enhancement request: make py3 read/write py2 pickle format

[Chris Angelico]

On Thu, Jun 11, 2015 at 8:10 AM, Devin Jeanpierre
<jeanpierreda at gmail.com> wrote:
> The problem is that there are two different ways repr might write out
> a dict equal to {'a': 1, 'b': 2}. This can make tests brittle -- e.g.
> it's why doctest fails badly at examples involving dictionaries. Text
> format protocol buffers output everything sorted, so that you can do
> textual diffs for compatibility tests and such.

With Python's JSON module [1], you can pass sort_keys=True to
stipulate that the keys be lexically ordered, which should make the
output "canonical". Pike's Standards.JSON.encode() [2] can take a flag
value to canonicalize the output, which currently has the same effect
(sort mappings by their indices). I did a quick check for Ruby and
didn't find anything in its standard library JSON module, but knowing
Ruby, it'll be available somewhere in a gem. A web search for 'perl
json' brought up a CPAN link [4] that has a canonicalize option for
sorting by keys. So that's three out of four definite, one uncertain,
where it's pretty easy to ensure that you get byte-for-byte identical
output from a JSON encoder.

Even though failing doctests are a separate problem, it's useful to
have canonical output. Your diffs get less noisy, for instance.
Coupled with a human-readability flag (eg "indent=4" in Python,
"Standards.JSON.HUMAN_READABLE" in Pike) that splits the result over
multiple lines, it can make a pretty easy to diff file. Definitely
worth doing... and definitely worth using a JSON encoder rather than
repr().

ChrisA

[1] https://docs.python.org/3/library/json.html#json.dump
[2] http://pike.lysator.liu.se/generated/manual/modref/ex/predef_3A_3A/Standards/JSON.html
[3] http://ruby-doc.org/stdlib-2.0.0/libdoc/json/rdoc/JSON.html
[4] http://search.cpan.org/~makamaka/JSON-2.90/lib/JSON.pm#PERL_-%3E_JSON