YAML 1.2 Boolean Parsing Transition for Legacy 1.1 Values

Direct answer

To preserve YAML 1.1 boolean behavior while using a YAML 1.2‑compliant parser, configure the parser to load the YAML 1.1 "implicit bool" schema (if the library offers it) or explicitly tag legacy values, then normalize the loaded strings in your application.

Confirmed facts

• The YAML 1.2 Core Schema defines only true and false as implicit booleans.
• Values like yes, no, on, off are therefore parsed as plain strings.
• Many YAML libraries (e.g., PyYAML, ruamel.yaml, libyaml‑based parsers) allow selecting an alternative schema such as the YAML 1.1 compatible schema or adding a custom resolver.

Likely explanation

If a parser does not expose a built‑in YAML 1.1 schema, the underlying cause is that the library’s resolver only registers the Core Schema’s bool tags. Adding a custom resolver for the legacy literals restores the 1.1 behavior without violating the YAML 1.2 specification, because you are explicitly providing an alternative schema.

Steps for this case

1. Identify the YAML parser/library you are using (e.g., PyYAML 5.1+, ruamel.yaml 0.17+, SnakeYAML, libyaml).
2. Check whether the library provides a "YAML 1.1" or "SafeLoader/FullLoader" option that includes the legacy bool resolver.
3. If available, instantiate the loader with that option:

   # PyYAML example
   import yaml
   data = yaml.load(stream, Loader=yaml.FullLoader)  # FullLoader includes YAML 1.1 bool resolver in recent versions
   

   For ruamel.yaml:

   from ruamel.yaml import YAML
   yaml = YAML(typ='rt')  # round‑trip loader preserves 1.1 implicit bool handling
   

4. If no built‑in 1.1 mode exists, add a custom resolver that maps the legacy literals to the !!bool tag:

   # PyYAML custom resolver
   import yaml
   import re
   def bool_constructor(loader, node):
       value = loader.construct_scalar(node).lower()
       return value in ('yes', 'on', 'true', '1')
   yaml.add_constructor('tag:yaml.org,2002:bool', bool_constructor)
   yaml.add_implicit_resolver(
       'tag:yaml.org,2002:bool',
       re.compile(r'''^(?:yes|y|no|n|true|false|on|off|1|0)$''', re.IGNORECASE),
       list('yYnNtTfFoO10')
   )
   

5. After loading, normalize any remaining string values that should be booleans:

   def normalize_bool(val):
       if isinstance(val, str):
           return val.lower() in ('yes', 'on', 'true', '1')
       return bool(val)
   # apply to config dict recursively
   

6. Validate the resulting configuration with unit tests that assert the expected type (bool) for each flag.

Missing diagnostic detail

If you cannot locate a YAML 1.1 compatibility mode in your library’s documentation, please specify the exact parser name and version you are using; that information determines whether a built‑in option exists or whether a custom resolver is required.