virtualenv

Virtualenv is a software tool used to create isolated Python environments. Each environment has its own independent Python interpreter and a separate set of installed packages, which helps prevent dependency conflicts between projects and enables reproducible deployments. Virtualenv has historically supported multiple Python versions and both Python 2 and Python 3, which contributed to its broad adoption.

How it works: Running virtualenv <env> creates a new directory containing a Python executable and a local site-packages directory. Activating the environment adjusts the shell’s PATH and related variables so that commands such as python and pip operate within that environment. This isolation allows project-specific packages to be installed without affecting the system Python. The activation script differs by platform: on Unix-like systems it is sourced from env/bin/activate, while on Windows it is typically env\Scripts\activate.bat or, for PowerShell, a corresponding script.

Usage: Virtualenv can be installed via a package installer, commonly pip install virtualenv. To create an environment, use virtualenv myenv. Activate the environment with source myenv/bin/activate on Unix-like systems or myenv\Scripts\activate on Windows. After activation, packages can be installed with pip, and separate requirements for the project can be captured with pip freeze. Deactivate returns you to the global Python environment.

Key options include selecting the Python interpreter with -p or --python to point to a specific python executable, or enabling access to the system-wide site-packages with --system-site-packages. While Python’s standard library provides a module called venv for creating environments, virtualenv remains a widely used alternative due to its broader compatibility and flexible options.