I encountered a problem when trying to test a module with Test::Unit. What I used to do is this:
my_module.rb:
class MyModule
def my_func
5 # return some value
end
end
test_my_module.rb:
require 'test/unit'
require 'my_module'
class TestMyModule < Unit::Test::TestCase
include MyModule
def test_my_func
assert_equal(5, my_func) # test the output value given the input params
end
end
Now the problem is, if my_module declares an initialize method, it gets included in the test class and this causes a bunch of problems since Test::Unit seems to override/generate an initialize method. So I'm wondering what is the best way to test a module?
I'm also wondering wether my module should become a class at this point since the initialize method is made for initializing the state of something. Opinions?
Thanks in advance !
Including an initialize method in a module feels very wrong to me, so I'd rethink that at the very least.
To answer your question about testing this as a module more directly, though, I would create a new, empty class, include your module in it, create an instance of that class, and then test against that instance:
class TestClass
include MyModule
end
class TestMyModule < Unit::Test::TestCase
def setup
#instance = TestClass.new
end
def test_my_func
assert_equal(5, #instance.my_func) # test the output value given the input params
end
end
Yeah, your initialize should definitely suggest that you're going towards a class. A module in ruby often feels like an interface in other languages, as long as you implement some basic things when you include the module you'll get a lot for free.
Enumerable is a great example, as long as you define [] and each when you include Enumerable you suddenly get pop, push, etc.
So my gut feeling about testing modules is that you should probably be testing classes that include the module rather than testing the module itself unless the module is designed to not be included in anything, it's simply a code storage mechanism.
Related
I am extending an existing library by creating a child class which extends to the library class.
In the child class, I was able to test most of functionality in initialize method, but was not able to mock super call. The child class looks like something like below.
class Child < SomeLibrary
def initialize(arg)
validate_arg(arg)
do_something
super(arg)
end
def validate_arg(arg)
# do the validation
end
def do_something
#setup = true
end
end
How can I write rspec test (with mocha) such that I can mock super call? Note that I am testing functionality of initialize method in the Child class. Do I have to create separate code path which does not call super when it is provided with extra argument?
You can't mock super, and you shouldn't. When you mock something, you are verifying that a particular message is received, and super is not a message -- it's a keyword.
Instead, figure out what behavior of this class will change if the super call is missing, and write an example that exercises and verifies that behavior.
As #myron suggested you probably want to test the behavior happening in super.
But if you really want to do this, you could do:
expect_any_instance_of(A).to receive(:instance_method).and_call_original
Assuming
class B < A
def instance_method
super
end
end
class A
def instance_method
#
end
end
Disclaimer expect_any_instance_of are a mark of weak test (see):
This feature is sometimes useful when working with legacy code, though
in general we discourage its use for a number of reasons:
The rspec-mocks API is designed for individual object instances, but
this feature operates on entire classes of objects. As a result there
are some semantically confusing edge cases. For example, in
expect_any_instance_of(Widget).to receive(:name).twice it isn't clear
whether a specific instance is expected to receive name twice, or if
two receives total are expected. (It's the former.)
Using this feature is often a design smell. It may be that your test is trying to do too much or that the object under test is too
complex.
It is the most complicated feature of rspec-mocks, and has historically received the most bug reports. (None of the core team
actively use it, which doesn't help.)
A good way to test this is to set an expectation of some action taken by the superclass - example :
class Some::Thing < Some
def instance_method
super
end
end
and the super class:
class Some
def instance_method
another_method
end
def self.another_method # not private!
'does a thing'
end
end
now test :
describe '#instance_method' do
it 'appropriately triggers the super class method' do
sawm = Some::Thing.new
expect(sawm).to receive(:another_method)
sawm.instance_method
end
end
All This Determines Is That Super Was Called On the Superclass
This pattern's usefulness is dependent on how you structure your tests/what expectations you have of the child/derivative class' mutation by way of the super method being applied.
Also - pay close attention to class and instance methods, you will need to adjust allows and expects accordingly
YMMV
A bit late to this party, but what you can also do is forego using the super keyword and instead do
class Parent
def m(*args)
end
end
class Child < Parent
alias super_m m
def m(*args)
super_m(*args)
end
end
That way your super method is accessible like any other method and can e.g. be stubbed like any other method. The main downside is that you have to explicitly pass arguments to the call to the super method.
I have a basic structure like this
class Automobile
def some_method
# this code sets up structure for child classes... I want to test this
end
end
class Car < Automobile
def some_method
super
# code specific to Car... it's tested elsewhere so I don't want to test this now
end
end
class CompactCar < Car
def some_method
super
# code specific to CompactCar... I want to test this
end
end
What is the recommended way to test CompactCar and Automobile without running the code from Car? Automobile#some_method provides the structure that is required by child classes, so I want to always test that, but Car's functionality is tested elsewhere and I don't want to duplicate efforts.
One solution is to use class_eval to overwrite Car#some_method, but this isn't ideal because the overwritten method stays in place for the duration of my testing (unless I re-load the original library file with setup/teardown methods... kind of an ugly solution). Also, simply stubbing the call to Car#some_method does not seem to work.
Is there a cleaner/more generally accepted way of doing this?
Just put the specific code into a separate method. You don't appear to be using anything from super. Unless you are?
class CompactCar < Car
def some_method
super
compact_car_specific_code
end
# Test this method in isolation.
def compact_car_specific_code
# code specific to CompactCar... I want to test this
end
end
For example.
When working with ruby on rails in cucumber you can do both
Example with a Class
class Awesome
def thing
end
end
World{ Awesome.new }
Example with a Module
module Awesome
def thing
end
end
World(Awesome)
Why would I use one over the other? What is the difference?
No difference because class's class is inherited from module's class. The only difference is that you can't instantiate the module. Usually in cucumber you don't need to do it so modules are just ok. If you somehow need to include something into the world and somewhere else you have to make an instance - you can go with class but I hardly can imagine such situation,
Let's say I have a bunch of related functions that have no persistent state, say various operations in a string differencing package. I can either define them in a class or module (using self) and they can be accessed the exact same way:
class Diff
def self.diff ...
def self.patch ...
end
or
module Diff
def self.diff ...
def self.patch ...
end
I can then do Diff.patch(...). Which is 'better' (or 'correct')?
The main reason I need to group them up is namespace issues, common function names are all used elsewhere.
Edit: Changed example from matrix to diff. Matrix is a terrible example as it does have state and everyone started explaining why it's better to write them as methods rather than answer the actual question. :(
In your two examples, you are not actually defining methods in a Class or a Module; you are defining singleton methods on an object which happens to be a Class or a Module, but could be just about any object. Here's an example with a String:
Diff = "Use me to access really cool methods"
def Diff.patch
# ...
end
You can do any of these and that will work, but the best way to group related methods is in a Module as normal instance methods (i.e. without self.):
module Diff
extend self # This makes the instance methods available to the Diff module itself
def diff ... # no self.
def patch ...
end
Now you can:
use this functionality from within any Class (with include Diff) or from any object (with extend Diff)
an example of this use is the extend self line which makes it possible to call Diff.patch.
even use these methods in the global namespace
For example, in irb:
class Foo
include Diff
end
Foo.new.patch # => calls the patch method
Diff.patch # => also calls Diff.patch
include Diff # => now you can call methods directly:
patch # => also calls the patch method
Note: the extend self will "modify" the Diff module object itself but it won't have any effect on inclusions of the module. Same thing happens for a def self.foo, the foo won't be available to any class including it. In short, only instance methods of Diff are imported with an include (or an extend), not the singleton methods. Only subclassing a class will provide inheritance of both instance and singleton methods.
When you actually want the inclusion of a module to provide both instance methods and singleton methods, it's not completely easy. You have to use the self.included hook:
module Foo
def some_instance_method; end
module ClassMethods
def some_singleton_method; end
end
def self.included(base)
base.send :extend, ClassMethods
end
def self.will_not_be_included_in_any_way; end
end
class Bar
include Foo
end
# Bar has now instance methods:
Bar.new.some_instance_method # => nil
# and singleton methods:
Bar.some_singleton_method # => nil
The main difference between modules and classes is that you can not instantiate a module; you can't do obj = MyModule.new. The assumption of your question is that you don't want to instantiate anything, so I recommend just using a module.
Still you should reconsider your approach: rather than using arrays of arrays or whatever you are doing to represent a Matrix, it would be more elegant to make your own class to represent a matrix, or find a good class that someone else has already written.
Ruby Modules are used to specify behaviour, pieces of related functionality.
Ruby Classes are used to specify both state and behaviour, a singular entity.
There is a maxim in software design that says that code is a liability, so use the less code possible. In the case of Ruby, the difference in code lines is cero. So you can use either way (if you don't need to save state)
If you want to be a purist, then use a Module, since you won't be using the State functionality. But I wouldn't say that using a class is wrong.
As a trivia info: In Ruby a Class is a kind of Module.
http://www.ruby-doc.org/core-1.9.3/Class.html
The following also works
Matrix = Object.new
def Matrix.add ...
def Matrix.equals ...
That's because so-called "class methods" are just methods added to a single object, and it doesn't really matter what that object class is.
As a matter of form, the Module is more correct. You can still create instances of the class, even if it has only class methods. You can think of a module here as a static class of C# or Java. Classes also always have the instance related methods (new, allocate, etc.). Use the Module. Class methods usually have something to do with objects (creating them, manipulating them).
I would like to put some code in module that throws an error if certain method is not defined.
This module relies on the external definition of this method, since this method's implementation is different for all classes. This code would help developers know early that they forgot to implement the method rather than when they tried to use features of the module.
module MyModule
def self.included(klass)
raise "MyModule: please `def my_method` on #{klass}" unless klass.respond_to?(:my_method)
end
end
I can easily raise an error in a module's included definition if a method is not defined, however since most modules are included at the top of a file, it's likely that my required method is defined in the class, but not before my module is included.
class MyClass
include MyModule
def self.my_method
# ...
end
end
This would still raise an error :(
Is it possible to raise an error only if the method truly is not defined in the class definition? Almost need a class.onload callback of sorts. If not, any other ideas for how to mitigate the possibilities that a programmer might include our module without defining this needed method?
Sounds like you want to make use of method_missing and define_method.
If you do use method_missing don't forget to:
call super for unhandled cases.
also implement a respond_to? method
look at this question, plus this and that.
Update:
It sounds the goal is to do static method checking like Java or c++ does. This is not really meaningful in ruby :-(
Since in ruby:
Each instance of an object has its own eigenclass. A given object may have the necessary methods mixed in at runtime. So just because Foo does not have a method at class load time is meaningless.
Frameworks like RoR hooks method_missing and dynamically create methods needed for the database query methods, so the method may exist (or not) when it is needed.
With regards to "class on load": A class definition is really executed. Try this:
class Foo
p "Hi"
end
You will see "Hi" the first and only the first time Foo is used. This is how things like devise hook into do their magic.
class User < ActiveRecord::Base
# **CALL 'devise' method**
devise :database_authenticatable, :registerable, :recoverable, :rememberable, :trackable, :validatable
# **CALL attr_accessible method**
attr_accessible :email, :password, :password_confirmation
end
So maybe by private convention have developers add a check_class method call to the bottom of the classes in question?
I understand the intent but it seems like fighting the way ruby is designed to work.
As a mostly Java person I appreciate the frustration. Let me guess: repeated cases of code getting pushed to production that had missing methods? :-P
Update2:
wrt onload In ruby barring use of frozen a class get new methods defined all the time. ( Or an instance can get new methods defined just for that instance. ) so checking for a method's nonexistence is only a snapshot check and not as definitive a check as a static language brings to the table. This is ruby's very own Halting problem
How about declaring a method with that name, which just raises an error, to make sure the user redefines the method?
module MyModule
def my_method
raise "Please implement me"
end
end
class MyClass
include MyModule
def my_method
# do something
end
end
Assuming your program requires all files when started and does not use any autoload and the like, you could use something like the following right after everything is required, but before the program actually starts:
classes_to_check = Object.constants.find_all do |const|
klass = Object.const_get(c)
klass.ancestors.include?(MyModule) if klass.kind_of?(Module)
end
classes_to_check.each do |klass|
raise "MyModule: please `def my_method` on #{klass}" \
unless klass.respond_to?(:my_method)
end
However, I personally always use Dogbert's solution.