#"):
        continue
    for op in ("==", ">=", "<="):
        if op in line:
            name, version = line.split(op)
            packages[name] = op + version
            break
    else:
        packages[line] = "any version"

for name, rule in packages.items():
    print(f"{name}: {rule}")
print(len(packages), "packages")