A Curious Use of For Loops In Ruby

I was thinking about the use of for loops in Ruby the other day, triggered by one of the Ruby problems over at rubeque.

Now, a for-loop in Ruby usually looks something like this:

for i in collection
  # i takes each value that's in the collection
end

Interesting things happen when the loop variable (i in the case above) is assigned to a value already before the loop. In particular, interesting things can happen if it’s not even a variable.

Let’s see what one can do with a for-loop in Ruby (or rather to a for-loop). Enter pry (or your favourite Ruby-REPL [Read-Evaluate-Print-Loop]):

pry(main)> for Object.new in [1,2,3]; end
NoMethodError: undefined method `new=’ for Object:Class
from (pry):3:in `block in __pry__’

Oh, that’s interesting: The interpreter does not complain about Object.new not being a variable, but rather that there’s not method ‘new=’ for class Object.

OK. let’s play with this… Let’s assume a class Foo:

class Foo
  def initialize foo
    @foo = foo
  end

  def foo= other
    @foo.unshift other
  end
end

Note that there’s no method named foo for class Foo, just ‘foo=’. Initialising a variable now with, say, an empty Array, there’s a curious way to reverse an Array:

pry(main)> f = Foo.new(res)
=> #<foo:0x007fd392c02368 @foo=[]>
pry(main)>
pry(main)> for f.foo in [:first, 2, 'third']
pry(main)* end
=> [:first, 2, "third"]
pry(main)> pp res
["third", 2, :first]
=> ["third", 2, :first]

There is a way to use an empty loop to reverse an Array. (Note. this does not solve the problem given at rubeque, since a class definition is not allowed inside a method definition). In fact there are more ways than this!

A few new things I learned about for loops in Ruby:

1. They do not require a variable name as the ‘loop variable’
2. If one uses what looks like a method call (the ‘f.foo’ in the example above) as the  ‘loop variable’ then…
   1. …the object f refers to, doesn’t even need to respond to that method, but
   2. …instead it needs to respond to ‘method_name=’ (‘foo=’ in the example above).

I don’t know what to think about this, but you probably shouldn’t rely on this behaviour or use it in production code.

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More Fun With Rakefiles

After Fun With Rakefiles, here’s another ‘behaviour’ I found in a Rakefile.
Some example code is available on GitHub: https://github.com/s2k/rake-loop.

The Setting

The project team worked on a larger number of modules, each in its own sub directory and a global rake task spec:all to run, well, all the specs for all the modules. Since they could run independently, we used the parallel gem to run RSpec in several processes, so our continuous integration system (or local machine) could use the available CPUs (as opposed to running in Ruby threads, which doesn’t utilise all processors [well, at the time of this writing]).

Also, while most of the RSpec code used the (now) old-style should-notation, some new specs were written using the new expect notation. For more on that topic see e.g. RSpec’s New Expectation Syntax by Myron Marston and the GitHub repo for RSpec expectations.

Trying To Fix A Hack

When working with the Rakefile and RSpec files I noticed something odd: The RSpec files using expect weren’t executed. The reason was some code in the Rakefile which (essentially) did this:

 run(rspec_filename) if File.read(rspec_filename).contains? 'should'

That explains why the newer RSpec code was not executed: It didn’t contain any shoulds. But what was going on?

Right after removing that code and re-running bundle exec rake spec:all, I found out that the command a) didn’t finish in any reasonable amount of time and b) over time there were more and more Ruby processes showing up in the system monitor. What was happening?

Why excluding RSpec files not containing ‘should’ helped

As far as the symptom was concerned, excluding RSpec files that didn’t contain the word ‘should’ helped avoiding the behaviour I observed. In any case, I wanted to also execute the other RSpec files, but I would not add to the hack by executing file which contained either ‘should’ or ‘expect’…

A Rake Feature

When rake is called from the command line in a directory that does not contain a Rakefile, then rake looks for a Rakefile in the parent directory. That’s a feature, since it allows you to have one description of a task in some directory, and use that task in all sub directories. (In our case however, something was going horribly wrong.)

Where It went wrong

I then figured out that the modules where the RSpec files didn’t contain the word ‘should’ also didn’t contain a Rakefile. So, when invoking rake in that case, it would move upwards the folder structure finally get to the project global Rakefile and call the task defined there.

Alas, the ‘global’ Rakefile not only contained a task definition for spec:all, but also a task called spec—which only invoked spec:all in a new shell:

namespace :spec do
  desc 'runs all rspec suites'
  task :all => :clean do
    # do stuff, in particular:
    sh "cd #{spec_dir}/../; bundle exec rake spec > #{report_folder}/#{module_name}.tmp 2>&1"
  end

 desc 'delete old report files'
 task :clean do
   FileUtils.rm_rf report_folder
 end
end

task :spec do
  sh 'bundle exec rake spec:all'
end

Notice that the global Rakefile expects a Rakefile containing a definition for the task ‘spec’ to be defined somewhere (see line 5 in the code snippet above), probably in the sub directory where rake is called.

Alas, if there is no Rakefile, rake will step up the directory structure, eventually find the global Rakefile call the task ‘spec’… and restart running ‘spec:all’ again, only to run into the exact same situation again and therefore start a loop.

Lessons Learned

• Using the parallel gem made a bad situation worse, but it also helped finding the real problem by maximising the effect.
• It’s probably a good idea to call rake tasks from within a rake task using the methods Rake itself provides (invoke and execute). There’s a discussion on stack overflow named How to run Rake tasks from within Rake tasks?
• If something goes wrong, fix the underlying issue, rather than working around the problem. It’ll save later generations a lot of work.
•  Generally speaking, it’s a good idea not to let specialised rake tasks (like running RSpec in some sub directory) end up invoking a more global rake task. That’s especially true if that more global task is going to invoke the more specialised one in turn.

Ruby Details: clone and dup

Another post that’s mostly a reminder to self. 😉

Over at Know Ruby: clone and dup Aaron Lasseigne explained the differences between dup and clone.

However there’s another difference, and that’s the way the methods reproduce the methods an object can respond to. As is so often the case, a short session in a Ruby REPL (like irb or pry) helps to see things in action:

[1] pry(main)> o = Object.new
=> #<Object:0x00000102d298a0>
[2] pry(main)> def o.meth
[2] pry(main)* puts "meth called on object #{self.inspect}"
[2] pry(main)* end
=> :meth
[3] pry(main)> o.meth
meth called on object #<Object:0x00000102d298a0>
=> nil
[4] pry(main)> o_dupped = o.dup
=> #<Object:0x00000102101eb0>
[5] pry(main)> o_dupped.meth
NoMethodError: undefined method `meth' for #
from (pry):7:in `__pry__'
[6] pry(main)> o_cloned = o.clone
=> #<Object:0x00000102138168>
[7] pry(main)> o_cloned.meth
meth called on object #<Object:0x00000102138168>
=> nil

Note that both, clone and dup, return a new object, but only the one returned by clone also has the method defined for object o.
In other words: dup does not replicate an objects singleton methods.

Fun With Rakefiles

I recently ran into an ‘interesting’ issue in a set of  Rakefiles.

Here’s a part of the global Rakefile, which imports the *.rake files from a tasks folder:

$LOAD_PATH.unshift File.join(File.dirname(__FILE__), 'lib')

Dir.glob('tasks/*.rake').each { |r| import r }

Some of these Rakefiles require other Ruby files (which is stored in the lib folder). Since executing one task involves loading all that code, there’s a chance that an error in one supporting Ruby file may cause an error in an entirely unrelated task.

Let’s look at an example, the code is available over at GitHub: https://github.com/s2k/fun_with_rakefiles

For simplicity there are just two rake files within tasks and a ruby file in lib. What we like to run is this:

rake make_it_so

However what we get is an error message pointing to some rather unrelated part of the code:

$rake make_it_so
rake aborted!
Errno::ENOENT: No such file or directory @ rb_sysopen - partlist.txt
…dev/fun_with_rakefiles/lib/thing.rb:7:in `readlines'
…dev/fun_with_rakefiles/lib/thing.rb:7:in `<class:Thing>'
…dev/fun_with_rakefiles/lib/thing.rb:1:in `<top (required)>'
…dev/fun_with_rakefiles/tasks/unrelated_thing_tasks.rake:1:in `require'
…dev/fun_with_rakefiles/tasks/unrelated_thing_tasks.rake:1:in `<top (required)>'
…/.rvm/gems/ruby-2.1.1/bin/ruby_executable_hooks:15:in `eval'
…/.rvm/gems/ruby-2.1.1/bin/ruby_executable_hooks:15:in `'
(See full trace by running task with --trace)

Hmmm… When I first saw the error message I really wondered why suddenly there’s a file partlist.txt missing and then I asked myself where that Thing class was used in the code.

In this example it’s relatively easy to see what’s going on: The ‘central’ Rakefile gathers all the other .rake files that may be necessary (which, in turn, require all the files they need). However, if there are a few (sub) rake files around things can get a bit more confusing.

In my case, it was simple enough to repair the issue, since the missing file could be added to source control and everything worked again. But still there are a few things to think about:

1. Should the be code in a Ruby class definition that’s executed while defining the class? Especially: Should it be there, in case you can avoid it?
   In my opinion it’s a good idea to only have code like that if it’s really necessary. In the example given, one could load the file when an instance of the class is created. Doing so will reduce the consequences of the missing file drastically: Only the rake tasks actually using that class would fail, not every rake task.
2. Could one avoid the failure and still be able to run the desired task?
   It turns out that you can:

   $rake -f tasks/what_we_want_to_do.rake  make_it_so
   Processing something…

   In some cases you may also need to supply this command line parameter, in order to allow rake to find other Ruby files:

   -I, --libdir LIBDIR      Include LIBDIR in the search path for required modules.

3. Can we check that nothing’s broken?
   In fact, we can:

   rake -T

   If that actually lists the available rake tasks and doesn’t exit with an error code, it looks OK, at least it didn’t break everything. However, if a file is missing form the version control system, it’s a works-on-my-machine situation.

In a project with a large number of rake files and supporting library code, it may be worth while to have rake -T as part of a smoke test on the continuous integration system.

 

 

List Cucumber Step Definitions

Yet another quick reminder to self: Here’s how to list the Cucumber step definitions:

cucumber -d -f stepdefs

Mac OS X, Parallels & the Back Slash

Yet another short note to my future self:

Running a Mac with a German keyboard layout (likely having the key sequence “7 8 9 0 ß ` <Backspace>” in the top row) and using a Parallels Desktop to run Windows and needing to enter a back slash (\) now & then?

Try the right-hand-side CRTL-ALT + ß (the key with the German ‘sz’ as the primary character).

osx-gcc-installer or XCode

1. How to remove Xcode completely from your system:

   sudo /Developer/Library/uninstall-devtools --mode=all

   Use at your own risk!

2. Where to get Xcode: The App store: https://developer.apple.com/xcode/
3. OSX-gcc-intstaller: https://github.com/kennethreitz/osx-gcc-installer

Addtional info: Another way is to get the “Command Line Tools for Xcode” at https://developer.apple.com/downloads/ (which requires an Apple developer account).

Happy hacking!

NoMethodError in Rails Tests — Fun With Fixtures

In case there’s this weird error message when running unit tests for a Rails app, chances are that your fixtures need some attention. Especially if the schema changed…

NoMethodError: undefined method ‘name’ for #

method method_missing in test_process.rb at line 511
method method_missing in test_case.rb at line 158
method rescue in run in setup_and_teardown.rb at line 26
method run in setup_and_teardown.rb at line 33
method block (2 levels) in run_test_suites in unit.rb at line 641
method each in unit.rb at line 635
method block in run_test_suites in unit.rb at line 635
method each in unit.rb at line 634
method run_test_suites in unit.rb at line 634
method run in unit.rb at line 594
method block in autorun in unit.rb at line 492

There’s no test method given, because, well the tests don’t even get that far: It’s likely that there’s a key (column name) given in the fixture, which is not in the DB schema (anymore).

More RVM, TextMate, Ruby and (!) Rails godness

Here’s another article on the topic of rvm, Textmate, Ruby & Rails: Blended Musings (http://blendedmusings.blogspot.com/2010/10/getting-rvm-textmate-ruby-187-and-192.html)

Another note to self: RVM, TextMate and Ruby 1.9.2

Getting rvm and Ruby 1.9.2 and TextMate can be a bit of work, especially if you’re using rake as well inside TextMate.

Good Thing there’s a solution already: Jim blogged about it at “RVM, ruby 1.9 and TextMate“. Thanks Jim!

 

Ruby, Sequel and Trees

While working on some tree structure a couple of unit tests failed, when creating some form of summary of said tree structure. First of all here’s a condensed form of the code:

require 'sequel'

DB = Sequel.sqlite
DB.create_table :items do
  primary_key :id
  String :name
  String :foo, :default => 'NOT SET'
  Integer :item_id
end

class Item < Sequel::Model
  one_to_many :items
  def summary
    if items.empty?
      return [ { self.name => self.foo } ]
    else
      items.inject( [] ){ | r, sub_res | r << sub_res.summary }
    end
  end
end

r1 = Item.create :name => 'R1'
r2 = Item.create :name => 'R2'

r1.add_item r2
r2.foo = 'Ding'
#r2.save

[ r1, r2 ].each{ |i|
  puts "Item    : #{ i.id }"
  puts "Direct Foo: #{ i.foo.inspect }"
  puts "Summary   : #{ i.summary.inspect }"
  puts
}

Notice, that I’ve commented out saving r2. The output is:

Item    : 1
Direct Foo: "NOT SET"
Summary   : [[{"R2"=>"NOT SET"}]]

Item    : 2
Direct Foo: "Ding"
Summary   : [{"R2"=>"Ding"}]

What I didn’t expect – and why the unit tests failed – is the ‘NOT SET’ in the r1 summary.

One way to end up with what I originally expected is to save r2. Another way is to set r2.foo before adding r1 to r1:

r2.foo = 'Ding'
r1.add_item r2

How ever I’m still not sure whether this is intended behaviour and why it should behave like this. (If it is, I’d like to know why.)
What do you think?

About 40 Years Ago And Metrics

The term “Software Engineering” is coming to age: It was coined slightly more than 40 years ago in Garmisch-Partenkirchen, Germany. Tom DeMarco wrote a very interesting article about his opinion about the topic, which is available online at the IEEE Software magazine: “Software Engineering: An Idea Whose Time Has Come and Gone?” Let me cite just this part:

This leads us to the odd conclusion that strict control is something that matters a lot on relatively useless projects and much less on useful projects. It suggests that the more you focus on control, the more likely you’re working on a project that’s striving to deliver something of relatively minor value.

I strongly recommend to read the whole article.

Remove files or directories from a git repository

Ever needed to remove a file or directory from a git repository? I had to: After pushing a change that included a rather large file to a remote repository, the repository couldn’t be cloned anymore (due to a memory limitation). The support is working on the issue, but being able to clone the repository seems more important to me than having the slides for a presentation (of the past) in this particular repository.

So here’s how: David Underhill has already done – and described – it: “Git: Forever Remove Files Or Folders From History”

Worked for me as well.

Sinatra and reloading

Since its 0.9.2 release on 18th of May 2009, Sinatra doesn’t automatically reload files anymore – not even in development mode.

To achieve this use ‘shotgun’

sudo gem install shotgun

and then start the server using:

shotgun <your_applicationname>.rb

The effect is essentially the same (apart from avoiding the issues the ‘traditional’ way of reloading apparently had – which is why it was removed). 

Thanks for pointing this out in this thread @ rubyforen.de.

A Pure Ruby Readline

 

As found on Github via RubyCorner on DEV_MEM.dump_to(:blog) – Multimedia systems blog.

For all of those who a running Ruby on a Windows machine (known not to be the ideal place to use the readline library) or on some flavour of Unix (where the library might not be available, at least not from scratch), there is now a pure Ruby implementation available: The pure Ruby readline on Github.

Euruko 2009 – back from Barcelona

These two days with Ruby enthusiasts form all over the world not just Europe was great. Nicely located at citilab, with very good public transportation to the city centre of Barcelona, there was enough space inside as well as out side the building to talk to the other attendees, just relax in the sun or work with the notebook. Two wi-fi networks inside and outside the building could deal well with all the notebooks and mobile phones connected to it. This worked very good indeed.

The presentations I liked most where the ones introducing really exciting and new stuff. First and foremost there’s Rhodes, which enables the creation of native mobile applications with Ruby and HTML. And of course Matz’ keynote was very entertaining as well. More really new stuff was about Adhearsion a way to build voice enabled application with Ruby (and Rails). Then some of the presentations were a really good show. Most notably Javier’s talk about gosu, a framework for games and Pablo’s presentation about Archaeopteryx which was probably the loudest presentation I’ve ever heard. 🙂

The only thing that was a tad bit disturbing for me was that I didn’t have one of the mobile microphones available and was kind of stuck behind the table as well as the phones themselves, as they seemed to go silent now and then. Aslak dealt with this best I think, repeatedly saying “Hello?” directly into the microphone whenever it went silent.

I think this year was the first time Twitter was heavily used during Euruko, reusing a bot made for Scotland on Rails. and this helped enhancing the communication and connecting people even more than in previous years. A special “Thank you” to all for the other attendees who gave me direct feedback about the presentation as well as the corresponding discussion about it, including but not limited to: Andrew Miller, Tim Becker, Mike Just, Dave Frey. Thanks a lot, indeed.

More summaries are available from Javier and Tomasz. 

Next year Euruko will take place in Krakow, Poland and I’m already looking forward to going there.

Get Going with Git

While still in the process switch ing from Subversion to Git, here are a few helpful links I found:

• How to set up a git server – one way to to it, by CommonThread
• Toolman Tim explains another way to serve git repositories (for those an a Mac)
• Another hint is available at Apple’s Support Site.
• Additionally you might need to chmod a+rx ~ and chmod a+rx ~/Sites (if Sites is where you put your stuff)

Once up and running there’s of course github, gitcentral and gitorious. For hosting open source projects currently github seems to be the choice, but in case you prefer not to share stuff (just yet) gitcentral offers free (as in no money) hosting of private projects using free (as in open source) software.