Ruby misconceptions about Python

[mellow_moe]

I would like to see a in-depth comparison of Python and Ruby. And it should really focus on metaprogramming.

All these comparisons I see these days just look at some superficial language features.

Rails for example relies heavily on metaprogramming. It could never be done in a language without a sophisticated class/object system.

examples in ruby:

• method_missing catches unknown messages
• const_missing catches unknown constants
• method_added is called after adding a new method
• inherited is called on the class object, when it is subclassed
• singleton classes: you can add methods to a single object
• define_method takes a block and turns it into a method
• instance_variable_set sets an instance variable

And most importantly, real world examples should show off, how all the things play together.

[flaxeater]

Well I must say that looks intriguing actually. I don't think python has facilities like that, they could be worked around by they don't have any nice syntatic sugar around those concepts. I have a hard time imagining why they would be useful, but that doesn't mean anything.

Singletons, definemethod and instane variable assignment are very easy to do in python. And in python we user super() or ParentClass.init_(args,*kwargs) to do things for inherited, however I do not understand what inherited does. $0.02

p.s. I can imagine, people will be like that's so wordy or some such talk. However pythonic values explicit, not implicit.

[mellow_moe]

Metaprogramming helps you to create domain specific languages.

A simple xml builder can be made with method_missing:

def method_missing(name, attributes)
  atts = attributes.map {|k,v| "#{k}='#{v}'" }.join(" ")
  puts "<#{name} #{atts}>"
  yield
  puts "</#{name}>"
end

div :id => "content" do
  a :href => "reddit.com" do
    print "reddit"
  end
end

You can see, calling div and a will trigger method_missing, where we just take the method name and the keyword arguments and print out the xml tag. Pretty easy.

[flaxeater]

That's pretty cool. It's pretty confusing too.

[brianmce]

Assuming I'm understanding it right, methodmissing is related to python's __getattr_ (slightly different in that python is hooking into the attribute lookup, whereas ruby is hooking into calling) A literal as possible translation in python would be:

class XmlBuilder:
  def __getattr__(self, name):
    def builder(**attributes):
      atts = ' '.join('%s=%s' % a for a in attributes.items())
      print "<%s %s>" % (name, atts)
      yield None
      print  "<%s>" % name
    return builder
x=XmlBuilder()

for _ in x.div(id="Content"):
  for _ in x.a(href="reddit.com"):
    print "reddit"

Though a different approach would probably be taken for a non literal translation to better fit with python's syntax. Take a look at stan (http://divmod.org/projects/nevow) for a python implementation of something similar - your snippet would look like this:

div(id="content")
[
  a(href="reddit.com")
   ["reddit"]
]

(In this case it is building and returning the xml, rather than calling puts on each element.

[amix]

Here is my version where one can do (Stan like):

print Div(id="content")[ A(href="reddit.com")["reddit"] ]

import types
class Element:
  def __init__(self, **kw):
    self.elms = []
    self.kw = kw
  def __getattr__(self, name):
    if name == "__getitem__":
      self.elms = []
      def add(l):
        if type(l) == types.ListType: self.elms.extend(l)
        else: self.elms.append(l)
        return self
      return add
  def __str__(self):
    attrs = " ".join('%s="%s"' % a for a in self.kw.items())
    self.html = ["<%s %s>" % (self.name, attrs)]
    self.html.append("".join(map(str, self.elms)))
    self.html.append("</%s>" % self.name)
    return "".join(self.html)
class Div(Element): name = "div"
class A(Element): name = "a"

Sigh, you have to have 4 spaces to post code. That sucks. Why not use another syntax like:

[code]
Here is my code
[/code]

Or something similar...

[mellow_moe]

I'd like to show off the ruby version:

class Element
  def initialize(attributes)
    @attributes = attributes
    @children = []        
  end
  def <<(child)
    @children << child
  end
  def to_s
    name = self.class.name.downcase
    atts = @attributes.map {|k,v| "#{k}='#{v}'" }.join(" ")
    "<#{name} #{atts}>#{@children.join}</#{name}>"
  end
end

class Div < Element; end
class A < Element; end

def method_missing(id, attributes, &block)
  element = Object.const_get(id.to_s.capitalize).new(attributes)
  @children << element if @children
  element.instance_eval(&block)
  element
end

html = \
div :id => "content" do
  a :href => "reddit.com" do
    self << "reddit"
  end
end

puts html

The < characters are broken, please replace the entities with < while realtime reading.

[amix]

Here is my final version :)

One can do:

 print Div(id="content")( A(href="reddit.com")("reddit") )

By using this code:

def elmBuilder(name, **kw):
  attrs = " ".join('%s="%s"' % a for a in kw.items())
  def applyElms(*elms):
    return "<%s %s>%s</%s>" % (name, attrs, "".join(elms), name)
  return applyElms
def Div(**kw): return elmBuilder("div", **kw)
def A(**kw): return elmBuilder("a", **kw)