python-peps/pep-0439.txt

PEP: 439
Title: Inclusion of pip bootstrap in Python installation
Version: $Revision$
Last-Modified: $Date$
Author: Richard Jones <richard@python.org>
BDFL-Delegate:  Nick Coghlan <ncoghlan@gmail.com>
Discussions-To: <distutils-sig@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 18-Mar-2013
Python-Version: 3.4
Post-History: 19-Mar-2013


Abstract
========

This PEP proposes the inclusion of a pip boostrap executable in the
Python installation to simplify the use of 3rd-party modules by Python
users.

This PEP does not propose to include the pip implementation in the
Python standard library.  Nor does it propose to implement any package
management or installation mechanisms beyond those provided by PEP
427 ("The Wheel Binary Package Format 1.0") and TODO distlib PEP.


Rationale
=========

Currently the user story for installing 3rd-party Python modules is
not as simple as it could be.  It requires that all 3rd-party modules
inform the user of how to install the installer, typically via a link
to the installer.  That link may be out of date or the steps required
to perform the install of the installer may be enough of a roadblock
to prevent the user from further progress.

Large Python projects which emphasise a low barrier to entry have
shied away from depending on third party packages because of the
introduction of this potential stumbling block for new users.

With the inclusion of the package installer command in the standard
Python installation the barrier to installing additional software is
considerably reduced.  It is hoped that this will therefore increase
the likelihood that Python projects will reuse third party software.

It is also hoped that this is reduces the number of proposals to
include more and more software in the Python standard library, and
therefore that more popular Python software is more easily upgradeable
beyond requiring Python installation upgrades.


Proposal
========

This proposal affects three components of packaging: `the pip bootstrap`_,
`setuptools`_ and, thanks to easier package installation, `modifications to
publishing packages`_.


The pip bootstrap
-----------------

The Python installation includes an executable called "pip3" (see PEP 394 for
naming rationale etc.) that attempts to import pip machinery.  If it can then
the pip command proceeds as normal.  If it cannot it will bootstrap pip by
downloading the pip implementation wheel file.  Once installed, the pip
command proceeds as normal. Once the bootstrap process is complete the "pip3"
command is no longer the bootstrap but rather the full pip command.

A boostrap is used in the place of a the full pip code so that we don't have
to bundle pip and also pip is upgradeable outside of the regular Python
upgrade timeframe and processes.

To avoid issues with sudo we will have the bootstrap default to
installing the pip implementation to the per-user site-packages
directory defined in PEP 370 and implemented in Python 2.6/3.0.  Since
we avoid installing to the system Python we also avoid conflicting
with any other packaging system (on Linux systems, for example.) If
the user is inside a virtual environment [1]_ then the pip
implementation will be installed into that virtual environment.

The bootstrap process will proceed as follows:

1. The user system has Python (3.4+) installed.  In the "scripts"
   directory of the Python installation there is the bootstrap script
   called "pip3".
2. The user will invoke a pip command, typically "pip3 install
   <package>", for example "pip3 install Django".
3. The boostrap script will attempt to import the pip implementation.
   If this succeeds, the pip command is processed normally.
4. On failing to import the pip implementation the bootstrap notifies
   the user that it is "upgrading pip" and contacts PyPI to obtain the
   latest download wheel file (see PEP 427.)
5. Upon downloading the file it is installed using the distlib
   installation machinery for wheel packages.  Upon completing the
   installation the user is notified that "pip3 has been upgraded."
   TODO how is it verified?
6. The pip tool may now import the pip implementation and continues to
   process the requested user command normally.

Users may be running in an environment which cannot access the public
Internet and are relying solely on a local package repository.  They
would use the "-i" (Base URL of Python Package Index) argument to the
"pip3 install" command.  This use case will be handled by:

1. Recognising the command-line arguments that specify alternative or
   additional locations to discover packages and attempting to
   download the package from those locations.
2. If the package is not found there then we attempt to donwload it
   using the standard "https://pypi.python.org/pypi/simple/pip" index.
3. If that also fails, for any reason, we indicate to the user the
   operation we were attempting, the reason for failure (if we know
   it) and display further instructions for downloading and installing
   the file manually.

Some users may have no Internet access suitable for fetching the pip
implementation file. Manual installation of the pip implementation will be
supported through the manual download of the wheel file and "pip3 install
<downloaded wheel file>". This installation - since it uses only the bootstrap
code - will not perform standard pip installation steps of saving the file to
a cache directory or updating any local database of installed files.

The download of the pip implementation install file should be performed
securely.  The transport from pypi.python.org will be done over HTTPS but the CA
certificate check will most likely not be performed, and therefore the download
would still be vulnerable to active MITM attacks.  To mitigate this
risk we will use the embedded signature support in the wheel format to validate
the downloaded file.

Beyond those arguments controlling index location and download
options, the "pip3" boostrap command may support further standard pip
options for verbosity, quietness and logging.

The "--no-install" option to the "pip3" command will not affect the
bootstrapping process.

setuptools
----------

The deprecation of requiring setuptools for installation is an existing goal of
the packaging comminity (TODO ref needed). Currently pip depends upon setuptools
functionality, and it is installed by the current pip boostrap. This PEP does
not propose installing setuptools during the new bootstrap.

It is intended that before Python 3.4 is shipped the functionlity required by
pip will be present in Python's standard library as the distlib module, and that
pip would be modified to use that functionality when present. TODO PEP reference
for distlib

Many existing "setup.py" files require setuptools to be installed (because one
of the first things they do is import setuptools). It is intended that pip's
behaviour will be either:

1. If setuptools is not present it can only install from wheel files and
   sdists with 2.0+ metadata, or
2. If setuptools is present it can also install from sdists with legacy
   metadata and eggs

By default, installing setuptools when necessary should be automatic so that
users are not inconvenienced, but advanced users should be able to ask that it
instead be treated as an error if no wheel is available to satisfy an
installation request or dependency (so they don't inadvertently install
setuptools on their production systems if they don't want to).


Modifications to publishing packages
------------------------------------

An additional new Python package is proposed, "pypublish", which
will be a tool for publishing packages to PyPI.  It would replace the
current "python setup.py register" and "python setup.py upload"
distutils commands.  Again because of the measured Python release
cycle and extensive existing Python installations these commands are
difficult to bugfix and extend.  Additionally it is desired that the
"register" and "upload" commands be able to be performed over HTTPS
with certificate validation.  Since shipping CA certificate keychains
with Python is not really feasible (updating the keychain is quite
difficult to manage) it is desirable that those commands, and the
accompanying keychain, be made installable and upgradeable outside of
Python itself.

The existing distutils mechanisms for package registration and upload would
remain, though with a deprecation warning.


Implementation
==============

The changes to pip required by this PEP are being tracked in that project's
issue tracker [2]_


Risks
=====

The key that is used to sign the pip implementation download might be
compromised and this PEP currently proposes no mechanism for key
revocation.

There is a Perl package installer also named "pip". It is quite rare and not
commonly used. The Fedora variant of Linux has historically named Python's
"pip" as "python-pip" and Perl's "pip" as "perl-pip".  This policy has been
altered[3] so that future and upgraded Fedora installations will use the name
"pip" for Python's "pip". Existing (non-upgraded) installations will still
have the old name for the Python "pip", though the potential for confusion is
now much reduced.


References
==========

.. [1] PEP 405, Python Virtual Environments
       http://www.python.org/dev/peps/pep-0405/

.. [2] pip issue tracking work needed for this PEP
       https://github.com/pypa/pip/issues/863

.. [3] Fedora's python-pip package does not provide /usr/bin/pip
       https://bugzilla.redhat.com/show_bug.cgi?id=958377


Acknowledgments
===============

Nick Coghlan for his thoughts on the proposal and dealing with the Red
Hat issue.

Jannis Leidel and Carl Meyer for their thoughts. Marcus Smith for feedback.

Marcela Mašláňová for resolving the Fedora issue.


Copyright
=========

This document has been placed in the public domain.



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