The package name must be decided the path on compile time but Go's binary doesn't have long package name. runtime package provide runtime.FuncForPC(pc).Name() but it's short name. It may be possible to pick-up package name from file name by runtime.Caller but it won't work for running go run command.

I understand go's package have name and path. (https://blog.golang.org/package-names)

And I found go/build package to obtain package name using go/ast. (https://golang.org/pkg/go/build/#Import)

But using it to log package is may too expensive.

It might be possible to back the package out of the runtime.Func's Name() method result, but in general we don't know the package. It's also unclear why the filename, which is only just a little longer than the package path, is too long but the package path is not. It really seems like in a huge project you'd want all the info you can get, namely the full file path.

The log flag seem to be working well for most people. We should probably avoid the complexity of new ones unless there is a repeated, compelling need.