From: Greg Willits <lists@...>
Date: 2009-07-25T20:27:45+09:00
Subject: Unit Testing and Instance Vars

I find myself often wondering about some design choices that result from
code which looks similar to the contrived example below.

class Places

  attr_reader :addresses

  def initialize(some_source)
    @addresses     = []
    @raw_addresses = []

    load_addresses(some_source)
  end

  def load_addresses(some_source)
    some_source.each do |this_source|
      @raw_addresses << this_source.address
    end
    clean_addresses
  end

  private

  def clean_addresses
    @raw_addresses.each do |this_address|
      @addresses << GisTools.normalize_address(this_address)
    end
  end
end

Testing for the final results of @addresses is easy as the instance var
is exposed through the attr_reader, but there's two problems:

A) Testing clean_addresses is dependent on running load_addresses first
in order to acquire data.

B) Seeing the intermediate results of @raw_addresses is not possible for
testing load_addresses independently because @raw_addresses is not
exposed.

To fix (A), we could insist on passing parameters. So, we would have:

  def load_addresses(some_source)
    some_source.each do |this_source|
      @raw_addresses << this_source.address
    end
    clean_addresses(@raw_addresses) # <-----  passed explicitly
  end

  private

  def clean_addresses(raw_addresses) # <----- passed explicitly
    raw_addresses.each do |this_address|
      @addresses << GisTools.normalize_address(this_address)
    end
  end

That would allow clean_addresses to be tested independently and be fed
data directly by the test which could be a good thing. Regardless of
that advantage, I've often wondered if requiring the passing of instance
vars as parameters really is a 'higher standard' or not. I see a lot of
code both ways. I tend to believe it's better to be explicit, but admit
it feels simpler in many cases to just use the hard-coded instance var.
I find myself flip flopping (always a bad thing).

Either way, I still can't see the results of @raw_addresses. I find
myself having to expose :raw_addresses so that tests can see the
results, but there really is no need for :raw_addresses to be exposed,
and it shouldn't be a part of the interface.

Questions:
1) Any consensus on the parameters issue (*always* pass params, or
*usually*)?
2) What's the 'best practice' approach to seeing the results of
@raw_addresses?

Many thanks for the discussion.

-- gw
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