[RFC/PATCH] CodingGuidelines: add Python code guidelines

[John Keeping <john@keeping.me.uk>]

These are kept short by simply deferring to PEP-8.  Most of the Python
code in Git is already very close to this style (some things in contrib/
are not).

Rationale for version suggestions:

 - Amongst the noise in [2], there isn't any disagreement about using
   2.6 as a base (see also [3]).

 - The Git INSTALL document currently says:

      Python version 2.6 or later is needed to use the git-p4
      interface to Perforce.

 - Restricting ourselves to 2.6+ makes aiming for Python 3 compatibility
   significantly easier [4].

 - Following Pete's comment [5] I tested Python 2.6.0 and it does
   support bytes literals, as suggested by [4] but contradicted by [6].

 - Advocating Python 3 support in all scripts is currently unrealistic
   because:

     - 'p4 -G' provides output in a format that is very hard to use with
       Python 3 (and its documentation claims Python 3 is unsupported).

     - Mercurial does not support Python 3.

     - Bazaar does not support Python 3.

 - But we should try to make new scripts compatible with Python 3
   because all new Python development is happening on version 3 and the
   Python community will eventually stop supporting Python 2 [7].

I chose to recommend `from __future__ import unicode_literals` since it
provides the widest range of compatibility (2.6+ and 3.0+) while
allowing us to be explicit about bytes vs. Unicode.  The alternative
would be to advocate using the 'u' prefix on Unicode strings but this
isn't available in versions 3.0 - 3.2 (it is reintroduced in version 3.3
as a no-op in order to make it easier to write scripts targeting a wide
range of Python versions without needing to use 2to3 [1]).  In reality I
doubt we will ever need to worry about this since ASCII strings will
just work in both Python 2 and Python 3.

[1] http://www.python.org/dev/peps/pep-0414/
[2] http://thread.gmane.org/gmane.comp.version-control.git/210329
[3] http://article.gmane.org/gmane.comp.version-control.git/210429
[4] http://docs.python.org/3.3/howto/pyporting.html#try-to-support-python-2-6-and-newer-only
[5] http://article.gmane.org/gmane.comp.version-control.git/213830
[6] http://docs.python.org/2.6/reference/lexical_analysis.html#literals
[7] http://www.python.org/dev/peps/pep-0404/
---
 Documentation/CodingGuidelines | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines
index 69f7e9b..baf3b41 100644
--- a/Documentation/CodingGuidelines
+++ b/Documentation/CodingGuidelines
@@ -179,6 +179,22 @@ For C programs:
  - Use Git's gettext wrappers to make the user interface
    translatable. See "Marking strings for translation" in po/README.
 
+For Python scripts:
+
+ - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
+
+ - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
+
+ - Where required libraries do not restrict us to Python 2, we try to
+   also be compatible with Python 3.  In this case we use
+   `from __future__ import unicode_literals` if we need to differentiate
+   Unicode string literals, rather than prefixing Unicode strings with
+   'u' since the latter is not supported in Python versions 3.0 - 3.2.
+
+ - We use the 'b' prefix for bytes literals.  Note that even though
+   the Python documentation for version 2.6 does not mention this
+   prefix it is supported since version 2.6.0.
+
 Writing Documentation:
 
  Every user-visible change should be reflected in the documentation.
-- 
1.8.1