You're right! In the background, ioutil.WriteFile uses os.OpenFile, and has been doing so since 2011, which doesn't create any intermediate directories.

I think that we could indeed change the example.
My 2¢ would be to add a couple of lines that create a directory in the example. We could also avoid the testdata name since it holds a special meaning in Go.

I'm not sure about documenting this behavior though since we don't explicitly mention anything about directories there.

/cc @dmitshur

The testdata directory exists on disk. However, the example doesn’t have an // Output comment, so it’s only meant to be compiled and displayed in documentation, but not executed:

$ go test -v -run ExampleWriteFile io/ioutil
testing: warning: no tests to run

It doesn’t work on the Go Playground because the testdata directory isn’t available in that environment. But given there isn’t an // Output comment, maybe there shouldn’t be a button to run it on the website.

@dmitshur the issue isn't really about the playground but that the example as written doesn't work without other steps that are not included here, like "must have this directory before running this code"

Seems like duplicate of #32916 fixed by https://golang.org/cl/284452