Bug: StrictYaml cannot serialize `None` (null value)

Typing the following into the python interpreter

Python 3.7.1 (v3.7.1:260ec2c36a, Oct 20 2018, 14:05:16) [MSC v.1915 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import strictyaml
>>> yaml = strictyaml.as_document({'a':None})

results in

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "D:\Temp\3\venv3\lib\site-packages\strictyaml\parser.py", line 258, in as_document
    return schema(YAMLChunk(schema.to_yaml(data), label=label))
  File "D:\Temp\3\venv3\lib\site-packages\strictyaml\any_validator.py", line 45, in to_yaml
    return schema_from_data(data).to_yaml(data)
  File "D:\Temp\3\venv3\lib\site-packages\strictyaml\compound.py", line 197, in to_yaml
    for key, value in data.items()
  File "D:\Temp\3\venv3\lib\site-packages\strictyaml\compound.py", line 200, in <listcomp>
    and value != self._defaults[key]
  File "D:\Temp\3\venv3\lib\site-packages\strictyaml\scalar.py", line 148, in to_yaml
    raise YAMLSerializationError("'{}' is not a string".format(data))
strictyaml.exceptions.YAMLSerializationError: 'None' is not a string

This appears to be a bug. (If it is not a bug, this gaping feature-hole is not documented anywhere.)

[crdoconnor]

I can add a NoneValidator which will accept values other than empty (defaulting to null) if that helps?

On Wed, 22 Apr 2020, 14:25 ArneBachmannDLR, [e-mail] wrote:

Here is my workaround:

from strictyaml.scalar import ScalarValidator class None_(ScalarValidator): ‘’’ Validator for strictyaml schemas expecting a None. ‘’’

def validate_scalar(_, chunk): if chunk.contents == ‘null’: return None chunk.expecting_but_found(“when expecting None”)

def to_yaml(_, data): return ‘null’

Make sure to put None_() before | Str() or other unions.

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[ArneBachmannDLR]

I think it would be useful to define a value to be optionally None (YAML’s null). The problem is that None is already taken in Python, so that None() is not available, therefore I went with None_(). An alternative might be NoneOr(Type()) or Nullable(Type).