RSpec: how to test file operations and file content

Question

In my app, I have the following code:

File.open "filename", "w" do |file|
  file.write("text")
end

I want to te

Answer 1

For very simple i/o, you can just mock File. So, given:

def foo
  File.open "filename", "w" do |file|
    file.write("text")
  end
end

then:

describe "foo" do

  it "should create 'filename' and put 'text' in it" do
    file = mock('file')
    File.should_receive(:open).with("filename", "w").and_yield(file)
    file.should_receive(:write).with("text")
    foo
  end

end

However, this approach falls flat in the presence of multiple reads/writes: simple refactorings which do not change the final state of the file can cause the test to break. In that case (and possibly in any case) you should prefer @Danny Staple's answer.