Add notes about dependencies.

oeps/oep-0007.rst

@@ -6,15 +6,15 @@ OEP-0007: Migrating to Python 3
 +---------------+-------------------------------------------+
 | Title         | Migrating to Python 3                     |
 +---------------+-------------------------------------------+
-| Last-Modified | 2016-08-12                                |
+| Last-Modified | 2016-09-23                                |
 +---------------+-------------------------------------------+
-| Author        | Cliff Dyer <cdyer@edx.org                 |
+| Author        | Cliff Dyer <cdyer@edx.org>                |
 +---------------+-------------------------------------------+
 | Arbiter       |                                           |
 +---------------+-------------------------------------------+
 | Status        | Draft                                     |
 +---------------+-------------------------------------------+
-| Type          |                                           |
+| Type          | Best Practices                            |
 +---------------+-------------------------------------------+
 | Created       | 2016-08-12                                |
 +---------------+-------------------------------------------+
@@ -26,14 +26,13 @@ Abstract
 ========
 
 With primary development of Python occurring on Python 3, and with Python 2
-scheduled for end-of-life in 2020, edX needs to plan for a transition of our
-code to Python 3.  Open edX is a large project, spanning many applications over
-even more github repos, with even more dependencies on third party libraries
-that may or may not support python 3.  This document outlines how we plan to
-scope the problem, socialize understanding of the differences between versions
-of python, migrate our code, and work with our Open edX community to ensure
-that the process is as painless as possible, and meets the needs of our
-stakeholders.
+scheduled for end-of-life in 2020, edX needs to plan for a transition of all of
+our code to Python 3.  Open edX is a large project, spanning many applications
+over even more github repos, with even more dependencies on third party
+libraries.  This document outlines how we plan to scope the problem, socialize
+understanding of the differences between versions of python, migrate our code,
+and work with our Open edX community to ensure that the process is as painless
+as possible, and meets the needs of our stakeholders.
 
 
 Rationale
@@ -78,8 +77,31 @@ look as much like native python 3 code as possible, with compatibility changes
 made at the top of the file, in the import headers.  This will make it easier
 to strip out compatibility shims when we are ready to drop python 2 support.
 
-We will follow the compatibility guidelines at
-http://python-future.org/compatible_idioms.html.
+Third-party dependencies
+------------------------
+
+Before converting a codebase to python 3, we need to make sure the code we
+depend on will also support python 3.  We can make a rough check for
+third-party library support using the [Can I Use Python 3 website or
+library](https://caniusepython3.com/).  This will sometimes show inaccurate
+results, as it depends upon self-reporting of library compatibility (via the
+library's `setup.py` classifiers), but will help guide our investigations and
+scope out the amount of work required.  Results can be tracked in the
+[Compatibility Audit wiki
+page](https://openedx.atlassian.net/wiki/display/ENG/Compatibility+Audit).
+
+If a required library does not support python 3, we have a few options:
+
+1. We can contribute a patch to support python 3 to the library.
+2. We can request python 3 support, and wait for the maintainers to implement it.
+3. We can find an alternative library that does support python 3.
+
+Which path is best may depend on the enthusiasm of the maintainers for
+supporting python 3, the amount of resources we want to commit to the project,
+and the availability and quality of alternatives.
+
+__future__ imports
+------------------
 
 All files should have the main `__future__` imports at the top to regularize
 some behaviors that differ by default between python 2 and 3. `from