# 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 🚀

# 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**.

Yaroslav Luchinsky <******@****.***> 1736187999 +0300

YungYueh ChanLee <******@****.***> 1733746858 +0800

Gerry Sabar <******@****.***> 1734039996 +0700

Sebastián Ramírez <******@****.***> 1735580804 +0000

Sebastián Ramírez <******@****.***> 1735583217 +0000

nillvitor <******@****.***> 1736455267 -0300

        } else if (first != null && second != null) {
            // Both non-null - should be same instance
            assertSame(first, second, "Second getSubject should return same cached instance as first");
        } else {
            fail("Inconsistent behavior: first=" + first + ", second=" + second);
        }

        // Third call should return the same cached Subject instance

	if err := DB.Joins("NamedPet", onQuery).Where("users.name = ?", user.Name).First(&user1).Error; err != nil {
		t.Fatalf("Failed to load with joins on, got error: %v", err)
	}

	AssertEqual(t, user1.NamedPet.Name, "joins-on_pet_1")

	onQuery2 := DB.Where(&Pet{Name: "joins-on_pet_2"})
	var user2 User
	if err := DB.Joins("NamedPet", onQuery2).Where("users.name = ?", user.Name).First(&user2).Error; err != nil {