testing: examples do not always capture stdout

[alexcesaro]

Examples do not capture stdout when os.Stdout is assigned to a variable outside the example body:

// foo_test.go
package foo

import "os"

var f = os.Stdout

func Example() {
    f.WriteString("a")
    // Output:
    // a
}

$ go test
a--- FAIL: Example (0.00s)
got:

want:
a
FAIL
exit status 1

[minux]

I think this is reasonable limitation, f is assigned before the testing package can replace it. If the example author does this, then it's probably intentional to bypass the output capture.

[gopherbot]

CL https://golang.org/cl/13927 mentions this issue.

[ianlancetaylor]

It doesn't work, so, don't do that. Examples are a convenience feature for documentation, not something that has to work no matter what the program does.