Wednesday, July 21, 2010

Stubbing only the nth instance's method, should or shouldn't ?

A few days back, cross-stub version 0.2.0 has been released, with this release, cross process instance method stubbing is now possible. While doing review of zan's work, we've discussed the possibility of having more control on which exact instance the stub should be applied to, as illustrated below:
1
2
 
Time.xstub(:day, :instance => 1)    # applies to ONLY 1st Time instance
Time.xstub(:day, :instance => 2..3) # applies to ONLY 2nd ~ 3rd Time instances

Initially, i was quite happy to have brought up this. But after some thinking, i'm convinced that this a LOUSY idea, as it requires the user to have intimate knowledge of how many times a class's instance have been created, which can be tedious (if not impossible) if the instance method is provided by ruby core/std libs, or popular gems. It also results in extremely brittle tests, as nth can easily beconme xth, or zth ...

I think the issue is very fundamental, it goes down to how the code is written. Given the following example:
1
2
3
4
5
6
7
8
9
10
11
12
13
 
  class Papa
    def free_today?
      # Any papa is free on weekend but not weekday
      !Time.now.strftime('%w').between?(0,5)
    end
  end

  class Mama
    def free_today?
      # Any mama is free on weekday but not weekend
      Time.now.strftime('%w').between?(0,5)
    end
  end

If i were to apply stubbing for Time#strftime, i would affect both Papa#free_today? & Mama#free_today?. Taking one step back, if we to affect only Papa#free_today?, we need to consider the context where the Time#strftime stub is-to-be applied, & do the following revision:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
 
class Papa

  def free_today?
    !today_dow.between?(0,5)
  end

  def today_dow
    Time.now.strftime('%w')
  end

end

class Mama

  def free_today?
    today_dow.between?(0,5)
  end

  def today_dow
    Time.now.strftime('%w')
  end

end

Therefore, by stubbing Papa#today_dow, we won't affect Mama#free_today?. Moreover, taking this approach gives us the chance to do some cleaning up:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
 
class Parent
  def today_dow
    Time.now.strftime('%w')
  end
end

class Papa < Parent
  def free_today?
    !today_dow.between?(0,5)
  end
end

class Mama < Parent
  def free_today?
    today_dow.between?(0,5)
  end
end

Imagine if we go the other approach of doing nth instance's method stubbing, though feasible, the test will be extremely brittle (nightmarish maintenance), as nth can easily be changed to xth, or zth.

CONCLUSION: No, we don't need better control over nth instance's method stubbing, cross-stub's instance method stubbing is going to remain as it is. If u need it, drop me a mail with an example to justify the need :]
Posted by at
Labels: , ,

No comments:

Post a Comment

Subscribe to: Post Comments (Atom)

Labels