We parse flags in main.go which is in main package, of course. Then we have another package where we want to read some flag's value.
flags.Args() work fine, it will return all non-flag values.
But I cannot figure out to how read already parsed value for a flag in a package other than main.
Is it possible?
Thanks
Amer
I had the same requirement recently and I wanted a solution that avoided calling flag.Parse repeatedly in init functions.
Perusing the flag package I found Lookup(name string) which returns a Flag which has a Value. Every built in Value implements the flag.Getter interface. The call chain looks like this:
flag.Lookup("httplog").Value.(flag.Getter).Get().(bool)
If you mistype the flag name or use the wrong type you get a runtime error. I wrapped the lookup in a function that I call directly where needed since the lookup and get methods are fast and the function is not called often. So the main package declares the flag.
// main.go
package main
import "flag"
var httplog = flag.Bool("httplog", false, "Log every HTTP request and response.")
func main() {
flag.Parse()
// ...
}
And the utility package, which is decoupled from main except for the flag name, reads the flag value.
// httpdiag.go
package utility
import "flag"
func logging() bool {
return flag.Lookup("httplog").Value.(flag.Getter).Get().(bool)
}
You can define the var storing the flag in the separate package, as an exported variable, then call the flag parsing in the main package to use that variable, like this:
mypackage/const.go
var (
MyExportedVar string
)
mainpackage/main.go
func init() {
flag.StringVar(&mypackage.MyExportedVar, "flagName", "defaultValue", "usage")
flag.Parse()
}
This way, everybody can access that flag, including that package itself.
Note:
this only works for exported variables.
You can define the flag in that package and call flag.Parse() in func init(), flag.Parse can be called multiple times.
However, if you want to access the flag's value from multiple packages you have to expose it or create an exposed function to check it.
for example:
// pkgA
var A = flag.Bool("a", false, "why a?")
func init() {
flag.Parse()
}
// main package
func main() {
flag.Parse()
if *pkgA.A {
// stuff
}
}
Also you can use FlagSet.Parse(os.Args) if you want to reparse the args.
When you parse your flags, parse them into global variables which start with an initial Capital so they are public, eg
package main
var Species = flag.String("species", "gopher", "the species we are studying")
func main() {
flag.Parse()
}
Then in your other package you can refer to them as
package other
import "/path/to/package/main"
func Whatever() {
fmt.Println(main.Species)
}
Related
If I have a requests package that defines an interface TextExtractor with a GetText method that returns a Text type, the implementations must fulfill the TextExtractor contract exactly, and they are forced to import the Text type.
I have two possible implementations of TextExtractor - one that uses AWS Comprehend and one that uses AWS Textract.
aws_comprehend.go
package aws_comprehend
type AwsComprehend struct{}
func (a *AwsComprehend) GetText() *Text {
// do some stuff with aws comprehend...
return &Text{
Text: "text",
Language: "language",
}
}
type Text struct {
Text string
Language string
}
request.go
package requests
import "fmt"
type TextExtractor interface {
GetText() *Text
}
type Text struct {
Text string
Language string
}
func HandleRequest(textExtractor TextExtractor) {
text := textExtractor.GetText()
fmt.Println(text)
}
main.go
package main
import (
"aws_comprehend"
"requests"
)
func main() {
textExtractor := new(aws_comprehend.AwsComprehend)
requests.HandleRequest(textExtractor)
// this does not work:
// cannot use textExtractor (variable of type *aws_comprehend.AwsComprehend) as
// requests.TextExtractor value in argument to requests.HandleRequest:
// *aws_comprehend.AwsComprehend does not implement requests.TextExtractor
// (wrong type for method GetText)
// have GetText() *aws_comprehend.Text
// want GetText() *requests.Text
}
I understand why this doesn't work. It's because GoLang doesn't support Covariant Result Types. But my question is, what is the standard way to code this situation? The fact that GoLang provides implicit interfaces means that isolating packages is very easy: the calling package defines the interfaces that it uses, and it is passed implementations that fulfill those interfaces. This means that packages don't have to reference each other at all. But if a package defines an interface that returns anything more than a primitive value, then you have to deliberately share those value types. The code above would be fine if GetText returned a string. But the fact that it returns a struct or another interface, means the code can't be written this way.
I want the requests package not to know anything about the aws_comprehend package. This is because I have two implementations of the TextExtractor interface: One that uses AWS Comprehend, and one that uses AWS Textract. I also would prefer not to include a "intermediate" package that has interfaces that both the requests package and the aws_comprehend package inherit from. If both packages have to inherit the same interface, then it seems like it's just indirect coupling to me and it ruins the idea of a implicit interfaces.
I understand that GoLang is very opinionated - So what is the standard way to solve this problem?
first, your file layout is not valid. you cant have two files in the same
folder, with different packages. so below is a corrected layout. also I removed
all pointers, because they aren't needed for this example and are just distracting.
Finally, I updated the method to return the correct type, so that the code
actually compiles:
aws_comprehend/aws_comprehend.go:
package aws_comprehend
import "hello/requests"
type AwsComprehend struct{}
func (AwsComprehend) GetText() requests.Text {
return requests.Text{}
}
requests/request.go:
package requests
import "fmt"
type TextExtractor interface {
GetText() Text
}
type Text struct {
Language string
Text string
}
func HandleRequest(textExtractor TextExtractor) {
text := textExtractor.GetText()
fmt.Println(text)
}
main.go:
package main
import (
"hello/aws_comprehend"
"hello/requests"
)
func main() {
var textExtractor aws_comprehend.AwsComprehend
requests.HandleRequest(textExtractor)
}
I'm testing the idea of putting most of my code in an internal package and then picking exactly which of the methods / types from there I'm exposing to the outside world. Code would look like:
/mypackage/internal/catapult
package catapult
func Load(boulder Boulder) {
// ...
}
func Trigger() *Launch {
// ...
}
// ...
Maybe Load is being called by other internal packages (/mypackage/internal/randomevents/boredsoldier and /mypackage/internal/actualattackstrategy) but shouldn't be allowed by users outside of internal. All those are allowed to do is Trigger the catapult once it's loaded.
So now I'd like to have a package above internal (/mypackage/general) where Trigger is exposed but not Load. I was hoping to do something like:
package general
const TriggerCatapult = catapult.Trigger
// ^ does not work because a function cannot be a const
var TriggerCatapult = catapult.Trigger
// ^ technically works but now the value of TriggerCatapult can be overwritten by any package user
func TriggerCatapult() *catapult.Launch {
return catapult.Trigger()
}
// ^ works. It's just "painful" to have to reproduce the entire function's signature every time
Is there a better way to do this?
No, there is no better way to do this that the way you provide:
func TriggerCatapult() *catapult.Launch {
return catapult.Trigger()
}
You shouldn't return unexported types though, and most linters would catch this for you.
If a user is going to interact directly with things in catapult, then that package should not be internal.
I am trying to use a global variable defined in parent package in the internal package. There are no errors but none of the logs from internal package are printed. My assumption on order of init is: parent.go global variables and then init() from internal package; which seems right when tested with fmt.Println(). But looks like the call to ReplaceGlobals is dropped during compilation. Is there a way to fix it so that I can use global variable from internal package?
Here is the zap package I am using: zap. The API in question is discussed here. Here is the code snippet.
~/parent/parent.go
package main
import (
"log"
"go.uber.org/zap"
"parent/internal"
)
var (
logger = GetMyLogger()
undo = zap.ReplaceGlobals(logger.Desugar())
)
func GetMyLogger() *zap.SugaredLogger {
xx, err := zap.NewProduction()
if err != nil {
log.Fatal("error")
}
sugar := xx.Sugar()
return sugar
}
func main() {
logger.Infof("logger in main")
internal.New()
}
Then in ~/parent/internal/internal.go
package internal
import (
"go.uber.org/zap"
)
var logger *zap.SugaredLogger
func init() {
logger = zap.L().Sugar()
logger.Infof("init() from internal: %v\n", logger)
logger = zap.S()
logger.Infof("init() from internal: %v\n", logger)
}
func New() {
logger.Infof("New() from internal")
}
Package initialization order is well defined, see Spec: Package initialization. Basically you can be sure that if you refer to a package, that package's initialization will be done first, recursively.
Since your internal package does not refer to your main, you have no guarantee if main is initialized before anything is executed from internal.
An easy solution in your case is to move the logger to your internal package, and have main refer / use that.
If you have more packages involved and you want to use the same logger from other packages, you may have a designated package holding the logger, and have everyone refer / use that.
I don't see the public method of the struct that I defined.
Can someone please let me understand why?
Here is the code:
// DataSaver.go:
package DataStorage
import (
"fmt"
"os"
)
type DataSaver struct {
// doesn't relevant to my question
fileName string
file *os.File
}
func PrintStr() {
fmt.Println("hello")
}
Then, I have a main method in other class. I initialized the struct and I wanted to call to PrintStr() function. However, I can't call to this method. Why?
Thank you!
That is not a method, it is just a function.
Import DataStorage (you should make it lower case) then you can call DataStorage.PrintStr()
I had a play around myself with this, and there are a few things to be careful of:
Make sure you import the package properly with respect to your GOPATH, e.g.
import "github.com/some-repo/datastorage"
And ensure that your packages/repos are in your GOPATH and in the correct directories (go is very fussy about how you do this)
Refer to your function using the package name like so:
func main(){
DataStorage.PrintStr()
}
The other poster is correct in that go conventions are to keep package names in lowercase.
Seems you dont have the concept of method receiver yet. Your method for the struct should be defined as below :
func (this DataSaver)PrintStr() {
fmt.Println("hello")
}
or
func (this *DataSaver)PrintStr() {
fmt.Println("hello")
}
which one you choose depends on you want to call the method on a pointer or not;
if I have
// types.go
type S string
func (s *S) Lower() *S {
*s = S(strings.ToLower(string(*s)))
return s
}
`
// in another file
import "u/types"
func main() {
s := types.S("asdf")
if s == "asdf" {
s.Lower()
}
}
Is there a way to shorten types.S("asdf") to just S("asdf")?
Is there a way to lowercase method calls from other files? e.g. s.Lower() => s.lower()?
It's not recommended for most cases but you can do import . "u/types" and all then skip the types prefix. . will import all the public symbols into your package for you allowing you to call them as if they were local to your package.
Not as long as that type is in a different package from where you're using it, without using dot-imports.
Yes, if the other file is still in the same package. Otherwise, no, because then the function won't be exported (visible to other packages). This is Go convention.