How to use virtual environments in Python

Python developers can use virtual environments to isolate project dependencies and avoid conflicts between incompatible package versions. The native `venv` tool, included in all supported Python versions, creates separate instances of the Python interpreter with their own sets of installed packages. This approach solves the common problem of competing package requirements across different projects, ensuring each project has access only to its designated dependencies.

Of all the reasons Python https://www.infoworld.com/article/2253770/what-is-python-powerful-intuitive-programming.html is a hit with developers, one of the biggest is its broad and ever-expanding selection of third-party packages. Convenient toolkits for everything from ingesting and formatting data to high-speed math and machine learning https://www.infoworld.com/article/2254843/what-is-machine-learning-intelligence-derived-from-data.html are just an import or pip install away. But what happens when those packages don’t play nice with each other? What do you do when different Python projects need competing or incompatible versions of the same add-ons? That’s where Python virtual environments come into play. A virtual environment is a way to have multiple, parallel instances of the Python interpreter, each with different sets of packages and different configurations. Each virtual environment contains a discrete copy of the Python interpreter, including copies of its support utilities such as the package manager pip . The packages installed in each virtual environment are seen only in that virtual environment and no other. Even large, complex packages with platform-dependent binaries can be corralled off from each other in virtual environments. There are a few common use cases for a virtual environment: Nothing says you can’t simply unpack a Python library into a subfolder of a project and use it that way. Likewise, you could download a standalone copy of the Python interpreter, unpack it into a folder, and use it to run scripts and packages devoted to it. But managing such cobbled-together projects soon becomes difficult. It only seems easier to do that at first. Working with packages that have binary components, or that rely on elaborate third-party dependencies, can be a nightmare. Worse, reproducing such a setup on someone else’s machine, or on a new machine you manage, is tricky. The best long-term solution is to use Python’s native mechanisms for creating, reproducing, and working with virtual environments. Python has native tooling for virtual environments that makes the whole process quite simple. This wasn’t always the case, but now all supported versions of Python use the native virtual environment tool, venv. To create a virtual environment in a given directory, type: python3 -m venv /path/to/venv For instance, to create the virtual environment in the current directory, using the subdirectory .venv type: python3 -m venv .venv On Microsoft Windows, you can use py instead of python3 to reliably access an installed Python version. See this article https://www.infoworld.com/article/2265603/how-to-use-pythons-py-launcher-for-windows.html for more about using the py launcher in Windows. The exact name for the virtual environment directory is arbitrary, but it’s typically .venv. The whole process of setting up the virtual environment may take a minute or two. When it’s finished, you should see that directory with a few subdirectories in it. The most important subdirectory is bin on Unix or Scripts on Windows, which is where you’ll find the copy of the Python interpreter for the virtual environment along with its utilities. Note that because each virtual environment contains its own copy of the Python interpreter, it can be fairly large. A Python 3.13 https://www.infoworld.com/article/2337441/the-best-new-features-and-fixes-in-python-313.html virtual environment will consume anywhere from 14MB to 26MB of disk space, depending on the operating system. If you are setting up a virtual environment in a project directory managed with some kind of version control system e.g., Git https://www.infoworld.com/article/2334697/what-is-git-version-control-for-collaborative-programming.html , exclude the environment directory from tracking before you make any commits after creating the venv. Venvs should not be tracked along with their associated code, as they are meant to be destroyed and recreated as needed. You should , however, track the requirements.txt or pyproject.toml files associated with the project, as those are used to describe what gets installed in the venv for that project. Before you can use this virtual environment, you need to explicitly activate it. Activation makes the virtual environment the default Python interpreter for the duration of a shell session. You’ll need to use different syntax for activating the virtual environment depending on which operating system and command shell you’re using. source /path/to/venv/bin/activate source /path/to/venv/bin/activate.csh source /path/to/venv/bin/activate.fish path/to/venv/Scripts/Activate.bat path/to/venv/Scripts/Activate.ps1 Note that the activated environment only works for the context it was activated in . For instance, if you launch two instances of PowerShell, A and B, and you activate the virtual environment in instance A, that environment will apply only to A, not B. Many Python IDEs https://www.infoworld.com/article/2250631/review-7-python-ides-compared.html will automatically detect and activate a virtual environment if one is found in the current project directory. Visual Studio Code https://www.infoworld.com/article/2335960/what-is-visual-studio-code-microsofts-extensible-code-editor.html , for instance, can do this when the Python extension is enabled https://code.visualstudio.com/docs/python/python-tutorial . Opening a terminal inside Visual Studio Code will automatically activate the selected virtual environment. PyCharm automatically creates a virtual environment for each new project and enables it automatically. Once you’ve activated the new virtual environment, you can use the pip package manager to add and change packages for it. You’ll find pip in the Scripts subdirectory of the virtual environment on Windows, and in the bin subdirectory on Unix OSes. If you’re already familiar with the way pip works, you’re set. It should be just the same in a virtual environment. Just make sure you’re using the instance of pip that manages packages for the virtual environment in the context where it was activated—e.g., the bash session or Windows CLI/PowerShell session. If you want to verify that you’re using the right pip and the right virtual environment, type pip -V and check that the path it displays points to a subdirectory of your virtual environment. Note that when you want to upgrade pip in a virtual environment, it’s best to use the command python3 -m pip install -U pip . This ensures the upgrade process is run in such a way that Python doesn’t lock crucial files. The command pip install -U pip may not be able to complete the upgrade properly. To use the virtual environment you created to run Python scripts, simply invoke Python from the command line in the context where you activated it. For instance, to run a script, just run python3 myscript.py . With PyCharm, you can use the IDE’s own package management interface https://www.jetbrains.com/help/pycharm/installing-uninstalling-and-upgrading-packages.html to manage the packages installed in your project. When you create a new virtual environment, pip will be installed, but that’s all. You’ll need to install any other packages you want to use in the environment. For projects with complex requirements, it is customary to keep in the root of the project a requirements.txt file that lists the requirements for the project. This way, if you need to recreate the virtual environment, you can reinstall all of the needed packages with the command pip install -r requirements.txt . More recently, a new project metadata format has emerged for Python projects, called pyproject.toml https://www.infoworld.com/video/2156937/pyproject-toml-the-modern-python-project-definition-file-explained.html . A pyproject.toml file contains the package requirements of the project, but also a great deal of other information about it. To install those requirements, you’d run pip install . in the same directory as the pyproject.toml file. Note that the copy of pip that lives in a virtual environment is local to that virtual environment . Each virtual environment has its own copy, which will need to be updated and maintained independently. This is why you may get warnings about pip being out of date in some virtual environments but not others; pip has to be updated in each virtual environment separately. When you’re done using the virtual environment, you can just terminate the session where you were using it. If you want to continue to work in the same session but with the default Python interpreter instead, type deactivate at the prompt. Windows users on the Command Prompt need to run deactivate.bat from the Scripts subdirectory, but Unix users and Windows users running PowerShell can simply type deactivate in any directory. Virtual environments are self-contained. When you no longer need the virtual environment, you can simply delete its directory. Just make sure you first close any running copies of Python that use the virtual environment. Whenever you want to refresh or recreate a virtual environment, you can simply delete the current environment directory and recreate the venv as described above see “Managing packages in Python virtual environments” . This is typically the easiest way to upgrade a project to a newer version of Python: delete or temporarily re-name the venv directory in case the new version doesn’t work , then create a new one and install the project requirements into it. If you have many older projects that aren’t being actively used, you can remove their environments to save space. They can be recreated easily whenever needed, although you will want to note which version of Python was used to create them along with the needed packages typically recorded in requirements.txt or pyproject.toml . It’s tempting to assume a virtual environment can be copied and moved around along with its project. Don’t do this. Virtual environments are tied to the location of the Python installation on the system where they’re created. If you want to move the project to another system, leave out the venv directory, and recreate the venv on the target machine. Do copy and move the requirements.txt or pyproject.toml file with the project , because those files are needed to recreate the venv on the other system. With Python 2, virtual environments aren’t a native feature of the language. Instead, you need to install third-party libraries to create and manage virtual environments. The most popular and widely used of these projects is virtualenv https://pypi.python.org/pypi/virtualenv , which handles creating the directory structure and copying the needed files into a virtual environment. To install virtualenv, just use pip install virtualenv . To create a virtual environment directory with it, type virtualenv /path/to/directory . Activating and deactivating the virtual environment works the same way as it does for virtual environments in Python 3 see above . Note that Python 2 should not be used for any new development. Virtual environments in Python 2, like Python 2 itself, should be used only for the maintenance of legacy projects that should eventually be migrated to Python 3. Normally, when you create a venv, it cannot work with the packages already present in the Python installation that created it. This is by design, as it ensures that packages installed globally don’t interfere with local ones. You can override this behavior, but only when you first create a virtual environment. If you pass the flag --system-site-packages to venv when you run it, the created venv will have access to the parent Python’s package directory. If you’re using Jupyter notebooks aka IPython notebooks , and you already have Jupyter installed systemwide, create your virtual environment and activate it. Then, from your virtual environment directory, run pip install ipykernel to add the needed components for IPython. Finally, run ipython kernel install —user —name= , where project name is a name you want to associate with that particular project. From there you should be able to launch Jupyter and switch to the IPython kernel you installed inside the virtual environment. When you upgrade a Python runtime on your system, virtual environments that use that version of Python aren’t automatically upgraded. That’s your responsibility. And that’s by design, because unwitting upgrades to Python versions can break their attendant packages. If you’ve upgraded an existing Python interpreter with a minor point upgrade—e.g., from Python 3.13.1 to Python 3.13.3—you can upgrade any corresponding virtual environments easily enough. From a command prompt in the project directory, enter: python -m venv /path/to/venv --upgrade Don’t activate the virtual environment beforehand, or the upgrade may not work. Alternatively, as noted above see “Removing the Python virtual environment” , you could elect to remove the venv completely and recreate it using your requirements.txt or pyproject.toml file. If you’ve installed a major new version of Python—e.g., you already have Python 3.10 and you now install Python 3.11 alongside it—you’ll need to create a new virtual environment that specifically uses the new major point version. Do not attempt to upgrade an existing virtual environment to a higher major point version of Python.