Currently reading a Ruby style guide and I came across an example:
def no_op; end
What is the purpose of empty body methods?
There are a number of reasons you might create an empty method:
Stub a method that you will fill in later.
Stub a method that a descendant class will override.
Ensure a class or object will #respond_to? a method without necessarily doing anything other than returning nil.
Undefine an inherited method's behavior while still allowing it to #respond_to? the message, as opposed to using undef foo on public methods and surprising callers.
There are possibly other reasons, too, but those are the ones that leapt to mind. Your mileage may vary.
There may be several reasons.
One case is when a class is expected to implement a specific interface (virtually speaking, given that in Ruby there are no interfaces), but in that specific class that method would not make sense. In this case, the method is left for consistency.
class Foo
def say
"foo"
end
end
class Bar
def say
"bar"
end
end
class Null
def say
end
end
In other cases, it is left as a temporary placeholder or reminder.
There are also cases where the method is left blank on purpose, as a hook for developers using that library. The method it is called somewhere at runtime, and developers using that library can override the blank method in order to execute some custom callback. This approach was used in the past by some Rails libraries.
Related
Some open source code I'm integrating in my application has some classes that include code to that effect:
class SomeClass < SomeParentClass
def self.new(options = {})
super().tap { |o|
# do something with `o` according to `options`
}
end
def initialize(options = {})
# initialize some data according to `options`
end
end
As far as I understand, both self.new and initialize do the same thing - the latter one "during construction" and the former one "after construction", and it looks to me like a horrible pattern to use - why split up the object initialization into two parts where one is obviously "The Wrong Think(tm)"?
Ideally, I'd like to see what is inside the super().tap { |o| block, because although this looks like bad practice, just maybe there is some interaction required before or after initialize is called.
Without context, it is possible that you are just looking at something that works but is not considered good practice in Ruby.
However, maybe the approach of separate self.new and initialize methods allows the framework designer to implement a subclass-able part of the framework and still ensure setup required for the framework is completed without slightly awkward documentation that requires a specific use of super(). It would be a slightly easier to document and cleaner-looking API if the end user gets functionality they expect with just the subclass class MyClass < FrameworkClass and without some additional note like:
When you implement the subclass initialize, remember to put super at the start, otherwise the magic won't work
. . . personally I'd find that design questionable, but I think there would at least be a clear motivation.
There might be deeper Ruby language reasons to have code run in a custom self.new block - for instance it may allow constructor to switch or alter the specific object (even returning an object of a different class) before returning it. However, I have very rarely seen such things done in practice, there is nearly always some other way of achieving the goals of such code without customising new.
Examples of custom/different Class.new methods raised in the comments:
Struct.new which can optionally take a class name and return objects of that dynamically created class.
In-table inheritance for ActiveRecord, which allows end user to load an object of unknown class from a table and receive the right object.
The latter one could possibly be avoided with a different ORM design for inheritance (although all such schemes have pros/cons).
The first one (Structs) is core to the language, so has to work like that now (although the designers could have chosen a different method name).
It's impossible to tell why that code is there without seeing the rest of the code.
However, there is something in your question I want to address:
As far as I understand, both self.new and initialize do the same thing - the latter one "during construction" and the former one "after construction"
They do not do the same thing.
Object construction in Ruby is performed in two steps: Class#allocate allocates a new empty object from the object space and sets its internal class pointer to self. Then, you initialize the empty object with some default values. Customarily, this initialization is performed by a method called initialize, but that is just a convention; the method can be called anything you like.
There is an additional helper method called Class#new which does nothing but perform the two steps in sequence, for the programmer's convenience:
class Class
def new(*args, &block)
obj = allocate
obj.send(:initialize, *args, &block)
obj
end
def allocate
obj = __MagicVM__.__allocate_an_empty_object_from_the_object_space__
obj.__set_internal_class_pointer__(self)
obj
end
end
class BasicObject
private def initialize(*) end
end
The constructor new has to be a class method since you start from where there is no instance; you can't be calling that method on a particular instance. On the other hand, an initialization routine initialize is better defined as an instance method because you want to do something specifically with a certain instance. Hence, Ruby is designed to internally call the instance method initialize on a new instance right after its creation by the class method new.
I am new to Ruby and I saw methods defined like:
def method_one
puts "method 1"
end
class MyClass
method_one
def method_two
puts "method 2"
end
end
The way method_one is used reminds me of Python decorators.The output of
c = MyClass.new
c.method_two
is
method 1
method 2
I have been trying to search for more information about this syntax/language feature in the Ruby documentation on the web but I don't know what keywords to search for.
What this is thing called?
TL;DR
This code doesn't do what you think it does. Don't do stuff like this.
Ruby's Top-Level Object
Ruby lets you define methods outside a class. These methods exist on a top-level object, which you can (generally) treat as a sort of catch-all namespace. You can see various posts like What is the Ruby Top-Level? for more details, but you shouldn't really need to care.
In your original post, method_one is just a method defined in the top-level. It is therefore available to classes and methods nested within the top-level, such as MyClass.
Methods in Classes
Despite what you think, the following doesn't actually declare a :method_one class or instance method on MyClass:
class MyClass
method_one
def method_two; end
end
Instead, Ruby calls the top-level ::method_one during the definition of the class, but it never becomes a class method (e.g. MyClass::method_one) or an instance method (e.g. MyClass.new.method_one). There might be a few use cases for doing this (e.g. printing debugging information, test injection, etc.) but it's confusing, error-prone, and generally to be avoided unless you have a really strong use case for it.
Better Options
In general, when you see something like this outside an academic lesson, the programmer probably meant to do one of the following:
Extend a class.
Add a singleton method to a class.
Include a module in a class.
Set up a closure during class definition.
The last gets into murky areas of metaprogramming, at which point you should probably be looking at updating your class initializer, or passing Proc or lambda objects around instead. Ruby lets you do all sorts of weird and wonderful things, but that doesn't mean you should.
I think you're a little mislead; the output of:
c = MyClass.new
c.method_two
is
#<MyClass:0x007feda41acf18>
"method 2"
You're not going to see method one until the class is loaded or if you're in IRB you enter the last end statement.
I would suggest looking into ruby's initialize method.
I have a typical OO pattern: one base abstract class (that defines abstract methods) and several classes that implement these abstract methods in class-specific way.
I'm used to write documentation only once in abstract methods and then it automatically propagates to several concrete classes (at least it works the following way in Javadoc, in Scaladoc, in Doxygen), i.e. I don't need to repeat the same description in all concrete classes.
However, I couldn't find how to do such propagation in YARD. I've tried, for example:
# Some description of abstract class.
# #abstract
class AbstractClass
# Some method description.
# #return [Symbol] some return description
# #abstract
def do_something
raise AbstractMethodException.new
end
end
class ConcreteClass < AbstractClass
def do_something
puts "Real implementation here"
return :foo
end
end
What I get:
Code works as expected - i.e. throws AbstractMethodException is called in abstract class, does the job in concrete class
In YARD, AbstractClass is clearly defined as abstract, ConcreteClass is normal
Method description and return type is good in AbstractClass
Method is said to throw AbstractMethodException in AbstractClass
Method has no description at all and generic Object return type in ConcreteClass, there's not a single notice of that an abstract method exists in base class.
What I expect to get:
Method's description and return type are inherited (i.e. copied) to ConcreteClass from info at AbstractClass
Ideally, this method is specified in "inherited" or "implemented" section of ConcreteClass description, with some reference link from ConcreteClass#do_something to AbstractMethod#do_something.
Is it possible to do so?
I think the issue boils down to what you're trying to do. It looks like you're trying to implement an Interface in Ruby, which makes sense if you're coming from Java or .NET, but isn't really how Ruby developers tend to work.
Here is some info about how the typical thought on Interfaces in Ruby: What is java interface equivalent in Ruby?
That said, I understand what you're trying to do. If you don't want your AbstractClass to be implemented directly, but you want to define methods that can be used in a class that behaves like the AbstractClass stipulates (as in Design by Contract), then you probably want to use a Module. Modules work very well for keeping your code DRY, but they don't quite solve your problem related to documenting overridden methods. So, at this point I think you can reconsider how you approach documentation, or at least approach it in a more Ruby-ish way.
Inheritance in Ruby is really (generally speaking from my own experience) only used for a few reasons:
Reusable code and attributes
Default behaviors
Specialization
There are obviously other edge cases, but honestly this is what inheritance tends to be used for in Ruby. That doesn't mean what you're doing won't work or violates some rule, it just isn't typical in Ruby (or most dynamically typed languages). This atypical behavior is probably why YARD (and other Ruby doc generators) doesn't do what you expect. That said, creating an abstract class that only defines the methods that must exist in a subclass really gains you very little from a code perspective. Methods not defined will result in a NoMethodError exception being thrown anyway, and you could programmatically check if an object will respond to a method call (or any message for that matter) from whatever calls the method, using #respond_to?(:some_method) (or other reflective tools for getting meta stuff). It all comes back Ruby's use of Duck Typing.
For pure documentation, why document a method that you don't actually use? You shouldn't really care about the class of the object being sent or received from calling a method, just what those objects respond to. So don't bother creating your AbstractClass in the first place if it adds no real value here. If it contains methods you actually will call directly without overriding, then create a Module, document them there, and run $ yardoc --embed-mixins to include methods (and their descriptions) defined in mixed-in Modules. Otherwise, document methods where you actually implement them, as each implementation should be different (otherwise why re-implement it).
Here is how I would something similar to what you're doing:
# An awesome Module chock-full of reusable code
module Stuff
# A powerful method for doing things with stuff, mostly turning stuff into a Symbol
def do_stuff(thing)
if thing.kind_of?(String)
return thing.to_sym
else
return thing.to_s.to_sym
end
end
end
# Some description of the class
class ConcreteClass
include Stuff
# real (and only implementation)
def do_something
puts "Real implementation here"
return :foo
end
end
an_instance = ConcreteClass.new
an_instance.do_somthing # => :foo
# > Real implementation here
an_instance.do_stuff("bar") # => :bar
Running YARD (with --embed-mixins) will include the methods mixed-in from the Stuff module (along with their descriptions) and you now know that any object including the Stuff module will have the method you expect.
You may also want to look at Ruby Contracts, as it may be closer to what you're looking for to absolutely force methods to accept and return only the types of objects you want, but I'm not sure how that will play with YARD.
Not ideal, but you can still use the (see ParentClass#method) construct (documented here). Not ideal because you have to type this manually for every overriding method.
That being said, I'm no Yard specialist but given its especially customizable architecture, I'd be surprised that there would be no easy way to implement what you need just by extending Yard, somewhere in the Templates department I guess.
Something that I see in a lot of code:
class Foo
attr_accessor :bar
# lots of code omitted
def baz
'qux' if bar
end
end
The exact form of the baz method is not too important - it's just that bar here is a reference to the getter method for the instance variable #bar, called from within the instance's class. I would favor retrieving the value from #bar explicitly. Are there any opinions on this?
I've never seen anything in the ruby style guide or similar covering this. I personally find that doing the former makes it harder to read and understand, especially when classes are over several hundred lines long.
Edit:
Perhaps to illustrate what I would consider to be the awkwardness of this design, let's re-evaluate a pretty standard initialize method:
class Foo
attr_accessor :bar, :qux
def initialize(bar, qux)
#bar = bar
#qux = qux
end
end
If we use the setter method, we cannot use bar = ? by analogy. Instead, we have:
class Foo
attr_accessor :bar, :qux
def initialize(bar, qux)
self.bar = bar
self.qux = qux
end
end
which has lost some of the elegance of the first. We have a more flexible design in that we are now free to rewrite our setter method and do away with attr_writer. But I've just some points in style, and it feels a lot like configuration over convention rather than the converse, something that Russ Olsen has declared a design 'pattern' not just of Rails but of Ruby too.
Accessing the attribute through the getter has the advantage of providing encapsulation. The use of an instance variable to store the value is an implementation detail in some respects. Whether that's appropriate is, of course, situational. I don't recall reading anything explicit this style issue, however.
Found https://softwareengineering.stackexchange.com/questions/181567/should-the-methods-of-a-class-call-its-own-getters-and-setters, which discusses the issue from a language-independent point of view. Also found https://www.ruby-forum.com/topic/141107, which is ruby-specific, although it doesn't break any new ground, let alone imply a Ruby standard.
Update: Just came across the following statement on page 24 of http://www.amazon.com/Practical-Object-Oriented-Design-Ruby-Addison-Wesley/dp/0321721330/ref=sr_1_1?s=books&ie=UTF8&qid=1376760915&sr=1-1, a well-respected book on Ruby: "Hide the variables, even from the class that defines them, by wrapping them in methods." (emphasis added). It goes on to give examples of methods in the class using the accessor methods for access.
I would favor retrieving the value from #bar explicitly. Are there any
opinions on this?
Yes, direct access is not as flexible of a design. Getters and setters can be used to transform values. That is why java programmers spend half their lives banging out do nothing setters and getters for their private variables--they want to present the setters and getters as their api, which allows them to change their code in the future to transform values on the way in or the way out without changing the api.
Then ruby came along with the neat attr_accessor method, which meant that writing do nothing setters and getters wasn't painful anymore.
Python goes one step further. In python, instance variables are public and you can directly access them, e.g.
print my_dog.age
A java programmer writing a python program would implement get_age() and set_age() methods:
class Dog:
def get_age(self):
return self.age
def set_age(self, age):
self.age = age
The java programmer would then fly over all the towns in the land and drop leaflets describing the getter and setter methods as the api for getting and setting the age instance variable, and they would warn people not to access the instance variables directly--or else things might break.
However, python has a feature that allows programmers to eliminate getters and setters until they are actually needed to do something useful--rather than dumbly getting or setting a value. Python allows you to transform direct access to instance variables by client code into method calls. To the client it's transparent. For instance, the client code may be accessing an instance variable in a class by writing:
my_dog.age
Python allows the writer of the class to subsequently implement a method named age(), and my_dog.age can be made to call that method instead of directly accessing the instance variable (note that in python, unlike in ruby, you can't normally call a method without the parentheses). The newly implemented age() method can then do anything it wants to the age instance variable before returning it to the client code, e.g. transform it into human years, or retrieve the age from a database.
It's actually faster to use a getter, mainly because attr_reader and attr_accessor are written in C instead of Ruby.
As someone who's been coding Ruby for a few years, I think using attr_* is much more readable. But that's probably just something I've gotten used to.
I have been a Java programmer for a while and I am trying to switch to ruby for a while. I was just trying to develop a small test program in ruby and my intention is something like following.
I want to create a simple linked list type of an object in ruby; where an instance variable in class points to another instance of same type.
I want to populate and link all nodes; before the constructor is called and only once. Something that we'd usually do in Java Static block.
Initialize method is a constructor signature in ruby. Are there any rules around them? Like in Java you cannot call another constructor from a constructor if its not the first line (or after calling the class code?)
Thanks for the help.
-Priyank
I want to create a simple linked list type of an object in ruby; where an instance variable in class points to another instance of same type.
Just a quick note: the word type is a very dangerous word in Ruby, especially if you come from Java. Due to an historic accident, the word is used both in dynamic typing and in static typing to mean two only superficially related, but very different things.
In dynamic typing, a type is a label that gets attached to a value (not a reference).
Also, in Ruby the concept of type is much broader than in Java. In Java programmer's minds, "type" means the same thing as "class" (although that's not true, since Interfaces and primitives are also types). In Ruby, "type" means "what can I do with it".
Example: in Java, when I say something is of type String, I mean it is a direct instance of the String class. In Ruby, when I say something is of type String, I mean it is either
a direct instance of the String class or
an instance of a subclass of the String class or
an object which responds to the #to_str method or
an object which behaves indistinguishably from a String.
I want to populate and link all nodes; before the constructor is called and only once. Something that we'd usually do in Java Static block.
In Ruby, everything is executable. In particular, there is no such thing as a "class declaration": a class body is just exectuable code, just like any other. If you have a list of method definitions in your class body, those are not declarations that are read by the compiler and then turned into a class object. Those are expressions that get executed by the evaluator one by one.
So, you can put any code you like into a class body, and that code will be evaluated when the class is created. Within the context of a class body, self is bound to the class (remember, classes are just objects like any other).
Initialize method is a constructor signature in ruby. Are there any rules around them? Like in Java you cannot call another constructor from a constructor if its not the first line (or after calling the class code?)
Ruby doesn't have constructors. Constructors are just factory methods (with stupid restrictions); there is no reason to have them in a well-designed language, if you can just use a (more powerful) factory method instead.
Object construction in Ruby works like this: object construction is split into two phases, allocation and initialization. Allocation is done by a public class method called allocate, which is defined as an instance method of class Class and is generally never overriden. It just allocates the memory space for the object and sets up a few pointers, however, the object is not really usable at this point.
That's where the initializer comes in: it is an instance method called initialize, which sets up the object's internal state and brings it into a consistent, fully defined state which can be used by other objects.
So, in order to fully create a new object, what you need to do is this:
x = X.allocate
x.initialize
[Note: Objective-C programmers may recognize this.]
However, because it is too easy to forget to call initialize and as a general rule an object should be fully valid after construction, there is a convenience factory method called Class#new, which does all that work for you and looks something like this:
class Class
def new(*args, &block)
obj = alloc
obj.initialize(*args, &block)
return obj
end
end
[Note: actually, initialize is private, so reflection has to be used to circumvent the access restrictions like this: obj.send(:initialize, *args, &block)]
That, by the way, is the reason why to construct an object you call a public class method Foo.new but you implement a private instance method Foo#initialize, which seems to trip up a lot of newcomers.
To answer your question: since an initializer method is just a method like any other, there are absolutely no restrictions as to what you can do whithin an initializer, in particular you can call super whenever, wherever, however and how often you want.
BTW: since initialize and new are just normal methods, there is no reason why they need to be called initialize and new. That's only a convention, although a pretty strong one, since it is embodied in the core library. In your case, you want to write a collection class, and it is quite customary for a collection class to offer an alternative factory method called [], so that I can call List[1, 2, 3] instead of List.new(1, 2, 3).
Just as a side note: one obvious advantage of using normal methods for object construction is that you can construct instances of anonymous classes. This is not possible in Java, for absolutely no sensible reason whatsoever. The only reason why it doesn't work is that the constructor has the same name as the class, and anonymous classes don't have a name, ergo there cannot be a constructor.
Although I am not quite sure why you would need to run anything before object creation. Unless I am missing something, shouldn't a list basically be
class List
def initialize(head=nil, *tail)
#head = head
#tail = List.new(*tail) unless tail.empty?
end
end
for a Lisp-style cons-list or
class List
def initialize(*elems)
elems.map! {|el| Element.new(el)}
elems.zip(elems.drop(1)) {|prv, nxt| prv.instance_variable_set(:#next, nxt)}
#head = elems.first
end
class Element
def initialize(this)
#this = this
end
end
end
for a simple linked list?
You can simply initialize your class variables in the class body, outside of any method declaration. It will behave like a static initializer in Java:
class Klass
##foo = "bar"
def sayFoo
puts ##foo
end
def self.sayFoo
puts ##foo
end
end
The class field ##foo is here initialized to "bar".
In ruby object creation works like this
class Class
def new(*args)
obj= self.allocate # get some memory
obj.send(:initialize) # call the private method initialize
end
end
Object#initialize is just an ordinary private method.
If you wan't something to happen before Object#initialize you have to write your own Class#new. But I see no reason why you would want to do that.
This is basically the same answer paradigmatic gave back in '09.
Here I want to illustrate that the "static initializer" can call other code. I'm simulating a scenario of loading a special user once, upon class initialization.
class Foo
def user
"Thomas"
end
end
class Bar
##my_user = Foo.new.user
def my_statically_defined_user
##my_user
end
end
b = Bar.new
puts b.my_statically_defined_user # ==> Thomas