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# Security - First Steps { #security-first-steps }

Let's imagine that you have your **backend** API in some domain.

And you have a **frontend** in another domain or in a different path of the same domain (or in a mobile application).

And you want to have a way for the frontend to authenticate with the backend, using a **username** and **password**.

We can use **OAuth2** to build that with **FastAPI**.

# First Steps { #first-steps }

The simplest FastAPI file could look like this:

{* ../../docs_src/first_steps/tutorial001.py *}

Copy that to a file `main.py`.

Run the live server:

<div class="termy">

```console
$ <font color="#4E9A06">fastapi</font> dev <u style="text-decoration-style:solid">main.py</u>

  <span style="background-color:#009485"><font color="#D3D7CF"> FastAPI </font></span>  Starting development server 🚀

import com.google.common.annotations.J2ktIncompatible;
import java.io.Serializable;
import org.jspecify.annotations.Nullable;

/**
 * {@code keySet()} implementation for {@link ImmutableMap}.
 *
 * @author Jesse Wilson
 * @author Kevin Bourrillion
 */
@GwtCompatible
final class ImmutableMapKeySet<K, V> extends IndexedImmutableSet<K> {
  private final ImmutableMap<K, V> map;

  ImmutableMapKeySet(ImmutableMap<K, V> map) {

Então, durante esse tempo, o computador pode ir e fazer outro trabalho, enquanto o "arquivo lento" 📝 termina.

    }

    /**
     * Throws {@link UnsupportedOperationException}. This method is inherited from {@link
     * ImmutableMap.Builder}, but it does not make sense for bimaps.
     *
     * @throws UnsupportedOperationException always
     * @deprecated This method does not make sense for bimaps and should not be called.
     * @since 31.1
     */
    @DoNotCall
    @Deprecated
    @Override

jobs:
  changes:
    runs-on: ubuntu-latest
    # Required permissions
    permissions:
      pull-requests: read
    # Set job outputs to values from filter step
    outputs:
      docs: ${{ steps.filter.outputs.docs }}
    steps:
    - uses: actions/checkout@v5
    # For pull requests it's not necessary to checkout the code but for the main branch it is
    - uses: dorny/paths-filter@v3
      id: filter
      with:

import com.google.j2objc.annotations.RetainedWith;
import java.util.function.BiConsumer;
import org.jspecify.annotations.Nullable;

/**
 * Implementation of {@link ImmutableMap} with exactly one entry.
 *
 * @author Jesse Wilson
 * @author Kevin Bourrillion
 */
@GwtCompatible
@SuppressWarnings("serial") // uses writeReplace(), not default serialization
final class SingletonImmutableBiMap<K, V> extends ImmutableBiMap<K, V> {

If you have a big application, you might end up accumulating **several tags**, and you would want to make sure you always use the **same tag** for related *path operations*.

In these cases, it could make sense to store the tags in an `Enum`.

**FastAPI** supports that the same way as with plain strings:

{* ../../docs_src/path_operation_configuration/tutorial002b.py hl[1,8:10,13,18] *}

import java.util.Map.Entry;
import java.util.SortedMap;
import org.jspecify.annotations.NullMarked;

/**
 * Generators of sorted maps and derived collections.
 *
 * @author Kevin Bourrillion
 * @author Jesse Wilson
 * @author Jared Levy
 * @author Hayward Chan
 * @author Chris Povirk
 * @author Louis Wasserman
 */
@GwtCompatible
@NullMarked
public class SortedMapGenerators {