YAML key consisting of a list of elements

Question

I need to have a list of elements as a key, so that I can check if several conditions are met. Example (don't know if this is possible and if the syntax is correct):

mapping:
  c_id:
    [pak, gb]: '4711'
    [pak, ch]: '4712'
    [pak]: '4713'
  d_id:
    .
    .
    .

Now I need to know if it is possible to have an approach as in the example.

Answer 1

In addition to Anthon's answer, here is how you can do it with PyYaml:

mapping:
  c_id:
    !!python/tuple [pak, gb]: '4711'
    !!python/tuple [pak, ch]: '4712'
    !!python/tuple [pak]: '4713'

The trick is to tell PyYaml that you want to load the keys into a tuple (since a list cannot be used as dict key). There is no way to do this implicitly, but this does not mean that it is not possible.

Explicit keys may be more readable in this case:

mapping:
  c_id:
    ? !!python/tuple [pak, gb]
    : '4711'
    ? !!python/tuple [pak, ch]
    : '4712'
    ? !!python/tuple [pak]
    : '4713'

This example is semantically equivalent to the first one.

Answer 2

The syntax for your YAML is correct. The only trick is that because in Python a key has to be immutable you need to specify access to the complex key as a tuple:

import ruamel.yaml

yaml_str = """\
mapping:
  c_id:
    [pak, gb]: '4711'
    [pak, ch]: '4712'
    [pak]: '4713'
"""

data = ruamel.yaml.round_trip_load(yaml_str)
print(data['mapping']['c_id'][('pak', 'gb')])

gives:

4711

Please note that this is not possible with PyYAML, as it doesn't support sequences as keys, you have to use ruamel.yaml (disclaimer: I am the author of that package)