id,summary,reporter,owner,description,type,status,priority,component,severity,resolution,keywords,cc
29,Keeping mapping keys ordered,edemaine@…,xi,"Would you be interested in adding the following kind of functionality to the public distribution of PyYAML?

{{{
>>> import yaml
>>> d = yaml.load('z: 1\ny: 2\nx: 3\n', Loader=yaml.order.OrderedLoader)
>>> d
yaml.order.odict([('z', 1), ('y', 2), ('z', 3)])
>>> for key, value in d.iteritems (): print key, value
z 1
y 2
x 3
>>> print yaml.dump(d, Dumper=yaml.order.OrderedDumper, default_flow_style=False),
z: 1
y: 2
x: 3
>>> s = yaml.dump(d, default_flow_style=False)
>>> print s,
!!omap
z: 1
y: 2
x: 3
>>> yaml.load(y)
yaml.order.odict([('z', 1), ('y', 2), ('z', 3)])
}}}

There are two things going on here:

1. Add real {{{!!omap}}} functionality.  When loading an {{{!!omap}}} object, create an odict object (defined by a new class that maintains a dictionary along with a key order), instead of the current behavior of creating a regular Python dictionary.  Conversely, when dumping such an object, preserve the key order (don't sort), and output an {{{!!omap}}} directive.  Both of these features seem quite desirable from a YAML standard point of view.

Perhaps, more generally, dumping could check for a special 'keys_in_order' attribute, in which case it follows the order of keys(), instead of sorting the keys as in the recent patch.

2. Add special {{{yaml.order.OrderedLoader}}}, which loads regular {{{!!map}}} values as if they were {{{!!omap}}} values, and {{{yaml.order.OrderedDumper}}}, which dumps {{{odict}}} types as regular {{{!!map}}} values (to avoid the ugly {{{!!omap}}} specifier).

Personally I would find this functionality very useful in many projects.  It would enable a computer program edit a human-written YAML file, without messing up all the key orders, so that the computer output looks pretty similar to what the human had just before.  I understand that YAML does not guarantee preservation of key order in a map type, or more precisely, it does not give it any significance to the order in absense of an {{{!!omap}}} or {{{!!pairs}}} type specification.  But this is a practically useful feature in some cases, so it seems natural to provide it as an optional functionality in {{{yaml.order}}}.

I'd be happy to write the code for all of this, because I need it myself.  My question is whether you'd consider including it in the PyYAML distribution.",enhancement,reopened,normal,pyyaml,normal,,,xi@… edemaine@… strombrg@…