# Using Dataclasses { #using-dataclasses }

FastAPI is built on top of **Pydantic**, and I have been showing you how to use Pydantic models to declare requests and responses.

But FastAPI also supports using <a href="https://docs.python.org/3/library/dataclasses.html" class="external-link" target="_blank">`dataclasses`</a> the same way:

{* ../../docs_src/dataclasses/tutorial001.py hl[1,7:12,19:20] *}

    assertSame(e, urlFormParameterEscaper());
    assertBasicUrlEscaper(e);

    /*
     * Specified as safe by RFC 2396 but not by java.net.URLEncoder. These tests will start failing
     * when the escaper is made compliant with RFC 2396, but that's a good thing (just change them
     * to assertUnescaped).
     */
    assertEscaping(e, "%21", '!');
    assertEscaping(e, "%28", '(');

		closeBitrotWriters(writers)
		if err != nil && !test.shouldFail {
			t.Errorf("Test %d: should pass but failed with: %v", i, err)
		}
		if err == nil && test.shouldFail {
			t.Errorf("Test %d: should fail but it passed", i)
		}
		for i, w := range writers {
			if w == nil {
				disks[i] = OfflineDisk
			}
		}
		if err == nil {

```JSON
{
    "name": "Foo",
    "description": "The pretender",
    "price": 42.0,
    "tax": 3.2
}
```

But you can also declare multiple body parameters, e.g. `item` and `user`:

{* ../../docs_src/body_multiple_params/tutorial002_py310.py hl[20] *}

    Path topDirectory();

    /**
     * Returns the root directory of the Maven invocation, if found. This is determined by the presence of a
     * {@code .mvn} directory or a POM with the root="true" property but is not always applicable (ie invocation
     * from outside a checkout).
     *
     * @return the root directory path, if present
     */
    @Nonnull
    Optional<Path> rootDirectory();

    /**

              <!-- FeatureUtilTest.*ExampleDerivedInterfaceTester, com.google.common.io.*Tester, incidentally FeatureSpecificTestSuiteBuilderTest.MyAbstractTester (but we don't care either way because it's not meant to run on its own but works OK if it does)... but not NullPointerTesterTest, etc. -->
              <exclude>%regex[.*Tester.class]</exclude>
              <!-- Anonymous TestCase subclasses in GeneratedMonitorTest -->

/**
 * Outer class that exists solely to let us write {@code Partially.GwtIncompatible} instead of plain
 * {@code GwtIncompatible}. This is more accurate for {@link Futures#catching}, which is available
 * under GWT but with a slightly different signature.
 *
 * <p>We can't use {@code PartiallyGwtIncompatible} because then the GWT compiler wouldn't recognize

## Install the Server Program { #install-the-server-program }

When you install FastAPI, it comes with a production server, Uvicorn, and you can start it with the `fastapi run` command.

But you can also install an ASGI server manually.

	AssertAssociationCount(t, users, "Account", 2, "")

	// Find
	var accounts []Account
	if DB.Model(&users).Association("Account").Find(&accounts); len(accounts) != 2 {
		t.Errorf("accounts count should be %v, but got %v", 3, len(accounts))
	}

	// Append
	DB.Model(&users).Association("Account").Append(
		&Account{Number: "account-slice-append-1"},
		&Account{Number: "account-slice-append-2"},

 *       typically because the type forbids nullable type arguments: For example, {@code
 *       ImmutableList.get} returns {@code E}, but that value is never {@code null}. (Accordingly,
 *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)
 *   <li>methods whose return type is a type variable but which can return {@code null} regardless