io/ioutil: WriteFile example returns "No such file or directory" because testdata does not exist

[KolesnikDmitriy]

What version of Go are you using (go version)?

$ go version
go1.12.6

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
https://play.golang.org/

What did you do?

Run example in https://golang.org/pkg/io/ioutil/#WriteFile

What did you expect to see?

Expected file created

What did you see instead?

2009/11/10 23:00:00 open testdata/hello: No such file or directory

Program exited.

[AlexRouSg]

Are you running the example in the playground or on your system?

EDIT: Sorry was looking at the wrong example

[av86743]

Indeed, that link https://golang.org/pkg/io/ioutil/#WriteFile above, which offers much better func TempFile choice, also with examples, deserves an issue on its own.

[agnivade]

Thanks. The code in the example assumes a testdata directory. Since it cannot find it, it throws an error. It worked so far because the source tree always had a testdata directory.

You are welcome to send a patch.

[darkLord19]

https://github.com/golang/go/tree/master/src/io/ioutil/testdata Isn't it still present?

[cuonglm]

@darkLord19 Examples are run in separate environment, without the directory structure as in source code. ReadFile has the same problem as it read testdata/hello file.

[gopherbot]

Change https://golang.org/cl/185037 mentions this issue: io/ioutil: make Read/WriteFile examples runnable in playground

[av86743]

Regarding https://golang.org/cl/185037 :

$ find go/src/github.com/av86743/go/src -type f -name 'example_test.go' -exec grep -e '"testdata' {} ';' -print 
	r, err := zip.OpenReader("testdata/readme.zip")
go/src/github.com/av86743/go/src/archive/zip/example_test.go
	cert, err := tls.LoadX509KeyPair("testdata/example-cert.pem", "testdata/example-key.pem")
go/src/github.com/av86743/go/src/crypto/tls/example_test.go
	content, err := ioutil.ReadFile("testdata/hello")
	err := ioutil.WriteFile("testdata/hello", message, 0644)
go/src/github.com/av86743/go/src/io/ioutil/example_test.go
$

[bcmills]

I believe this is a specific case of the broader issue #19825.

[fgm]

Does the fix need to bring the testdata/hello file by changing the playground implementation, or could it just read one of the few files actually available, readable and non-empty test files on the playground, currently just /etc/group and /etc/hosts ? The latter would actually be demonstrating the purpose of ioutil.ReadFile, without touching anything else anywhere.