Generating anchors with PyYAML.dump()?

Question

I'd like to be able to generate anchors in the YAML generated by PyYAML's dump() function. Is there a way to do this? Ideally the anchors would have the same name as the YAML nodes.

Example:

import yaml
yaml.dump({'a': [1,2,3]})
'a: [1, 2, 3]\n'

What I'd like to be able to do is generate YAML like:

import yaml
yaml.dump({'a': [1,2,3]})
'a: &a [1, 2, 3]\n'

Can I write a custom emitter or dumper to do this? Is there another way?

Answer 1

By default, anchors are only emitted when it detects a reference to an object previously seen:

>>> import yaml
>>>
>>> foo = {'a': [1,2,3]}
>>> doc = (foo,foo)
>>>
>>> print yaml.safe_dump(doc, default_flow_style=False)
- &id001
  a:
  - 1
  - 2
  - 3
- *id001

If you want to override how it is named, you'll have to customize the Dumper class, specifically the generate_anchor() function. ANCHOR_TEMPLATE may also be useful.

In your example, the node name is simple, but you need to take into account the many possibilities for YAML values, ie it could be a sequence rather than a single value:

>>> import yaml
>>>
>>> foo = {('a', 'b', 'c'): [1,2,3]}
>>> doc = (foo,foo)
>>>
>>> print yaml.dump(doc, default_flow_style=False)
!!python/tuple
- &id001
  ? !!python/tuple
  - a
  - b
  - c
  : - 1
    - 2
    - 3
- *id001

Answer 2

This is not so easy. Unless the data that you want to use for the anchor is inside the node. This is because the anchor gets attached to the node contents, in your example '[1,2,3]' and doesn't know that this value is associated with key 'a'.

l = [1, 2, 3]
foo = {'a': l, 'b': l}
class SpecialAnchor(yaml.Dumper):

    def generate_anchor(self, node):
        print('Generating anchor for {}'.format(str(node)))
        anchor =  super().generate_anchor(node)
        print('Generated "{}"'.format(anchor))
        return anchor

y1 = yaml.dump(foo, Dumper=Anchor)

Gives you:

Generating anchor for SequenceNode(tag='tag:yaml.org,2002:seq', value=[ScalarNode(tag='tag:yaml.org,2002:int', value='1'), ScalarNode(tag='tag:yaml.org,2002:int', value='2'), ScalarNode(tag='tag:yaml.org,2002:int', value='3')])
Generated "id001"
a: &id001 [1, 2, 3]
b: *id001

So far I haven't found a way to get the key 'a' given the node...