pip
===

pip installs Python packages.  It is a replacement for
``easy_install``.  Documentation follows; to download `visit the PyPI
entry <http://pypi.python.org/pypi/pip>`_ (or the `repository
<http://bitbucket.org/ianb/pip/>`_).

.. toctree::

   news
   requirement-format

.. comment: split here

Introduction
------------

pip is a replacement for `easy_install
<http://peak.telecommunity.com/DevCenter/EasyInstall>`_.  It uses mostly the
same techniques for finding packages, so packages that were made
easy_installable should be pip-installable as well.

pip is meant to improve on easy_install.  Some of the improvements:

* All packages are downloaded before installation.  Partially-completed
  installation doesn't occur as a result.

* Care is taken to present useful output on the console.

* The reasons for actions are kept track of.  For instance, if a package is
  being installed, pip keeps track of why that package was required.

* Error messages should be useful.

* The code is relatively concise and cohesive, making it easier to use
  programmatically.

* Packages don't have to be installed as egg archives, they can be installed
  flat (while keeping the egg metadata).

* Native support for other version control systems (Git, Mercurial and Bazaar)

* Maybe uninstallation will get added. (It might get added to easy_install,
  but I think the chance for pip is higher.)

* Simple to define fixed sets of requirements and reliably reproduce a
  set of packages.

pip is complementary with `virtualenv
<http://pypi.python.org/pypi/virtualenv>`_, and it is encouraged that you use
virtualenv to isolate your installation.

Community
---------

The homepage for pip is temporarily located `on PyPI
<http://pypi.python.org/pypi/pip>`_ -- a more proper homepage will
follow.  Bugs can go on the `pip Trac instance
<http://oss.openplans.org/pip/>`_.  Discussion should happen on the
`virtualenv email group
<http://groups.google.com/group/python-virtualenv?hl=en>`_.

Differences From easy_install
-----------------------------

pip cannot install some packages.  Specifically:

* It cannot install from eggs.  It only installs from source.  (Maybe this will
  be changed sometime, but it's low priority.)

* It doesn't understand Setuptools extras (like ``package[test]``).  This should
  be added eventually.

* It is incompatible with some packages that customize distutils or setuptools
  in their ``setup.py`` files.

* Maybe it doesn't work on Windows.  At least, the author doesn't test on
  Windows often.

* It also has some extra features.  Extra features the author thinks are great.

.. _`requirements file`:

Requirements Files
------------------

When installing software, and Python packages in particular, it's common that
you get a lot of libraries installed.  You just did ``easy_install MyPackage``
and you get a dozen packages.  Each of these packages has its own version.

Maybe you ran that installation and it works.  Great!  Will it keep working? 
Did you have to provide special options to get it to find everything?  Did you
have to install a bunch of other optional pieces?  Most of all, will you be able
to do it again?

If you've ever tried to setup an application on a new system, or with slightly
updated pieces, and had it fail, pip requirements are for you.  If you
haven't had this problem then you will eventually, so pip requirements are
for you too -- requirements make explicit, repeatable installation of packages.

So what are requirements files?  They are very simple: lists of packages to
install.  Instead of running something like ``pip MyApp`` and getting
whatever libraries come along, you can create a requirements file something like::

    MyApp
    Framework==0.9.4
    Library>=0.2

Then, regardless of what MyApp lists in ``setup.py``, you'll get a specific
version of Framework and at least the 0.2 version of Library.  (You might think
you could list these specific versions in ``setup.py`` -- try it and you'll
quickly see why that doesn't work.)  You can add optional libraries and support
tools that MyApp doesn't strictly require.

You can also include "editable" packages -- packages that are checked out from
Subversion, Git, Mercurial and Bazaar.  These are just like using the ``-e``
option to pip.  They look like::

    -e svn+http://myrepo/svn/MyApp#egg=MyApp

You have to start the URL with ``svn+`` (``git+``, ``hg+`` or ``bzr+``), and
you have to include ``#egg=Package`` so pip knows what to expect at that URL.
You can also include ``@rev`` in the URL, e.g., ``@275`` to check out
revision 275.

Freezing Requirements
---------------------

So you have a working set of packages, and you want to be able to install them
elsewhere.  `Requirements files`_ let you install exact versions, but it won't
tell you what all the exact versions are.

To create a new requirements file from a known working environment, use::

    $ pip freeze > stable-req.txt

This will write a listing of *all* installed libraries to ``stable-req.txt``
with exact versions for every library.  You may want to edit the file down after
generating (e.g., to eliminate unnecessary libraries), but it'll give you a
stable starting point for constructing your requirements file.

You can also give it an existing requirements file, and it will use that as a
sort of template for the new file.  So if you do::

    $ pip freeze -r devel-req.txt > stable-req.txt

it will keep the packages listed in ``devel-req.txt`` in order and preserve
comments.

Bundles
-------

Another way to distribute a set of libraries is a bundle format (specific to
pip).  This format is not stable at this time (there simply hasn't been
any feedback, nor a great deal of thought).  A bundle file contains all the
source for your package, and you can have pip install them all together. 
Once you have the bundle file further network access won't be necessary.  To
build a bundle file, do::

    $ pip bundle MyApp.pybundle MyApp

(Using a `requirements file`_ would be wise.)  Then someone else can get the
file ``MyApp.pybundle`` and run::

    $ pip install MyApp.pybundle

This is *not* a binary format.  This only packages source.  If you have binary
packages, then the person who installs the files will have to have a compiler,
any necessary headers installed, etc.  Binary packages are hard, this is
relatively easy.

Using pip With virtualenv
-------------------------

pip is most nutritious when used with `virtualenv
<http://pypi.python.org/pypi/virtualenv>`_.  One of the reasons pip
doesn't install "multi-version" eggs is that virtualenv removes much of the need
for it.

pip does not have to be installed to use it, you can run ``python
path/to/pip.py`` and it will work.  This is intended to avoid the
bootstrapping problem of installation.  You can also run pip inside
any virtualenv environment, like::

    $ virtualenv new-env/
    ... creates new-env/ ...
    $ pip install -E new-env/ MyPackage

This is exactly equivalent to::

    $ ./new-env/bin/python path/to/pip.py install MyPackage

Except, if you have ``virtualenv`` installed and the path ``new-env/``
doesn't exist, then a new virtualenv will be created.

Using pip with buildout
-----------------------

If you are using `zc.buildout
<http://pypi.python.org/pypi/zc.buildout>`_ you should look at
`gp.recipe.pip <http://pypi.python.org/pypi/gp.recipe.pip>`_ as an
option to use pip and virtualenv in your buildouts.

Using pip with virtualenvwrapper
---------------------------------

If you are using `virtualenvwrapper
<http://www.doughellmann.com/projects/virtualenvwrapper/>`_, you might
want pip to automatically create its virtualenvs in your
``$WORKON_HOME``.

You can tell pip to do so by defining ``PIP_VIRTUALENV_BASE`` in your
environment and setting it to the same value as that of
``$WORKON_HOME``.

Do so by adding the line::

    export PIP_VIRTUALENV_BASE=$WORKON_HOME

in your .bashrc under the line starting with ``export WORKON_HOME``.
