```Python hl_lines="1  7-12  19-20"
{!../../../docs_src/dataclasses/tutorial001.py!}
```

This is still supported thanks to **Pydantic**, as it has <a href="https://docs.pydantic.dev/latest/concepts/dataclasses/#use-of-stdlib-dataclasses-with-basemodel" class="external-link" target="_blank">internal support for `dataclasses`</a>.

mkdir $GOCACHE

# Building a trivial non-main package should run compiler the first time.
go build -x -gcflags=-m lib.go
stderr 'compile( |\.exe"?)'
stderr 'lib.go:2.* can inline f'

# ... but not the second, even though it still prints the compiler output.
go build -x -gcflags=-m lib.go
! stderr 'compile( |\.exe"?)'
stderr 'lib.go:2.* can inline f'

# Building a trivial main package should run the compiler and linker the first time.

# cmd/doc should use GOROOT to locate the 'go' command,
# not use whatever is in $PATH.

# Remove 'go' from $PATH. (It can still be located via $GOROOT/bin/go, and the
# test script's built-in 'go' command still knows where to find it.)
env PATH=''
[GOOS:plan9] env path=''

go doc p.X

-- go.mod --
module example

go 1.19

require example.com/p v0.1.0

replace example.com/p => ./pfork
-- example.go --
package example

	MULL	CX
	SHRL	$22, DX
	MOVL	DX, BX
	IMULL	$10000000, DX
	MOVL	SI, CX
	SUBL	DX, CX

	// DI = sec/100 (still)
	// BX = (nano/100%1e9)/1e7 = (nano/1e9)%100 = sec%100
	// CX = (nano/100%1e9)%1e7 = (nano%1e9)/100 = nsec/100
	// store nsec for return
	IMULL	$100, CX
	MOVL	CX, nsec+8(FP)

	// DI = sec/100 (still)
	// BX = sec%100
	// construct DX:AX = 64-bit sec and store for return
	MOVL	$0, DX
	MOVL	$100, AX
	MULL	DI

app = FastAPI(
    servers=[
        {"url": "/", "description": "Default, relative server"},
        {
            "url": "http://staging.localhost.tiangolo.com:8000",
            "description": "Staging but actually localhost still",
        },
        {"url": "https://prod.example.com"},
    ]
)


@app.get("/foo")
def foo():
    return {"message": "Hello World"}


client = TestClient(app)


def test_app():

# (But asking for exactly v2.0.0+incompatible should still succeed.)
go list -m github.com/russross/blackfriday@v2.0.0+incompatible
stdout '^github.com/russross/blackfriday v2\.0\.0\+incompatible$'


# However, if the latest compatible version does not include a go.mod file,
# +incompatible versions should still be listed, as they may still reflect the
# intent of the module author.

# This is a test that if one of the main modules replaces the other
# the vendor consistency checks still pass. The replacement is ignored
# because it is of a main module, but it is still recorded in
# vendor/modules.txt.

go work vendor
go list all # make sure the consistency checks pass
! stderr .

# Removing the replace causes consistency checks to fail
cp a_go_mod_no_replace a/go.mod
! go list all # consistency checks fail

type i4 struct {
	a, b, c, d int
}

//go:noinline
func spills(px *i4) {
}

//go:registerparams
//go:noinline
func F(x i4) i4 {
	ab := x.a + x.b
	bc := x.b + x.c
	cd := x.c + x.d
	ad := x.a + x.d
	ba := x.a - x.b
	cb := x.b - x.c
	dc := x.c - x.d
	da := x.a - x.d
	i := i4{ab*bc + da, cd*ad + cb, ba*cb + ad, dc*da + bc}
	spills(&i)
	return i
}

func main() {
	x := i4{1, 2, 3, 4}

go get example.com/c@v0.1.0
go list -m all
stdout '^example.com/a v0.1.0 '
stdout '^example.com/b v0.2.0 '
stdout '^example.com/c v0.1.0 '

# At this point, b.2 is still an explicit root, so its dependency on c
# is still tracked, and it will still be downgraded away if we remove c.
# ('go get' never makes a root into a non-root. Only 'go mod tidy' does that.)

go get example.com/c@none
go list -m all
! stdout '^example.com/a '

// 2. given a previous hash, we can retrieve a previous state from the map, if it is still retained.
type Ledger interface {
	// Put adds or overwrites a key in the Ledger
	Put(key, value string) (string, error)
	// Delete removes a key from the Ledger, which may still be read using GetPreviousValue
	Delete(key string) error
	// Get returns a the value of the key from the Ledger's current state