oyaml.py

import yaml
import yaml.constructor

###############################################################
# Procedures to preserve order of config file for readability #
###############################################################

# http://stackoverflow.com/questions/5121931/in-python-how-can-you-load-yaml-mappings-as-ordereddicts
try:
    # included in standard lib from Python 2.7
    from collections import OrderedDict
except ImportError:
    # try importing the backported drop-in replacement
    # it's available on PyPI
    from ordereddict import OrderedDict

class OrderedDictYAMLLoader(yaml.Loader):
    """
    A YAML loader that loads mappings into ordered dictionaries.
    """

    def __init__(self, *args, **kwargs):
        yaml.Loader.__init__(self, *args, **kwargs)

        self.add_constructor(u'tag:yaml.org,2002:map', type(self).construct_yaml_map)
        self.add_constructor(u'tag:yaml.org,2002:omap', type(self).construct_yaml_map)

    def construct_yaml_map(self, node):
        data = OrderedDict()
        yield data
        value = self.construct_mapping(node)
        data.update(value)

    def construct_mapping(self, node, deep=False):
        if isinstance(node, yaml.MappingNode):
            self.flatten_mapping(node)
        else:
            raise yaml.constructor.ConstructorError(None, None,
                'expected a mapping node, but found %s' % node.id, node.start_mark)

        mapping = OrderedDict()
        for key_node, value_node in node.value:
            key = self.construct_object(key_node, deep=deep)
            try:
                hash(key)
            except TypeError, exc:
                raise yaml.constructor.ConstructorError('while constructing a mapping',
                    node.start_mark, 'found unacceptable key (%s)' % exc, key_node.start_mark)
            value = self.construct_object(value_node, deep=deep)
            mapping[key] = value
        return mapping


# https://gist.github.com/miracle2k/3184458
"""Make PyYAML output an OrderedDict.
It will do so fine if you use yaml.dump(), but that generates ugly, 
non-standard YAML code.
To use yaml.safe_dump(), you need the following.
"""

def represent_odict(dump, tag, mapping, flow_style=None):
    """Like BaseRepresenter.represent_mapping, but does not issue the sort().
    """
    value = []
    node = yaml.MappingNode(tag, value, flow_style=flow_style)
    if dump.alias_key is not None:
        dump.represented_objects[dump.alias_key] = node
    best_style = True
    if hasattr(mapping, 'items'):
        mapping = mapping.items()
    for item_key, item_value in mapping:
        node_key = dump.represent_data(item_key)
        node_value = dump.represent_data(item_value)
        if not (isinstance(node_key, yaml.ScalarNode) and not node_key.style):
            best_style = False
        if not (isinstance(node_value, yaml.ScalarNode) and not node_value.style):
            best_style = False
        value.append((node_key, node_value))
    if flow_style is None:
        if dump.default_flow_style is not None:
            node.flow_style = dump.default_flow_style
        else:
            node.flow_style = best_style
    return node
    
# Sample of how it would be dumped and not; will update later to be more of a python module.  Just dumping to gist for now    
if __name__ == "__main__":
      # Write out ordered YAML
      with open("/config.yaml", "w") as f:
         yaml.SafeDumper.add_representer(OrderedDict,
             lambda dumper, value: represent_odict(dumper, u'tag:yaml.org,2002:map', value))
         f.write(yaml.safe_dump(config))
         
      # Read in ordered YAML
      with open("/config.yaml") as f:
        config = yaml.load(f, OrderedDictYAMLLoader)