MEP11: Third-party dependencies

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This MEP attempts to improve the way in which third-party dependencies

in matplotlib are handled.

Status

**Completed** -- needs to be merged

Branches and Pull requests

#1157: Use automatic dependency resolution

#1290: Debundle pyparsing

#1261: Update six to 1.2

Abstract

One of the goals of matplotlib has been to keep it as easy to install

as possible. To that end, some third-party dependencies are included

in the source tree and, under certain circumstances, installed

alongside matplotlib. This MEP aims to resolve some problems with

that approach, bring some consistency, while continuing to make

installation convenient.

At the time that was initially done, `setuptools`, `easy_install` and

`PyPI` were not mature enough to be relied on. However, at present,

we should be able to safely leverage the "modern" versions of those

tools, `distribute` and `pip`.

While matplotlib has dependencies on both Python libraries and C/C++

libraries, this MEP addresses only the Python libraries so as to not

confuse the issue. C libraries represent a larger and mostly

orthogonal set of problems.

Detailed description

matplotlib depends on the following third-party Python libraries:

- Numpy

- dateutil (pure Python)

- pytz (pure Python)

- six -- required by dateutil (pure Python)

- pyparsing (pure Python)

- PIL (optional)

- GUI frameworks: pygtk, gobject, tkinter, PySide, PyQt4, wx (all

optional, but one is required for an interactive GUI)

Current behavior

When installing from source, a `git` checkout or `pip`:

- `setup.py` attempts to `import numpy`. If this fails, the

installation fails.

- For each of `dateutil`, `pytz` and `six`, `setup.py` attempts to

import them (from the top-level namespace). If that fails,

matplotlib installs its local copy of the library into the

top-level namespace.

- `pyparsing` is always installed inside of the matplotlib

namespace.

This behavior is most surprising when used with `pip`, because no

`pip` dependency resolution is performed, even though it is likely to

work for all of these packages.

The fact that `pyparsing` is installed in the matplotlib namespace has

reportedly (#1290) confused some users into thinking it is a

matplotlib-related module and import it from there rather than the

top-level.

When installing using the Windows installer, `dateutil`, `pytz` and

`six` are installed at the top-level *always*, potentially overwriting

already installed copies of those libraries.

TODO: Describe behavior with the OS-X installer.

When installing using a package manager (Debian, RedHat, MacPorts

etc.), this behavior actually does the right thing, and there are no

special patches in the matplotlib packages to deal with the fact that

we handle `dateutil`, `pytz` and `six` in this way. However, care

should be taken that whatever approach we move to continues to work in

that context.

Maintaining these packages in the matplotlib tree and making sure they

are up-to-date is a maintenance burden. Advanced new features that

may require a third-party pure Python library have a higher barrier to

inclusion because of this burden.

Desired behavior

Third-party dependencies are downloaded and installed from their

canonical locations by leveraging `pip`, `distribute` and `PyPI`.

`dateutil`, `pytz`, and `pyparsing` should be made into optional

dependencies -- though obviously some features would fail if they

aren't installed. This will allow the user to decide whether they

want to bother installing a particular feature.

Implementation

For installing from source, and assuming the user has all of the

C-level compilers and dependencies, this can be accomplished fairly

easily using `distribute` and following the instructions `here

<http://packages.python.org/distribute/>`_. The only anticipated

change to the matplotlib library code will be to import `pyparsing`

from the top-level namespace rather than from within matplotlib. Note

that `distribute` will also allow us to remove the direct dependency

on `six`, since it is, strictly speaking, only a direct dependency of

`dateutil`.

For binary installations, there are a number of alternatives (here

ordered from best/hardest to worst/easiest):

1. The distutils wininst installer allows a post-install script to

run. It might be possible to get this script to run `pip` to

install the other dependencies. (See `this thread

<http://grokbase.com/t/python/distutils-sig/109bdnfhp4/distutils-ann-setuptools-post-install-script-for-bdist-wininst>`_

for someone who has trod that ground before).

2. Continue to ship `dateutil`, `pytz`, `six` and `pyparsing` in

our installer, but use the post-install-script to install them

*only* if they can not already be found.

3. Move all of these packages inside a (new) `matplotlib.extern`

namespace so it is clear for outside users that these are

external packages. Add some conditional imports in the core

matplotlib codebase so `dateutil` (at the top-level) is tried

first, and failing that `matplotlib.extern.dateutil` is used.

2 and 3 are undesirable as they still require maintaining copies of

these packages in our tree -- and this is exacerbated by the fact that

they are used less -- only in the binary installers. None of these 3

approaches address Numpy, which will still have to be manually

installed using an installer.

TODO: How does this relate to the Mac OS-X installer?

Backward compatibility

At present, matplotlib can be installed from source on a machine

without the third party dependencies and without an internet

connection. After this change, an internet connection (and a working

PyPI) will be required to install matplotlib for the first time.

(Subsequent matplotlib updates or development work will run without

accessing the network).

Alternatives

Distributing binary `eggs` doesn't feel like a usable solution. That

requires getting `easy_install` installed first, and Windows users

generally prefer the well known `.exe` or `.msi` installer that works

out of the box.

MEP11