In Python, how can you load YAML mappings as OrderedDicts?

Question

I'd like to get PyYAML's loader to load mappings (and ordered mappings) into the Python 2.7+ OrderedDict type, instead of the vanilla dict and the list of pairs it currently uses.

What's the best way to do that?

Answer 1

Update: In python 3.6+ you probably don't need OrderedDict at all due to the new dict implementation that has been in use in pypy for some time (although considered CPython implementation detail for now).

Update: In python 3.7+, the insertion-order preservation nature of dict objects has been declared to be an official part of the Python language spec, see What's New In Python 3.7.

I like @James' solution for its simplicity. However, it changes the default global yaml.Loader class, which can lead to troublesome side effects. Especially, when writing library code this is a bad idea. Also, it doesn't directly work with yaml.safe_load().

Fortunately, the solution can be improved without much effort:

import yaml
from collections import OrderedDict

def ordered_load(stream, Loader=yaml.Loader, object_pairs_hook=OrderedDict):
    class OrderedLoader(Loader):
        pass
    def construct_mapping(loader, node):
        loader.flatten_mapping(node)
        return object_pairs_hook(loader.construct_pairs(node))
    OrderedLoader.add_constructor(
        yaml.resolver.BaseResolver.DEFAULT_MAPPING_TAG,
        construct_mapping)
    return yaml.load(stream, OrderedLoader)

# usage example:
ordered_load(stream, yaml.SafeLoader)

For serialization, I don't know an obvious generalization, but at least this shouldn't have any side effects:

def ordered_dump(data, stream=None, Dumper=yaml.Dumper, **kwds):
    class OrderedDumper(Dumper):
        pass
    def _dict_representer(dumper, data):
        return dumper.represent_mapping(
            yaml.resolver.BaseResolver.DEFAULT_MAPPING_TAG,
            data.items())
    OrderedDumper.add_representer(OrderedDict, _dict_representer)
    return yaml.dump(data, stream, OrderedDumper, **kwds)

# usage:
ordered_dump(data, Dumper=yaml.SafeDumper)