order in dict is not preserved

Python 3.6.3 import yaml document = """ b: c: 3 d: 4 a: 1 """ print(yaml.dump(yaml.load(document), default_flow_style=False)) a: 1 b: c: 3 d: 4

[sigmavirus24]

This is a property of Python, not PyYAML. Python does not preserve the order of dictionaries and so we cannot either. To do so, you’d have to yaml.load into an OrderedDict (a dictionary implementation that preserves order). Cheers!

[shoogle]

the yaml-specs do not guarantee an order.

Correct.

As PyYAML is a yaml parser, guaranteeing order seems like a slight breach of the yaml specs.

Wrong. Since the spec doesn’t guarantee an order, that means any order is valid. PyYAML could return dict keys in any arbitrary order (alphabetical, reverse-alphabetical, shortest first, random, order of creation, etc.) and it would still be perfectly consistent with the YAML specification.

In practice, the only ordering that makes any sense is the order in which they were created, because if they are returned in a different order then the information about which was created first is lost forever. If the user requires any other form of ordering (alphabetical, etc.), then he/she is able to sort the dict themself after it has been returned in creation order. However, if the dict is not returned in creation order then the user can never put it back in creation order (except by a lucky guess).

It is for this very reason that, since Python 3.7, dictionaries are ordered by default as a feature of the language (and not just as an implementation detail as they were in 3.6).

This is why I think returning in creation order should be the default in PyYAML (at least for Python >= 3.7) and not just an option, though I understand the desire to ensure backwards compatibility. (It should be noted, however, that nobody complained when Python dicts became ordered by default, even though it could be seen as a backwards-incompatible change.)