### Top-level application { #top-level-application }

First, create the main, top-level, **FastAPI** application, and its *path operations*:

{* ../../docs_src/sub_applications/tutorial001.py hl[3, 6:8] *}

### Sub-application { #sub-application }

Then, create your sub-application, and its *path operations*.

This sub-application is just another standard FastAPI application, but this is the one that will be "mounted":

    /**
     * Validate that the constructor masks the supplied value to its
     * lowest 8 bits (NdrShort incorrectly masks to 8 bits even though it's a short).
     */
    @ParameterizedTest
    @ValueSource(ints = { 0, 1, 255, -1, 256, 65535 })
    void constructorMasksValue(int input) {
        NdrShort ns = new NdrShort(input);
        // NdrShort masks to 0xFF (8 bits) in its constructor
        int expected = input & 0xFF;

The history of **FastAPI** is in great part the history of its predecessors.

As said in the section [Alternatives](alternatives.md){.internal-link target=_blank}:

<blockquote markdown="1">

**FastAPI** wouldn't exist if not for the previous work of others.

There have been many tools created before that have helped inspire its creation.

  /** Maps a bucket to the "entry" of its first element. */
  private transient int[] hashTableKToV;

  /** Maps a bucket to the "entry" of its first element. */
  private transient int[] hashTableVToK;

  /** Maps an "entry" to the "entry" that follows it in its bucket. */
  private transient int[] nextInBucketKToV;

  /** Maps an "entry" to the "entry" that follows it in its bucket. */

## Using the `jsonable_encoder` in a `Response` { #using-the-jsonable-encoder-in-a-response }

Because **FastAPI** doesn't make any changes to a `Response` you return, you have to make sure its contents are ready for it.

For example, you cannot put a Pydantic model in a `JSONResponse` without first converting it to a `dict` with all the data types (like `datetime`, `UUID`, etc) converted to JSON-compatible types.

        uses: actions/cache@0400d5f644dc74513175e3cd8d07132dd4860809 # v4
        with:
          path: ~/.mimir/local
          key: mimir-${{ runner.os }}-its-${{ hashFiles('**/pom.xml') }}
          restore-keys: |
            mimir-${{ runner.os }}-its-
            mimir-${{ runner.os }}-

      - name: Download Maven distribution
        uses: actions/download-artifact@634f93cb2916e3fdff6788551b99b062d0335ce0 # v4

          source.readData(data)
        }

        in 5..7 -> {
          data.writeByte('\n'.code) // 'data' on a line of its own.
        }

        in 8..9 -> {
          id = source.readUtf8LineStrict().takeIf { it.isNotEmpty() }
        }

        in 10..12 -> {
          id = null // 'id' on a line of its own.
        }

        in 13..14 -> {
          type = source.readUtf8LineStrict().takeIf { it.isNotEmpty() }

        // Although we cannot directly verify the super constructor call with Mockito for a real class,
        // we can infer its correct behavior if the object is successfully constructed and its own fields are correct.
        // The super constructor is called with (domainHandle, 0, acct_flags, null, 0)

   * stream whose elements are non-null. However, the method goes out of its way to still handle
   * nulls in the stream. This means that the method can safely be used with a stream that contains
   * nulls as long as the *last* element is *not* null.
   *
   * (To "go out of its way," the method tracks a `set` bit so that it can distinguish "the final

   * other methods are supported by the multimap and its views. When adding a key that doesn't
   * satisfy the predicate, the multimap's {@code put()}, {@code putAll()}, and {@code
   * replaceValues()} methods throw an {@link IllegalArgumentException}.
   *
   * <p>When methods such as {@code removeAll()} and {@code clear()} are called on the filtered