How to find where a method is defined at runtime?

I'm coming late to this thread, and am surprised that nobody mentioned Method#owner.

class A; def hello; puts "hello"; end end
class B < A; end
b = B.new
b.method(:hello).owner
=> A

You can actually go a bit further than the solution above. For Ruby 1.8 Enterprise Edition, there is the __file__ and __line__ methods on Method instances:

require 'rubygems'
require 'activesupport'

m = 2.days.method(:ago)
# => #<Method: Fixnum(ActiveSupport::CoreExtensions::Numeric::Time)#ago>

m.__file__
# => "/Users/james/.rvm/gems/ree-1.8.7-2010.01/gems/activesupport-2.3.8/lib/active_support/core_ext/numeric/time.rb"
m.__line__
# => 64

For Ruby 1.9 and beyond, there is source_location (thanks Jonathan!):

require 'active_support/all'
m = 2.days.method(:ago)
# => #<Method: Fixnum(Numeric)#ago>    # comes from the Numeric module

m.source_location   # show file and line
# => ["/var/lib/gems/1.9.1/gems/activesupport-3.0.6/.../numeric/time.rb", 63]

This is really late, but here's how you can find where a method is defined:

http://gist.github.com/76951

# How to find out where a method comes from.
# Learned this from Dave Thomas while teaching Advanced Ruby Studio
# Makes the case for separating method definitions into
# modules, especially when enhancing built-in classes.
module Perpetrator
  def crime
  end
end

class Fixnum
  include Perpetrator
end

p 2.method(:crime) # The "2" here is an instance of Fixnum.
#<Method: Fixnum(Perpetrator)#crime>

If you're on Ruby 1.9+, you can use source_location

require 'csv'

p CSV.new('string').method(:flock)
# => #<Method: CSV#flock>

CSV.new('string').method(:flock).source_location
# => ["/path/to/ruby/1.9.2-p290/lib/ruby/1.9.1/forwardable.rb", 180]

Note that this won't work on everything, like native compiled code. The Method class has some neat functions, too, like Method#owner which returns the file where the method is defined.

EDIT: Also see the __file__ and __line__ and notes for REE in the other answer, they're handy too. -- wg