12. Ambientes virtuais e pacotes

12.1. Introdução

Aplicações em Python normalmente usam pacotes e módulos que não vêm como parte da instalação padrão. Aplicações as vezes necessitam uma versão específica de uma biblioteca, porque ela requer que algum problema em particular tenha sido consertado ou foi escrita utilizando-se de uma versão obsoleta da interface da biblioteca.

Isso significa que talvez não seja possível que uma instalação Python preencha os requisitos de qualquer aplicação. Se uma aplicação A necessita a versão 1.0 de um módulo particular mas a aplicação B necessita a versão 2.0, os requisitos entrarão em conflito e instalar qualquer uma das duas versões 1.0 ou 2.0 fará com que uma das aplicações não consiga executar.

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. Criando ambientes virtuais

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

Isso irá criar o diretório tutorial-env se ele não existir e também criará diretórios dentro dele contendo uma cópia do interpretador Python, a biblioteca padrão e diversos arquivos de suporte.

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

No Windows, execute:

tutorial-env/Scripts/activate

No Unix ou no MacOS, executa:

source tutorial-env/bin/activate

(Este script é escrito para o shell bash. Se você usa shells csh ou fish, existem scripts alternativos activate.csh e activate.fish para utilização.)

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. Gerenciando pacotes com o 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 tem uma série de subcomandos: “search”, “install”, “uninstall”, “freeze”, etc. (Consulte o guia Instalando Módulos Python para a documentação completa do 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 seguido do nome de um ou mais pacotes irá remover os pacotes do ambiente virtual.

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 tem inúmeras outras opções. Consulte o guia Instalando Módulos Python para a documentação completa do pip. Quando você escrever um pacote e desejar deixá-lo disponível no Python Package Index, consulte o guia Distribuindo Módulos Python.