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Python Notes : Virtual Environments and Packages
Introduction
Python applications will often use packages and modules that don't come as part of the standard library
apps will sometimes need a specific version of a library
an application may require that a particular bug has been fixed
an app may be written using an obsolete version of the library's interface

it may not be possible for one Python installation to meet the requirements of every application
if app A needs version 1.0 of a particular module but app B needs version 2.0 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
a virtual environment is a self-contained directory tree that contains a specific 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 virtual environment with version 2.0
If application B requires a library be upgraded to version 3.0, this will not affect application A's environment.

Creating Virtual Environments
The module used to create and manage virtual environments is called venv
venv will install the Python version from which the command was run (as reported by the --version option)
executing the command with python3.12 will install version 3.12

to create a virtual environment

  1. decide upon a directory location
  2. run the venv module as a script with the directory path
    Note : run from OS-level shell aka DOS prompt 
python -m venv e:\python\tutorial-env
will create the tutorial-env directory if it doesn’t exist
also create directories inside it containing a copy of the Python interpreter and various supporting files
activate environment on Windows from DOS prompt 
# <path>\tutorial-env\Scripts\activate
e:\python\tutorial-env\Scripts\activate
to enter the interpreter enter py at the prompt
to deactivate the environment enter deactivate at the DOS prompt
Managing Packages with pip
can install, upgrade, and remove packages using pip
by default pip will install packages from the Python Package Index
pip has a number of subcommands: “install”, “uninstall”, “freeze”, etc.
consult the Installing Python Modules guide for complete documentation for pip

install the latest version of a package 

(tutorial-env) $ python -m 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
can also install a specific version of a package by giving the package name followed by == and the version number 
(tutorial-env) $ python -m 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
to upgrade the package to the latest version 
(tutorial-env) $ python -m 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
to remove one or more packages from the virtual environment 
(tutorial-env) $ python -m pip uninstall <packageName0>,...,<packageNameX>
to display information about a particular package 
(tutorial-env) $ python -m 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: [email protected]
License: Apache 2.0
Location: /Users/akuchling/envs/tutorial-env/lib/python3.4/site-packages
Requires:
display all of the packages installed in the virtual environment 
(tutorial-env) $ python -m pip list
novas (3.1.1.3)
numpy (1.9.2)
pip (7.0.3)
requests (2.7.0)
setuptools (16.0)
python -m pip freeze will produce a similar list of the installed packages
the output uses the format that python -m pip install expects
common convention is to put this list in a requirements.txt file 
(tutorial-env) $ python -m pip freeze > requirements.txt
(tutorial-env) $ cat requirements.txt
novas==3.1.1.3
numpy==1.9.2
requests==2.7.0
requirements.txt can be committed to version control and shipped as part of an app
users can install all the necessary packages with install -r 
(tutorial-env) $ python -m 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
to make a package available on the Python Package Index consult the Python packaging user guide
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