Can PyYAML dump dict items in non-alphabetical order?

Question

I\'m using yaml.dump to output a dict. It prints out each item in alphabetical order based on the key.

>>> d = {\"z\":0,\"y\":0,\"x\":         


        

Answer 1

If safe_dump (i.e. dump with Dumper=SafeDumper) is used, then calling yaml.add_representer has no effect. In such case it is necessary to call add_representer method explicitly on SafeRepresenter class:

yaml.representer.SafeRepresenter.add_representer(
    OrderedDict, ordered_dict_representer
)

Answer 2

The following setting makes sure the content is not sorted in the output:

yaml.sort_base_mapping_type_on_output = False

Answer 3

There's probably a better workaround, but I couldn't find anything in the documentation or the source.

---

Python 2 (see comments)

I subclassed OrderedDict and made it return a list of unsortable items:

from collections import OrderedDict

class UnsortableList(list):
    def sort(self, *args, **kwargs):
        pass

class UnsortableOrderedDict(OrderedDict):
    def items(self, *args, **kwargs):
        return UnsortableList(OrderedDict.items(self, *args, **kwargs))

yaml.add_representer(UnsortableOrderedDict, yaml.representer.SafeRepresenter.represent_dict)

And it seems to work:

>>> d = UnsortableOrderedDict([
...     ('z', 0),
...     ('y', 0),
...     ('x', 0)
... ])
>>> yaml.dump(d, default_flow_style=False)
'z: 0\ny: 0\nx: 0\n'

---

Python 3 or 2 (see comments)

You can also write a custom representer, but I don't know if you'll run into problems later on, as I stripped out some style checking code from it:

import yaml

from collections import OrderedDict

def represent_ordereddict(dumper, data):
    value = []

    for item_key, item_value in data.items():
        node_key = dumper.represent_data(item_key)
        node_value = dumper.represent_data(item_value)

        value.append((node_key, node_value))

    return yaml.nodes.MappingNode(u'tag:yaml.org,2002:map', value)

yaml.add_representer(OrderedDict, represent_ordereddict)

But with that, you can use the native OrderedDict class.

Answer 4

I was also looking for an answer to the question "how to dump mappings with the order preserved?" I couldn't follow the solution given above as i am new to pyyaml and python. After spending some time on the pyyaml documentation and other forums i found this.

You can use the tag

!!omap

to dump the mappings by preserving the order. If you want to play with the order i think you have to go for keys:values

The links below can help for better understanding.

https://bitbucket.org/xi/pyyaml/issue/13/loading-and-then-dumping-an-omap-is-broken

http://yaml.org/type/omap.html

Answer 5

This is really just an addendum to @Blender's answer. If you look in the PyYAML source, at the representer.py module, You find this method:

def represent_mapping(self, tag, mapping, flow_style=None):
    value = []
    node = MappingNode(tag, value, flow_style=flow_style)
    if self.alias_key is not None:
        self.represented_objects[self.alias_key] = node
    best_style = True
    if hasattr(mapping, 'items'):
        mapping = mapping.items()
        mapping.sort()
    for item_key, item_value in mapping:
        node_key = self.represent_data(item_key)
        node_value = self.represent_data(item_value)
        if not (isinstance(node_key, ScalarNode) and not node_key.style):
            best_style = False
        if not (isinstance(node_value, ScalarNode) and not node_value.style):
            best_style = False
        value.append((node_key, node_value))
    if flow_style is None:
        if self.default_flow_style is not None:
            node.flow_style = self.default_flow_style
        else:
            node.flow_style = best_style
    return node

If you simply remove the mapping.sort() line, then it maintains the order of items in the OrderedDict.

Another solution is given in this post. It's similar to @Blender's, but works for safe_dump. The common element is the converting of the dict to a list of tuples, so the if hasattr(mapping, 'items') check evaluates to false.

Update:

I just noticed that The Fedora Project's EPEL repo has a package called python2-yamlordereddictloader, and there's one for Python 3 as well. The upstream project for that package is likely cross-platform.

Answer 6

For Python 3.7+, dicts preserve insertion order. Since PyYAML 5.1.x, you can disable the sorting of keys (#254). Unfortunately, the sorting keys behaviour does still default to True.

>>> import yaml
>>> yaml.dump({"b":1, "a": 2})
'a: 2\nb: 1\n'
>>> yaml.dump({"b":1, "a": 2}, sort_keys=False)
'b: 1\na: 2\n'

My project oyaml is a monkeypatch/drop-in replacement for PyYAML. It will preserve dict order by default in all Python versions and PyYAML versions.

>>> import oyaml as yaml  # pip install oyaml
>>> yaml.dump({"b":1, "a": 2})
'b: 1\na: 2\n'

Additionally, it will dump the collections.OrderedDict subclass as normal mappings, rather than Python objects.

>>> from collections import OrderedDict
>>> d = OrderedDict([("b", 1), ("a", 2)])
>>> import yaml
>>> yaml.dump(d)
'!!python/object/apply:collections.OrderedDict\n- - - b\n    - 1\n  - - a\n    - 2\n'
>>> yaml.safe_dump(d)
RepresenterError: ('cannot represent an object', OrderedDict([('b', 1), ('a', 2)]))
>>> import oyaml as yaml
>>> yaml.dump(d)
'b: 1\na: 2\n'
>>> yaml.safe_dump(d)
'b: 1\na: 2\n'