How do I write an RSpec test to unit-test this interesting metaprogramming code?

Ruby
1

I’m a little stumped by this problem. Here’s some simple code that,
for each argument specified, will add specific get/set methods named
after that argument. If you write attr_option :foo, :bar, then you
will see #foo/foo= and #bar/bar= instance methods on Config:

module Configurator
  class Config
    def initialize()
      @options = {}
    end

    def self.attr_option(*args)
      args.each do |a|
        if not self.method_defined?(a)
          define_method "#{a}" do
            @options[:"#{a}"] ||= {}
          end

          define_method "#{a}=" do |v|
            @options[:"#{a}"] = v
          end
        else
          throw Exception.new("already have attr_option for #{a}")
        end
      end
    end
  end
end

So far, so good. I want to write some RSpec tests to verify this code
is actually doing what it’s supposed to. But there’s a problem! If I
invoke attr_option :foo in one of the test methods, that method is
now forever defined in Config. So a subsequent test will fail when it
shouldn’t, because foo is already defined:

  it "should support a specified option" do
    c = Configurator::Config
    c.attr_option :foo
    # ...
  end

  it "should support multiple options" do
    c = Configurator::Config
    c.attr_option :foo, :bar, :baz   # Error! :foo already defined
                                     # by a previous test.
    # ...
  end

Is there a way I can give each test an anonymous “clone” of the
Config class which is independent of the others?

2

On 5/23/10, James W. [email protected] wrote:

        else

now forever defined in Config. So a subsequent test will fail when it
c.attr_option :foo, :bar, :baz # Error! :foo already defined
# by a previous test.
# …
end

Is there a way I can give each test an anonymous “clone” of the
Config class which is independent of the others?

As I see it, you have several options:

  1. remove the exception you are raising at the end of attr_option.
  2. intercept and ignore that exception when you call attr_option in your
    tests.
  3. remove the methods you added at the end of each test. Something
    like this should work:
    c.send :undef_method, :foo
  4. (what you were asking about) make a copy of Configurator::Config
    before changing it. This should work:
    c=Configurator::Config.clone

PS: c is an especially unlucky choice for a local variable name, I
have found. If you ever have to run your program under one of the
console-mode debuggers (I do this all the time) it will get confused
with the continue command, which is abbreviated c, often with highly
frustrating results.

3

On Sun, May 23, 2010 at 4:00 PM, James W. [email protected] wrote:

       else

now forever defined in Config. So a subsequent test will fail when it
c.attr_option :foo, :bar, :baz # Error! :foo already defined
# by a previous test.
# …
end
Caleb is right, c is not a good name ;), however

lambda{ c.attr_option… }.should raise_error( WhatWasIt)

I call it WhatWasIt because you really should define your own Exception,
e.g.
IllegalMonitorState = Class::new RuntimeError

please take care to subclass RuntimeError, subclassing Exception is
waaaay toooo general.

HTH
R.

4

On May 23, 2010, at 07:00 , James W. wrote:

 end

 it "should support multiple options" do
   c = Configurator::Config
   c.attr_option :foo, :bar, :baz   # Error! :foo already defined
                                    # by a previous test.
   # ...
 end

Caleb and Robert are nit-picking… ‘c’ is a perfectly fine name for a
variable in a 4 line test/spec.

The problem you’re having is easily solved by using anonymous
subclasses:

 it "should support a specified option" do
   c = Class.new(Configurator::Config)
   c.attr_option :foo
   # ...
 end

That makes a throwaway class that has all the same features of the
superclass without any of the infectious properties of calling your attr
methods on the real thing.