函数返回运动员时，为什么Go会“声明并不使用运动员”？

I am using the declared variable to return a struct. Why is the compiler says I'm not using the declared variable?

I put log.Printf statements to help debug the error. Why doesn't the log.Printf count as using the variable?

import (  
"github.com/gorilla/sessions"
)

func profileFromSession(r *http.Request) *workout.Athlete {

session, err := workout.SessionStore.Get(r, defaultSessionID)
log.Printf("$$$$$$$$$$$ session contains %#v", session)
if err != nil {
    log.Print("$$$$$$$$ error SessionStore.Get %s", err)
    return nil
}

// retrieve Athlete struct and type-assert it.
valAthlete := session.Values[currentAthleteKey]
var athlete = &workout.Athlete{}
log.Printf("before type check athlete is %T", athlete)

if athlete, ok := valAthlete.(*workout.Athlete); !ok {
    log.Print("athlete is not the correct Type")
}
log.Printf("profile athlete has %#s", athlete)

    return athlete
}

ERROR 2019-07-26 22:00:32,931 instance_factory.py:243] Failed to build Go application

/var/folders/mw/0y88j8_54bjc93d_lg3120qw0000gp/T/tmpAqfXoRappengine-go-bin/auth.go:265:33: athlete declared and not used

Using dev_appserver.py go version go1.11.2 darwin/amd64

The := statement declares a new variable. In particular, variables declared using := within an if condition are scoped to only the corresponding if and else blocks, not the rest of the surrounding scope.

So you have two distinct athlete variables there: one scoped to the function, and a second one scoped to the if-statement within that function.

To assign to the function-scoped variable instead of declaring a new local variable, use = instead of :=. In order to do that, you may need to also declare the ok variable ahead of that assignment:

    var ok bool
    if athlete, ok = valAthlete.(*workout.Athlete); !ok {