Ruby execute code in class getting inherited to

Question

I'm trying to be able to have a global exception capture where I can add extra information when an error happens. I have two classes, "crawler" and "amazon". What I want to do is be able to call "crawl", execute a function in amazon, and use the exception handling in the crawl function.

Here are the two classes I have:

require 'mechanize'

class Crawler
  Mechanize.html_parser = Nokogiri::HTML

  def initialize
    @agent = Mechanize.new
  end

  def crawl
    puts "crawling"

    begin
      #execute code in Amazon class here?
    rescue Exception => e
      puts "Exception: #{e.message}"
      puts "On url: #{@current_url}"
      puts e.backtrace
    end
  end

  def get(url)
    @current_url = url
    @agent.get(url)
  end
end

class Amazon < Crawler
  #some code with errors
  def stuff
    page = get("http://www.amazon.com")
    puts page.parser.xpath("//asldkfjasdlkj").first['href']
  end
end

a = Amazon.new
a.crawl

Is there a way I can call "stuff" inside of "crawl" so I can use that exception handling over the entire stuff function? Is there a better way to accomplish this?

EDIT: This is when I ended up doing

require 'mechanize'

class Crawler
  Mechanize.html_parser = Nokogiri::HTML

  def initialize
    @agent = Mechanize.new
  end

  def crawl
    yield
  rescue Exception => e
    puts "Exception: #{e.message}"
    puts "On url: #{@current_url}"
    puts e.backtrace
  end

  def get(url)
    @current_url = url
    @agent.get(url)
  end
end

c = Crawler.new

c.crawl do
  page = c.get("http://www.amazon.com")
  puts page.parser.xpath("//asldkfjasdlkj").first['href']
end

I want to be able to call "stuff" from crawler so i can the exception handling in crawler. — AdamB, Apr 06 '12 at 22:40
your code doesn't work on my PC... so the code is just an rough idea? — Siwei, Apr 06 '12 at 22:56

score 0 · Answer 1 · answered Apr 06 '12 at 22:53

I managed to get the desired functionality with "super" and a placeholder function. Is there still a better way to do this?

require 'mechanize'

class Crawler
  Mechanize.html_parser = Nokogiri::HTML

  def initialize
    @agent = Mechanize.new
  end

  def stuff
  end

  def crawl
    stuff
  rescue Exception => e
    puts "Exception: #{e.message}"
    puts "On url: #{@current_url}"
    puts e.backtrace
  end

  def get(url)
    @current_url = url
    @agent.get(url)
  end
end

class Amazon < Crawler
  #some code with errors
  def stuff
    super
    page = get("http://www.amazon.com")
    puts page.parser.xpath("//asldkfjasdlkj").first['href']
  end
end

a = Amazon.new
a.crawl

score 0 · Accepted Answer · answered Apr 06 '12 at 23:24

0

You could have crawl accept a code block:

def crawl
  begin
    yield
  rescue Exception => e
    # handle exceptoin
  end
end

def stuff
  crawl do
    # implementation of stuff
  end
end

I'm not crazy about having a method with no body. A code block might make more sense here. May also eliminate the need for subclassing depending on what you want to do.

answered Apr 06 '12 at 23:24

Alex Rockwell

1,454
9
13

I like that, its much cleaner. – AdamB Apr 07 '12 at 00:02

score 0 · Answer 3 · answered Apr 07 '12 at 00:21

If you want it another way, take a look at "strategy" design patten:

# test_mach.rb
require 'rubygems'
require 'mechanize'

# this is the context class,which calls the different strategy implementation
class Crawler
  def initialize(some_website_strategy)
    @strategy = some_website_strategy
  end

  def crawl
    begin
      @strategy.crawl
      #execute code in Amazon class here?
    rescue Exception => e
      puts "==== starts this exception comes from Parent Class"
      puts e.backtrace
      puts "==== ends  this exception comes from Parent Class"
    end
  end
end

# strategy class for Amazon
class Amazon
  def crawl
    puts "now crawling amazon"
    raise "oh ... some errors when crawling amazon"
  end
end

# strategy class for taobao.com 
class Taobao
  def crawl
    puts "now crawling taobao"
    raise "oh ... some errors when crawling taobao"
  end
end

then run this code:

amazon = Crawler.new(Amazon.new)
amazon.crawl
taobao = Crawler.new(Taobao.new)
taobao.crawl

result:

now crawling amazon
==== starts this exception comes from Parent Class
test_mach.rb:27:in `crawl'
test_mach.rb:13:in `crawl'
test_mach.rb:38
==== ends  this exception comes from Parent Class
now crawling taobao
==== starts this exception comes from Parent Class
test_mach.rb:34:in `crawl'
test_mach.rb:13:in `crawl'
test_mach.rb:40
==== ends  this exception comes from Parent Class

BTW. for your implementation, basically I did the same way with you. except

# my implementation
class Crawler
  def stuff
    raise "abstract method called"
  end
end

If you want it another way, take a look at "around alias" (<< metaprogramming ruby>>, page 155 ) . However I think "around alias" is the revert case from strategy.

( I mean,

in strategy pattern, you at first implement the "context" class, then you need to implement the "strategy" class.
but in "around alias" spell, you need to first of all get an "strategy" implemented, then you write a new class that "around alias" the "strategy"...

err... hope I didn't make you confused ^_^ )

Ruby execute code in class getting inherited to

3 Answers3