Repository: pydantic/pydantic

Issue:
Empty tuple type does not generate valid JSON schema
### Checks

* [x] I added a descriptive title to this issue
* [x] I have searched (google, github) for similar issues and couldn't find anything
* [x] I have read and followed [the docs](https://pydantic-docs.helpmanual.io/) and still think this is a bug

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# Bug

Output of `python -c "import pydantic.utils; print(pydantic.utils.version_info())"`:
```
 python -c "import pydantic.utils; print(pydantic.utils.version_info())"
             pydantic version: 1.8.1
            pydantic compiled: True
                 install path: /Users/smacpher/.virtualenvs/nucleus/lib/python3.6/site-packages/pydantic
               python version: 3.6.9 (default, Oct 16 2019, 11:14:39)  [GCC 4.2.1 Compatible Apple LLVM 10.0.0 (clang-1000.10.44.4)]
                     platform: Darwin-17.7.0-x86_64-i386-64bit
     optional deps. installed: ['typing-extensions']
```
<!-- or if you're using pydantic prior to v1.3, manually include: OS, python version and pydantic version -->

<!-- Please read the [docs](https://pydantic-docs.helpmanual.io/) and search through issues to
confirm your bug hasn't already been reported. -->

<!-- Where possible please include a self-contained code snippet describing your bug: -->

```py
import jsonschema
import pydantic

from typing import Tuple

class Model(pydantic.BaseModel):
    f: Tuple[()]

jsonschema.Draft7Validator.check_schema(Model.schema())
```

This fails with:

```
SchemaError: [] is not valid under any of the given schemas

Failed validating 'anyOf' in metaschema['properties']['properties']['additionalProperties']['properties']['items']:
    {'anyOf': [{'$ref': '#'}, {'$ref': '#/definitions/schemaArray'}],
     'default': True}

On schema['properties']['f']['items']:
    []
```

The generated schema is:

```py
{'title': 'Model',
 'type': 'object',
 'properties': {'f': {'title': 'F', 'type': 'array', 'items': []}},
 'required': ['f']}
```

Is this expected behavior? Would the desired behavior be to set `maxItems: 0` and remove `items: []` like such (that would make the generated schema valid)?

```
{'title': 'Model',
 'type': 'object',
 'properties': {'f': {'title': 'F', 'type': 'array', 'maxItems': 0}},
 'required': ['f']}
```

BTW, glad to see that empty tuples (`Tuple[()]`) are now supported (https://github.com/samuelcolvin/pydantic/issues/2318)! Thank you 🙏 .


Discussion:

Resolving patch:
diff --git a/docs/build/schema_mapping.py b/docs/build/schema_mapping.py
--- a/docs/build/schema_mapping.py
+++ b/docs/build/schema_mapping.py
@@ -81,10 +81,17 @@
         'JSON Schema Validation',
         'And equivalently for any other sub type, e.g. `List[int]`.'
     ],
+    [
+        'Tuple[str, ...]',
+        'array',
+        {'items': {'type': 'string'}},
+        'JSON Schema Validation',
+        'And equivalently for any other sub type, e.g. `Tuple[int, ...]`.'
+    ],
     [
         'Tuple[str, int]',
         'array',
-        {'items': [{'type': 'string'}, {'type': 'integer'}]},
+        {'items': [{'type': 'string'}, {'type': 'integer'}], 'minItems': 2, 'maxItems': 2},
         'JSON Schema Validation',
         (
             'And equivalently for any other set of subtypes. Note: If using schemas for OpenAPI, '
diff --git a/pydantic/schema.py b/pydantic/schema.py
--- a/pydantic/schema.py
+++ b/pydantic/schema.py
@@ -490,15 +490,19 @@ def field_type_schema(
             definitions.update(sf_definitions)
             nested_models.update(sf_nested_models)
             sub_schema.append(sf_schema)
-        if len(sub_schema) == 1:
-            if field.shape == SHAPE_GENERIC:
-                f_schema = sub_schema[0]
-            else:
-                f_schema = {'type': 'array', 'items': sub_schema[0]}
-        else:
-            f_schema = {'type': 'array', 'items': sub_schema}
+
+        sub_fields_len = len(sub_fields)
         if field.shape == SHAPE_GENERIC:
-            f_schema = {'allOf': [f_schema]}
+            all_of_schemas = sub_schema[0] if sub_fields_len == 1 else {'type': 'array', 'items': sub_schema}
+            f_schema = {'allOf': [all_of_schemas]}
+        else:
+            f_schema = {
+                'type': 'array',
+                'minItems': sub_fields_len,
+                'maxItems': sub_fields_len,
+            }
+            if sub_fields_len >= 1:
+                f_schema['items'] = sub_schema
     else:
         assert field.shape in {SHAPE_SINGLETON, SHAPE_GENERIC}, field.shape
         f_schema, f_definitions, f_nested_models = field_singleton_schema(
@@ -835,7 +839,15 @@ def field_singleton_schema(  # noqa: C901 (ignore complexity)
             ref_template=ref_template,
             known_models=known_models,
         )
-        f_schema.update({'type': 'array', 'items': list(sub_schema['properties'].values())})
+        items_schemas = list(sub_schema['properties'].values())
+        f_schema.update(
+            {
+                'type': 'array',
+                'items': items_schemas,
+                'minItems': len(items_schemas),
+                'maxItems': len(items_schemas),
+            }
+        )
     elif not hasattr(field_type, '__pydantic_model__'):
         add_field_type_to_schema(field_type, f_schema)
 


Test patch:
diff --git a/tests/test_annotated_types.py b/tests/test_annotated_types.py
--- a/tests/test_annotated_types.py
+++ b/tests/test_annotated_types.py
@@ -79,6 +79,8 @@ class Model(BaseModel):
                     {'title': 'X', 'type': 'integer'},
                     {'title': 'Y', 'type': 'integer'},
                 ],
+                'minItems': 2,
+                'maxItems': 2,
             },
             'pos2': {
                 'title': 'Pos2',
@@ -87,6 +89,8 @@ class Model(BaseModel):
                     {'title': 'X'},
                     {'title': 'Y'},
                 ],
+                'minItems': 2,
+                'maxItems': 2,
             },
             'pos3': {
                 'title': 'Pos3',
@@ -95,6 +99,8 @@ class Model(BaseModel):
                     {'type': 'integer'},
                     {'type': 'integer'},
                 ],
+                'minItems': 2,
+                'maxItems': 2,
             },
         },
         'required': ['pos1', 'pos2', 'pos3'],
diff --git a/tests/test_schema.py b/tests/test_schema.py
--- a/tests/test_schema.py
+++ b/tests/test_schema.py
@@ -526,34 +526,36 @@ class Model(BaseModel):
 
 
 @pytest.mark.parametrize(
-    'field_type,expected_schema',
+    'field_type,extra_props',
     [
-        (tuple, {}),
+        (tuple, {'items': {}}),
         (
             Tuple[str, int, Union[str, int, float], float],
-            [
-                {'type': 'string'},
-                {'type': 'integer'},
-                {'anyOf': [{'type': 'string'}, {'type': 'integer'}, {'type': 'number'}]},
-                {'type': 'number'},
-            ],
+            {
+                'items': [
+                    {'type': 'string'},
+                    {'type': 'integer'},
+                    {'anyOf': [{'type': 'string'}, {'type': 'integer'}, {'type': 'number'}]},
+                    {'type': 'number'},
+                ],
+                'minItems': 4,
+                'maxItems': 4,
+            },
         ),
-        (Tuple[str], {'type': 'string'}),
+        (Tuple[str], {'items': [{'type': 'string'}], 'minItems': 1, 'maxItems': 1}),
+        (Tuple[()], {'maxItems': 0, 'minItems': 0}),
     ],
 )
-def test_tuple(field_type, expected_schema):
+def test_tuple(field_type, extra_props):
     class Model(BaseModel):
         a: field_type
 
-    base_schema = {
+    assert Model.schema() == {
         'title': 'Model',
         'type': 'object',
-        'properties': {'a': {'title': 'A', 'type': 'array'}},
+        'properties': {'a': {'title': 'A', 'type': 'array', **extra_props}},
         'required': ['a'],
     }
-    base_schema['properties']['a']['items'] = expected_schema
-
-    assert Model.schema() == base_schema
 
 
 def test_bool():
@@ -1923,6 +1925,8 @@ class Config:
                     {'exclusiveMinimum': 0, 'type': 'integer'},
                     {'exclusiveMinimum': 0, 'type': 'integer'},
                 ],
+                'minItems': 3,
+                'maxItems': 3,
             },
         ),
         (
@@ -2312,6 +2316,8 @@ class LocationBase(BaseModel):
                 'default': Coordinates(x=0, y=0),
                 'type': 'array',
                 'items': [{'title': 'X', 'type': 'number'}, {'title': 'Y', 'type': 'number'}],
+                'minItems': 2,
+                'maxItems': 2,
             }
         },
     }
@@ -2403,11 +2409,19 @@ class Model(BaseModel):
                 'examples': 'examples',
             },
             'data3': {'title': 'Data3', 'type': 'array', 'items': {}},
-            'data4': {'title': 'Data4', 'type': 'array', 'items': {'$ref': '#/definitions/CustomType'}},
+            'data4': {
+                'title': 'Data4',
+                'type': 'array',
+                'items': [{'$ref': '#/definitions/CustomType'}],
+                'minItems': 1,
+                'maxItems': 1,
+            },
             'data5': {
                 'title': 'Data5',
                 'type': 'array',
                 'items': [{'$ref': '#/definitions/CustomType'}, {'type': 'string'}],
+                'minItems': 2,
+                'maxItems': 2,
             },
         },
         'required': ['data0', 'data1', 'data2', 'data3', 'data4', 'data5'],