Windowed comparison for differences in the middle of long strings

combas · July 14, 2010, 9:17pm

When two strings fail to match, if the difference is somewhere in the
middle of the strings, it can be annoying/impossible to track down the
actual difference. I’ve written a little Comparison object that
provides this kind of error message:

Strings differ at position 12:
expected: …“efghijklmnopqrst”…
actual: …“efghijklXnopqrst”…

It shows a “prelude” of a few characters, then the difference, on
successive lines so they’re easy to visually scan. It also does the
right thing (or tries to) if the difference is near the beginning or
end of the string (i.e. does or doesn’t show ellipses).

gist.github.com

https://gist.github.com/474363

comparison.rb

class Comparison
  @@window = 64
  @@prelude = 12

  def self.window
    @@window
  end

  def self.window=(val)
    @@window = val

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comparison_spec.rb

here = File.dirname(__FILE__); require File.expand_path("#{here}/../spec_helper")

describe Comparison do
  before do
    @old_window = Comparison.window
    Comparison.window = 16
    @old_prelude = Comparison.prelude
    Comparison.prelude = 8
  end

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For people who can’t wait for this to get incorporated into RSpec
proper, you can require comparison.rb and it’ll override the existing
RSpec == matcher. Or wait for

github.com/rspec/rspec-expectations

Windowed comparison for differences in the middle of long strings

opened 07:10PM - 14 Jul 10 UTC

closed 07:38AM - 23 Feb 13 UTC

cohuman

When two strings fail to match, if the difference is somewhere in the middle of …the strings, it can be annoying/impossible to track down the actual difference. I've written a little Comparison object that provides this kind of error message: Strings differ at position 12: expected: ..."efghijklmnopqrst"... actual: ..."efghijklXnopqrst"... It shows a "prelude" of a few characters, then the difference, on successive lines so they're easy to visually scan. It also does the right thing (or tries to) if the difference is near the beginning or end of the string (i.e. does or doesn't show ellipses). http://gist.github.com/474363 For people who can't wait for this to get incorporated into RSpec proper, you can require comparison.rb and it'll override the existing RSpec == matcher. One open question is whether the exception message should show the full actual string as well as the comparison... On one hand, it adds to screen clutter, but on the other hand, it can be important in tracking down the problem, especially if the prelude is ambiguous.

to be pulled into version 2.1 (maybe).

One open question is whether the exception message should show the
full actual string as well as the comparison… On one hand, it adds
to screen clutter, but on the other hand, it can be important in
tracking down the problem, especially if the prelude is ambiguous.