Let's see how that works and how to change it if you need to do that.

## Pydantic Models for Input and Output { #pydantic-models-for-input-and-output }

Let's say you have a Pydantic model with default values, like this one:

{* ../../docs_src/separate_openapi_schemas/tutorial001_py310.py ln[1:7] hl[7] *}

specific language governing permissions and limitations
under the License.
-->

<model>
  <id>supplementalModel</id>
  <name>SupplementalDataModel</name>
  <description>Provides access to supplemental POM data models.</description>
  <defaults>
    <default>
      <key>package</key>
      <value>org.apache.maven.plugin.resources.remote</value>
    </default>
  </defaults>
  <classes>

        Doc(
            """
            Pydantic's `include` parameter, passed to Pydantic models to set the
            fields to include.
            """
        ),
    ] = None,
    exclude: Annotated[
        Optional[IncEx],
        Doc(
            """
            Pydantic's `exclude` parameter, passed to Pydantic models to set the
            fields to exclude.
            """
        ),
    ] = None,

    /**
     * Gets the level of validation to perform on processed models.
     *
     * @return The level of validation to perform on processed models.
     */
    int getValidationLevel();

    /**
     * Sets the level of validation to perform on processed models. For building of projects,

        <configuration>
          <velocityBasedir>${project.basedir}/../../src/mdo</velocityBasedir>
          <version>1.2.0</version>
          <models>
            <model>src/main/mdo/metadata.mdo</model>
          </models>
          <templates>
            <template>model.vm</template>
          </templates>
          <params>
            <param>packageModelV4=org.apache.maven.api.metadata</param>

You can declare the type used for the response by annotating the *path operation function* **return type**.

You can use **type annotations** the same way you would for input data in function **parameters**, you can use Pydantic models, lists, dictionaries, scalar values like integers, booleans, etc.

{* ../../docs_src/response_model/tutorial001_01_py310.py hl[16,21] *}

FastAPI will use this return type to:

* **Validate** the returned data.

        <groupId>org.codehaus.modello</groupId>
        <artifactId>modello-maven-plugin</artifactId>
        <configuration>
          <version>1.2.0</version>
          <models>
            <model>src/main/mdo/core-extensions.mdo</model>
          </models>
          <templates>
            <template>model.vm</template>
          </templates>
          <params>
            <param>locationTracking=true</param>

        <configuration>
          <version>4.2.0</version>
          <velocityBasedir>${project.basedir}/../../src/mdo</velocityBasedir>
          <models>
            <model>src/main/mdo/maven.mdo</model>
          </models>
          <templates>
            <template>model.vm</template>
          </templates>
          <params>
            <param>packageModelV4=org.apache.maven.api.model</param>

                "input": "XL",
            }
        ],
        "body": {"title": "towel", "size": "XL"},
    }


def test_post():
    data = {"title": "towel", "size": 5}
    response = client.post("/items/", json=data)
    assert response.status_code == 200, response.text
    assert response.json() == data


def test_openapi_schema():

        <configuration>
          <version>2.0.0</version>
          <velocityBasedir>${project.basedir}/../../src/mdo</velocityBasedir>
          <models>
            <model>src/main/mdo/settings.mdo</model>
          </models>
          <templates>
            <template>model.vm</template>
          </templates>
          <params>
            <param>packageModelV4=org.apache.maven.api.settings</param>