virtualenv - Fast Python Environment Manager

Everyone knows Python's built-in venv exists, but virtualenv creates environments noticeably faster and doesn't suck when you need to work with different Python versions. I learned this the hard way after spending 3 hours debugging why my Django 4.2 project broke my Django 3.2 project.

The Problem Everyone Hits

You're working on Project A with Django 3.2. Everything works. Then you start Project B that needs Django 4.1. You pip install django==4.1 and suddenly Project A won't start because some middleware changed. Sound familiar?

Without virtual environments, you get dependency hell. Your system Python becomes a clusterfuck of conflicting packages. I've seen senior developers completely reinstall Python trying to fix this mess.

Why virtualenv Beats venv

Speed: Creating environments is noticeably faster than venv - maybe 3x faster on my machine. Our CI builds went from 45 seconds to maybe 25 seconds just from this switch. virtualenv caches stuff locally instead of hitting PyPI every time.

Multi-Python Support: Need Python 3.11 for one project and 3.9 for another? virtualenv handles this. venv only works with whatever Python version you're running.

Updates: virtualenv updates with pip install --upgrade virtualenv. venv updates when Python updates, which is never when you need it.

Real Version Numbers That Matter

Latest virtualenv is 20.34.0 as of August 2025. They dropped Python 3.7 support in 2024 with version 20.27.0 because CI environments stopped supporting it anymore. If you're still on Python 3.7 in 2025, what are you even doing?

Oh, and stay away from virtualenv 20.0.22 - it was yanked for breaking plugins. That version was a clusterfuck for about two weeks until they fixed it in 20.0.23.

Python 3.8+ is supported, but honestly, if you're not on 3.10+ by now, upgrade already.

When I Actually Use It

I create virtualenvs for every single Python project. Period. Even stupid one-file scripts. Because I learned that "quick script that doesn't need packages" turns into "production system with 15 dependencies" faster than you can say "technical debt."

For CI/CD, the speed difference matters. Our builds got way faster after switching from venv to virtualenv. Not gonna lie and say I timed it scientifically, but it's the difference between 'wait for it' and 'done already'.

But knowing why virtualenv matters is just the start. Every developer hits the same basic problems when getting started.

Install It The Right Way

Don't overthink this. Just run:

pip install virtualenv

Everyone suggests pipx. I just use pip because it works and I don't need the complexity unless something's actually broken.

Windows PATH Hell (The Real Problem)

Windows users, you will hit this. After installing, you'll get "virtualenv: command not found" because Windows doesn't add Python Scripts to PATH automatically.

Quick fix: Use python -m virtualenv myenv instead of virtualenv myenv. Works every time.

Permanent fix: Add C:\Users\YourName\AppData\Roaming\Python\Python311\Scripts to your PATH environment variable. Replace "Python311" with your actual version.

Creating Your First Environment

## Basic environment
virtualenv myproject

## Specific Python version (if you have multiple)
virtualenv -p python3.11 myproject

## If you're paranoid about the name
virtualenv venv

I use venv as the directory name for every project. It's in my muscle memory now.

Activation That Actually Works

Linux/Mac:

source venv/bin/activate

Windows Command Prompt:

virtualenv\Scripts\activate

Windows PowerShell (if you hate yourself):

virtualenv\Scripts\Activate.ps1

Note: PowerShell might block this with execution policy bullshit. Run Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser once to fix it.

Shit That Will Break

Don't put virtualenvs on network drives - I did this at my first job because our home directories were on NFS. It took forever to create an environment and I couldn't figure out why until someone told me network latency kills everything. Felt like an idiot for a week.

Don't use --system-site-packages - This lets your virtualenv access globally installed packages. Sounds convenient, breaks everything mysteriously in production.

Don't forget your .gitignore:

venv/
env/
.venv/
*.pyc
__pycache__/

Don't try to move virtualenvs - They have hardcoded paths everywhere. I once spent an entire Saturday debugging why my environments weren't working. Turns out I had virtualenvs nested inside other virtualenvs because I was an idiot. Just delete and recreate when you need to move stuff.

Speed It Up

If virtualenv creation is slow on Windows, it's probably your antivirus. Add your Python directory to Windows Defender exclusions. Made virtualenv creation way faster - from 'time to get coffee' slow to actually usable.

Real CI/CD Usage

Skip the fancy GitHub Actions examples. This actually works:

python -m pip install virtualenv
virtualenv venv
source venv/bin/activate  # or venv\Scripts\activate on Windows
pip install -r requirements.txt

That's it. No fancy YAML, no weird edge cases. Just works.