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# 12. Virtual Environments and Packages

## 12.1. Introduction

Python applications will often use packages and modules that don’t come as part of the standard library. Applications will sometimes need a specific version of a library, because the application may require that a particular bug has been fixed or the application may be written using an obsolete version of the library’s interface.

This means it may not be possible for one Python installation to meet the requirements of every application. If application A needs version 1.0 of a particular module but application B needs version 2.0, then the requirements are in conflict and installing either version 1.0 or 2.0 will leave one application unable to run.

The solution for this problem is to create a virtual environment (often shortened to “virtualenv”), a self-contained directory tree that contains a Python installation for a particular version of Python, plus a number of additional packages.

Different applications can then use different virtual environments. To resolve the earlier example of conflicting requirements, application A can have its own virtual environment with version 1.0 installed while application B has another virtualenv with version 2.0. If application B requires a library be upgraded to version 3.0, this will not affect application A’s environment.

## 12.2. Creating Virtual Environments

The script used to create and manage virtual environments is called pyvenv. pyvenv will usually install the most recent version of Python that you have available; the script is also installed with a version number, so if you have multiple versions of Python on your system you can select a specific Python version by running `pyvenv-3.4` or whichever version you want.

To create a virtualenv, decide upon a directory where you want to place it and run pyvenv with the directory path:

    pyvenv tutorial-env

This will create the `tutorial-env` directory if it doesn’t exist, and also create directories inside it containing a copy of the Python interpreter, the standard library, and various supporting files.

Once you’ve created a virtual environment, you need to activate it.

On Windows, run:

    tutorial-env/Scripts/activate

On Unix or MacOS, run:

    source tutorial-env/bin/activate

(This script is written for the bash shell. If you use the csh or fish shells, there are alternate `activate.csh` and `activate.fish` scripts you should use instead.)

Activating the virtualenv will change your shell’s prompt to show what virtualenv you’re using, and modify the environment so that running `python` will get you that particular version and installation of Python. For example:

    -> source ~/envs/tutorial-env/bin/activate
    (tutorial-env) -> python
    Python 3.4.3+ (3.4:c7b9645a6f35+, May 22 2015, 09:31:25)
      ...
    >>> import sys
    >>> sys.path
    ['', '/usr/local/lib/python34.zip', ...,
    '~/envs/tutorial-env/lib/python3.4/site-packages']
    >>>

## 12.3. Managing Packages with pip

Once you’ve activated a virtual environment, you can install, upgrade, and remove packages using a program called pip. By default `pip` will install packages from the Python Package Index, <https://pypi.python.org/pypi>. You can browse the Python Package Index by going to it in your web browser, or you can use `pip`‘s limited search feature:

    (tutorial-env) -> pip search astronomy
    skyfield               - Elegant astronomy for Python
    gary                   - Galactic astronomy and gravitational dynamics.
    novas                  - The United States Naval Observatory NOVAS astronomy library
    astroobs               - Provides astronomy ephemeris to plan telescope observations
    PyAstronomy            - A collection of astronomy related tools for Python.
    ...

`pip` has a number of subcommands: “search”, “install”, “uninstall”, “freeze”, etc. (Consult the Installing Python Modules guide for complete documentation for `pip`.)

You can install the latest version of a package by specifying a package’s name:

    -> pip install novas
    Collecting novas
      Downloading novas-3.1.1.3.tar.gz (136kB)
    Installing collected packages: novas
      Running setup.py install for novas
    Successfully installed novas-3.1.1.3

You can also install a specific version of a package by giving the package name followed by `==` and the version number:

    -> pip install requests==2.6.0
    Collecting requests==2.6.0
      Using cached requests-2.6.0-py2.py3-none-any.whl
    Installing collected packages: requests
    Successfully installed requests-2.6.0

If you re-run this command, `pip` will notice that the requested version is already installed and do nothing. You can supply a different version number to get that version, or you can run `pip install --upgrade` to upgrade the package to the latest version:

    -> pip install --upgrade requests
    Collecting requests
    Installing collected packages: requests
      Found existing installation: requests 2.6.0
        Uninstalling requests-2.6.0:
          Successfully uninstalled requests-2.6.0
    Successfully installed requests-2.7.0

`pip uninstall` followed by one or more package names will remove the packages from the virtual environment.

`pip show` will display information about a particular package:

    (tutorial-env) -> pip show requests
    ---
    Metadata-Version: 2.0
    Name: requests
    Version: 2.7.0
    Summary: Python HTTP for Humans.
    Home-page: http://python-requests.org
    Author: Kenneth Reitz
    Author-email: me@kennethreitz.com
    License: Apache 2.0
    Location: /Users/akuchling/envs/tutorial-env/lib/python3.4/site-packages
    Requires:

`pip list` will display all of the packages installed in the virtual environment:

    (tutorial-env) -> pip list
    novas (3.1.1.3)
    numpy (1.9.2)
    pip (7.0.3)
    requests (2.7.0)
    setuptools (16.0)

`pip freeze` will produce a similar list of the installed packages, but the output uses the format that `pip install` expects. A common convention is to put this list in a `requirements.txt` file:

    (tutorial-env) -> pip freeze > requirements.txt
    (tutorial-env) -> cat requirements.txt
    novas==3.1.1.3
    numpy==1.9.2
    requests==2.7.0

The `requirements.txt` can then be committed to version control and shipped as part of an application. Users can then install all the necessary packages with `install -r`:

    -> pip install -r requirements.txt
    Collecting novas==3.1.1.3 (from -r requirements.txt (line 1))
      ...
    Collecting numpy==1.9.2 (from -r requirements.txt (line 2))
      ...
    Collecting requests==2.7.0 (from -r requirements.txt (line 3))
      ...
    Installing collected packages: novas, numpy, requests
      Running setup.py install for novas
    Successfully installed novas-3.1.1.3 numpy-1.9.2 requests-2.7.0

`pip` has many more options. Consult the Installing Python Modules guide for complete documentation for `pip`. When you’ve written a package and want to make it available on the Python Package Index, consult the Distributing Python Modules guide.

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