Controlling Yaml Serialization Order in Python

Question

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

Answer 1

The last time I checked, Python's dictionaries weren't ordered. If you really want them to be, I strongly recommend using a list of key/value pairs.

[
    ('key', 'value'),
    ('key2', 'value2')
]

Alternatively, define a list with the keys and put them in the right order.

keys = ['key1', 'name', 'price', 'key2'];
for key in keys:
    print obj[key]

Answer 2

New Solution (as of 2020 and PyYAML 5.1)

You can dump a dictionary in its current order by simply using

yaml.dump(data, default_flow_style=False, sort_keys=False)

Answer 3

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

Answer 4

I think the problem is when you dump the data. I looked into the code of PyYaml and there is a optional argument called sort_keys, setting that value to False seems to do the trick.