# Learn { #learn }

Here are the introductory sections and the tutorials to learn **FastAPI**.

You would have **one single `engine` object** for all your code to connect to the same database.

{* ../../docs_src/sql_databases/tutorial001_an_py310.py ln[14:18] hl[14:15,17:18] *}

Using `check_same_thread=False` allows FastAPI to use the same SQLite database in different threads. This is necessary as **one single request** could use **more than one thread** (for example in dependencies).

Nevertheless, it is already being used as a "standard" by several tools. This greatly improves interoperability, as you could switch Uvicorn for any other ASGI server (like Daphne or Hypercorn), or you could add ASGI compatible tools, like `python-socketio`.

///

/// check | **FastAPI** uses it to

Handle all the core web parts. Adding features on top.

After that, you would need to **install** FastAPI and any other **packages** you want to use.

To install packages you would normally use the `pip` command that comes with Python (or similar alternatives).

Nevertheless, if you just use `pip` directly, the packages would be installed in your **global Python environment** (the global installation of Python).

        MAX_FD=$( ulimit -H -n ) ||
            warn "Could not query maximum file descriptor limit"
    esac
    case $MAX_FD in  #(
      '' | soft) :;; #(
      *)
        # In POSIX sh, ulimit -n is undefined. That's why the result is checked to see if it worked.
        # shellcheck disable=SC2039,SC3045
        ulimit -n "$MAX_FD" ||
            warn "Could not set maximum file descriptor limit to $MAX_FD"
    esac
fi

You could have turns as in the burgers example, first the living room, then the kitchen, but as you are not waiting 🕙 for anything, just cleaning and cleaning, the turns wouldn't affect anything.

It would take the same amount of time to finish with or without turns (concurrency) and you would have done the same amount of work.

If it was in a type annotation we could have used the vertical bar, as:

```Python
some_variable: PlaneItem | CarItem
```

But if we put that in the assignment `response_model=PlaneItem | CarItem` we would get an error, because Python would try to perform an **invalid operation** between `PlaneItem` and `CarItem` instead of interpreting that as a type annotation.

    return backingMap;
  }

  @SuppressWarnings("unchecked") // value could not get in if not a T
  private <T extends B> @Nullable T trustedPut(
      TypeToken<@NonNull T> type, @ParametricNullness T value) {
    return (T) backingMap.put(type, value);
  }

  @SuppressWarnings("unchecked") // value could not get in if not a T
  private <T extends @NonNull B> @Nullable T trustedGet(TypeToken<T> type) {

	}{Version: healthInfo.Version}

	enc := json.NewEncoder(gzWriter)
	if e := enc.Encode(header); e != nil {
		internalLogIf(ctx, fmt.Errorf("Could not encode health info header: %w", e))
		return nil
	}

	if e := enc.Encode(healthInfo); e != nil {
		internalLogIf(ctx, fmt.Errorf("Could not encode health info: %w", e))
		return nil
	}

	gzWriter.Flush()
	gzWriter.Close()

	return b.Bytes()

You import it and create an "instance" the same way you would with the class `FastAPI`:

```Python hl_lines="1  3" title="app/routers/users.py"
{!../../docs_src/bigger_applications/app/routers/users.py!}
```

### *Path operations* with `APIRouter` { #path-operations-with-apirouter }

And then you use it to declare your *path operations*.

Use it the same way you would use the `FastAPI` class: