PyYaml can't parse mappings with lists as keys

Here is an example of valid YAML from the YAML spec (example 2.11):

? [ New York Yankees,
    Atlanta Braves ]
: [ 2001-07-02, 2001-08-12,
    2001-08-14 ]

However, PyYaml can’t parse this:

yaml.safe_load("""? [ New York Yankees,
    Atlanta Braves ]
: [ 2001-07-02, 2001-08-12,
    2001-08-14 ]""")

yaml.constructor.ConstructorError: while constructing a mapping
  in "<unicode string>", line 1, column 1:
    ? [ New York Yankees,
    ^
found unhashable key
  in "<unicode string>", line 1, column 3:
    ? [ New York Yankees,
      ^

This must be a bug, as it violates the spec. It could be fixed by parsing the list as a tuple.

[perlpunk]

It would be cool if someone could have a look at my PR (#159). It still might need some work (what if we have circular aliases?), but if there’s something wrong in general with it, I’d like to know.

[perlpunk]

I did a PR for this last year: #159