Creating a spec fails if schemas contain non-JSON-serializable objects

Start at openapi_core/schema/schemas/util.py, specifically dicthash, and trace how create_spec reaches schema hashing through the parameter and schema factories. Reproduce the YAML example with an unquoted date, then verify that creating the spec succeeds while schema hashing still distinguishes schema dictionaries consistently.

Creating a spec fails if schemas in the spec dict contain non-JSON-serializable objects. That's for example the case when generating the spec dict from a deserialized OpenAPI template in YAML format and having dates in the schema as shown in the example below:

#!/usr/bin/env python3

from openapi_core import create_spec
import yaml

template = """
openapi: "3.0.0"
info:
  title: Minimal valid OpenAPI specification
  version: "0.1"
paths:
  /status:
    get:
      parameters:
        - name: date
          in: query
          schema:
            type: string
            format: date
            example: 2020-06-23
      responses:
        default:
          description: Return the API status.
"""

spec_dict = yaml.safe_load(template)
create_spec(spec_dict)

Running this example code results in the following exception:

Traceback (most recent call last):
  File "./bar.py", line 27, in <module>
    create_spec(spec_dict)
  File "/home/user/.local/lib/python3.8/site-packages/openapi_core/schema/shortcuts.py", line 12, in create_spec
    return spec_factory.create(spec_dict, spec_url=spec_url)
  File "/home/user/.local/lib/python3.8/site-packages/openapi_core/schema/specs/factories.py", line 54, in create
    info, list(paths), servers=list(servers), components=components,
  File "/home/user/.local/lib/python3.8/site-packages/openapi_core/schema/paths/generators.py", line 38, in generate
    path_name, list(operations), parameters=list(parameters),
  File "/home/user/.local/lib/python3.8/site-packages/openapi_core/schema/operations/generators.py", line 68, in generate
    http_method, path_name, responses, list(parameters),
  File "/home/user/.local/lib/python3.8/site-packages/openapi_core/schema/parameters/generators.py", line 23, in generate_from_list
    parameter = self.parameter_factory.create(parameter_spec)
  File "/home/user/.local/lib/python3.8/site-packages/openapi_core/schema/parameters/factories.py", line 26, in create
    schema, _ = self.schemas_registry.get_or_create(schema_spec)
  File "/home/user/.local/lib/python3.8/site-packages/openapi_core/schema/schemas/registries.py", line 19, in get_or_create
    schema_hash = dicthash(schema_spec)
  File "/home/user/.local/lib/python3.8/site-packages/openapi_core/schema/schemas/util.py", line 15, in dicthash
    return hash(dumps(d, sort_keys=True))
  File "/usr/lib/python3.8/json/__init__.py", line 234, in dumps
    return cls(
  File "/usr/lib/python3.8/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python3.8/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/usr/lib/python3.8/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type date is not JSON serializable

It doesn't matter if PyYAML or ruamel.yaml is used, as both convert dates from the YAML files into date/datetime objects.

The reason this fails is because openapi-core tries to json.dumps schema dicts:https://github.com/p1c2u/openapi-core/blob/97b568dc5535f5acbe719eed540c8888984d4732/openapi_core/schema/schemas/util.py#L14-L15

As a workaround converting the date into a string in the YAML file by explicitly adding quotes (example: "2020-06-23") works, however if I'm reading the OpenAPI specification correctly the example field of parameter objects can be of any type (https://swagger.io/specification/#parameter-object), potentially also creating problems with other types of objects which aren't JSON-serializable.