gistfile1.py

# idea from http://www.codemud.net/~thinker/GinGin_CGI.py/show_id_doc/201
def log(logfile):
    def assign_call(call):
        return lambda *args, **kws: logfile.write(call.__name__) and call(*args, **kws)
    return assign_call

class Foo(object):
    @log(file('my.log',"w"))
    def bar():
        pass

f = Foo()
f.bar()
f.bar()

gistfile2.rb

class Class
  def log(name, logfile)
    original_c_m = instance_method(name)
    
    define_method name do |*args|
      logfile.write caller
      original_c_m.bind(self).call(*args)
    end
    
  end
end	

class Foo
  
  def foo
  end
  
  log :foo, File.new("my.log", "w")
end

f = Foo.new
f.foo
f.foo