[rspec-users] Mocking expectations on I/O operations

Andrew Premdas apremdas at gmail.com
Thu Sep 15 17:47:39 EDT 2011

On 14 September 2011 11:51, Rob Aldred <rob at stardotstar.com> wrote:
> I'm speccing a small lib which manipulates image files using mini_magick.
> The lib creates various temporary files during the process, the lib then cleans up the temporary files at the end.
> I'm trying to mock expectations that the calls are made to File#delete
> Eg...
> File.should_receive(:delete).once.with("#{generator.tmp_path}-full\.png")
> File.should_receive(:delete).once.with("#{generator.tmp_path}-screen\.png")
> File.should_receive(:delete).once.with("#{generator.tmp_path}-shadow\.png")
> The method in the lib that does this is:
> Class IconGenerator
> ...
>  private
>  def cleanup
>  # remove temporary files created during
>  # image manipulation
>  ['-full','_screen','_shadow','_back_shadow'].each do |f|
>  File.delete "#{tmp_path}#{f}.png" if File.exists? "#{tmp_path}#{f}.png"
>  end
>  end
> ...
> end
> The expectations work fine, however, I also have an after block which also calls File.delete to clean up the final version of the manipulated test image
> after(:each) do
>  # cleanup manipulated image
>  File.delete out_dir.join('MenuIcon.png') if File.exists? out_dir.join('MenuIcon.png')
>  end
> The File.delete call in the after block fails because its calling the mocked version of File.delete
> This results in the following error:
> Failure/Error: File.delete out_dir.join('MenuIcon.png') if File.exists? out_dir.join('MenuIcon.png')
> NoMethodError:
> undefined method `=~' for #<Pathname:/Users/robaldred/Sites/egg/tmp/MenuIcon.png>
> My next step was to stub the write method for every instance of MiniMagick::Image
> However I will then no longer be able to test the cleanup because the delete method will never be called if the files don't exist.
> I'm pretty sure I'm going about this the wrong way and making it difficult for myself here.
> Appreciate any help & feedback
> Thanks
> --
> Rob
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> rspec-users at rubyforge.org
> http://rubyforge.org/mailman/listinfo/rspec-users

Hi Rob,

First of all perhaps you are testing the wrong thing in the wrong place.

As cleanup is a private method, why are you specifying (in this spec
how it works). As far as IconGenerator is concerned currently all you
care about is that it calls cleanup. You're not interested in how that
cleanup is implemented. If you want to specify how the cleanup works
then make cleanup public - either in this class, or perhaps better yet
in another class. e.g.

describe "Cleanup.clean"
  it "should delete files"

describe IconGenerator
   it should cleanup
      Cleanup.should_receive(:clean) ...

Once you separate the cleanup spec from the IconGenerator spec then
the clash should disappear! You'll never have to mock File.delete



Andrew Premdas

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