add frozenset to schema.py/field_class_to_schema

[wangpeibao]

Feature Request

         pydantic version: 1.5.1
        pydantic compiled: False
             install path: /home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/pydantic
           python version: 3.6.5 (default, May 11 2018, 13:30:17)  [GCC 7.3.0]
                 platform: Linux-4.15.0-29deepin-generic-x86_64-with-Deepin-15.11-stable
 optional deps. installed: []

Use pydantic to validate frozenset in FastApi, when I open SwaggerUI in Chrome, error occurs.
Add frozenset type in schema.py/field_class_to_schema to resolve the error.

```exception
Traceback (most recent call last):
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/uvicorn/protocols/http/httptools_impl.py", line 385, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/uvicorn/middleware/proxy_headers.py", line 45, in __call__
    return await self.app(scope, receive, send)
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/fastapi/applications.py", line 148, in __call__
    await super().__call__(scope, receive, send)  # pragma: no cover
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/starlette/applications.py", line 102, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/starlette/middleware/errors.py", line 181, in __call__
    raise exc from None
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/starlette/middleware/errors.py", line 159, in __call__
    await self.app(scope, receive, _send)
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/starlette/exceptions.py", line 82, in __call__
    raise exc from None
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/starlette/exceptions.py", line 71, in __call__
    await self.app(scope, receive, sender)
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/starlette/routing.py", line 550, in __call__
    await route.handle(scope, receive, send)
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/starlette/routing.py", line 227, in handle
    await self.app(scope, receive, send)
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/starlette/routing.py", line 41, in app
    response = await func(request)
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/fastapi/applications.py", line 103, in openapi
    return JSONResponse(self.openapi())
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/fastapi/applications.py", line 95, in openapi
    openapi_prefix=self.openapi_prefix,
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/fastapi/openapi/utils.py", line 291, in get_openapi
    flat_models=flat_models, model_name_map=model_name_map
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/fastapi/utils.py", line 84, in get_model_definitions
    model, model_name_map=model_name_map, ref_prefix=REF_PREFIX
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/pydantic/schema.py", line 456, in model_process_schema
    model, by_alias=by_alias, model_name_map=model_name_map, ref_prefix=ref_prefix, known_models=known_models
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/pydantic/schema.py", line 492, in model_type_schema
    f, by_alias=by_alias, model_name_map=model_name_map, ref_prefix=ref_prefix, known_models=known_models
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/pydantic/schema.py", line 191, in field_schema
    known_models=known_models or set(),
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/pydantic/schema.py", line 418, in field_type_schema
    known_models=known_models,
  File "/home/wang/PycharmProjects/FastApiLearn/venv/lib/python3.6/site-packages/pydantic/schema.py", line 686, in field_singleton_schema
    raise ValueError(f'Value not declarable with JSON Schema, field: {field}')
ValueError: Value not declarable with JSON Schema, field: name='a' type=frozenset required=True


```schema.py 

field_class_to_schema: Tuple[Tuple[Any, Dict[str, Any]], ...] = (
    (Path, {'type': 'string', 'format': 'path'}),
    (datetime, {'type': 'string', 'format': 'date-time'}),
    (date, {'type': 'string', 'format': 'date'}),
    (time, {'type': 'string', 'format': 'time'}),
    (timedelta, {'type': 'number', 'format': 'time-delta'}),
    (IPv4Network, {'type': 'string', 'format': 'ipv4network'}),
    (IPv6Network, {'type': 'string', 'format': 'ipv6network'}),
    (IPv4Interface, {'type': 'string', 'format': 'ipv4interface'}),
    (IPv6Interface, {'type': 'string', 'format': 'ipv6interface'}),
    (IPv4Address, {'type': 'string', 'format': 'ipv4'}),
    (IPv6Address, {'type': 'string', 'format': 'ipv6'}),
    (str, {'type': 'string'}),
    (bytes, {'type': 'string', 'format': 'binary'}),
    (bool, {'type': 'boolean'}),
    (int, {'type': 'integer'}),
    (float, {'type': 'number'}),
    (Decimal, {'type': 'number'}),
    (UUID, {'type': 'string', 'format': 'uuid'}),
    (dict, {'type': 'object'}),
    (list, {'type': 'array', 'items': {}}),
    (tuple, {'type': 'array', 'items': {}}),
    (set, {'type': 'array', 'items': {}, 'uniqueItems': True}),
    (frozenset, {'type': 'array', 'items': {}, 'uniqueItems': True}),  # my addition
)

[samuelcolvin]

Could you fix your formatting in the issue?

It would be useful to see a demonstration of the problem: e.g. code that raises an exception.

[wangpeibao]

I have added my exception.

[samuelcolvin]

I meant a self contained example that demonstrates the problem.

E.g.:

from pydantic import BaseModel

...

[wangpeibao]

from pydantic import BaseModel
class Item(BaseModel):
    item1: frozenset

This piece of code is ok. But when I read "SwaggerUI" in web framework "FastApi", the above exception occured.

[samuelcolvin]

That's not a demonstration of the problem. The error can be caused with:

from pydantic import BaseModel


class Item(BaseModel):
    item1: frozenset


item = Item(item1={1, 2, 3})
print(item.schema())

exception:

Traceback (most recent call last):
  File "test.py", line 9, in <module>
    print(item.schema())
  File "/Users/samuel/code/pydantic/pydantic/main.py", line 558, in schema
    s = model_schema(cls, by_alias=by_alias)
  File "/Users/samuel/code/pydantic/pydantic/schema.py", line 137, in model_schema
    m_schema, m_definitions, nested_models = model_process_schema(
  File "/Users/samuel/code/pydantic/pydantic/schema.py", line 465, in model_process_schema
    m_schema, m_definitions, nested_models = model_type_schema(
  File "/Users/samuel/code/pydantic/pydantic/schema.py", line 501, in model_type_schema
    f_schema, f_definitions, f_nested_models = field_schema(
  File "/Users/samuel/code/pydantic/pydantic/schema.py", line 190, in field_schema
    f_schema, f_definitions, f_nested_models = field_type_schema(
  File "/Users/samuel/code/pydantic/pydantic/schema.py", line 417, in field_type_schema
    f_schema, f_definitions, f_nested_models = field_singleton_schema(
  File "/Users/samuel/code/pydantic/pydantic/schema.py", line 733, in field_singleton_schema
    raise ValueError(f'Value not declarable with JSON Schema, field: {field}')
ValueError: Value not declarable with JSON Schema, field: name='item1' type=frozenset required=True

This is what I was looking for.

PR welcome to to fix this.