How do you control how the order in which PyYaml outputs key/value pairs when serializing a Python dictionary?
I’m using Yaml as a simple serialization format in a Python script. My Yaml serialized objects represent a sort of “document”, so for maximum user-friendliness, I’d like my object’s “name” field to appear first in the file. Of course, since the value returned by my object’s __getstate__ is a dictionary, and Python dictionaries are unordered, the “name” field will be serialized to a random location in the output.
e.g.
>>> import yaml
>>> class Document(object):
... def __init__(self, name):
... self.name = name
... self.otherstuff = 'blah'
... def __getstate__(self):
... return self.__dict__.copy()
...
>>> doc = Document('obj-20111227')
>>> print yaml.dump(doc, indent=4)
!!python/object:__main__.Document
otherstuff: blah
name: obj-20111227
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Answer
Took me a few hours of digging through PyYAML docs and tickets, but I eventually discovered this comment that lays out some proof-of-concept code for serializing an OrderedDict as a normal YAML map (but maintaining the order).
e.g. applied to my original code, the solution looks something like:
>>> import yaml
>>> from collections import OrderedDict
>>> def dump_anydict_as_map(anydict):
... yaml.add_representer(anydict, _represent_dictorder)
...
>>> def _represent_dictorder( self, data):
... if isinstance(data, Document):
... return self.represent_mapping('tag:yaml.org,2002:map', data.__getstate__().items())
... else:
... return self.represent_mapping('tag:yaml.org,2002:map', data.items())
...
>>> class Document(object):
... def __init__(self, name):
... self.name = name
... self.otherstuff = 'blah'
... def __getstate__(self):
... d = OrderedDict()
... d['name'] = self.name
... d['otherstuff'] = self.otherstuff
... return d
...
>>> dump_anydict_as_map(Document)
>>> doc = Document('obj-20111227')
>>> print yaml.dump(doc, indent=4)
!!python/object:__main__.Document
name: obj-20111227
otherstuff: blah