Ruby Best Practices

Capturing the Essence of a Defect

Before you can begin to hunt down a bug, you need to be able to reproduce it in isolation. The main idea is that if you remove all the extraneous code that is unrelated to the issue, it will be easier to see what is really going on. As you continue to investigate an issue, you may discover that you can reduce the example more and more based on what you learn. Because I have a real example handy from one of my projects, we can look at this process in action to see how it plays out.

What follows is some Prawn code that was submitted as a bug report. The problem it’s supposed to show is that every text span() resulted in a page break happening, when it wasn’t supposed to:

Prawn::Document.generate("span.pdf") do

  span(350, :position => :center) do
    text "Here's some centered text in a 350 point column. " * 100
  end

  text "Here's my sentence."

  bounding_box([50,300], :width => 400) do
    text "Here's some default bounding box text. " * 10
    span(bounds.width,
      :position => bounds.absolute_left - margin_box.absolute_left) do
      text "The rain in Spain falls mainly on the plains. " * 300
    end
  end

  text "Here's my second sentence."

end

Without a strong knowledge of Prawn, this example may already seem fairly reduced. After all, the text represents a sort of abstract problem definition rather than some code that was ripped out of an application, and that is a good start. But upon running this code, I noticed that the defect was present whenever a span() call was made. This ...