How to I temporarily override print and puts from a class? - ruby

Question

I'm working on a shell for my project. I want the Shell class to override all print functions while the shell is running, so I've done this:
# WARNING: Blocks until the user exits.
def start
# Override Kernel print functions.
master_print = Kernel.method :print
master_puts = Kernel.method :puts
Kernel.module_exec {
define_method(:print) { |text = ""|
self.send(:print_override, master_print, text)
}
define_method(:puts) { |text = ""|
self.send(:puts_override, master_puts, text)
}
define_method(:puts_padded) { |text = ""|
self.send(:puts_override, master_puts, "")
self.send(:puts_override, master_puts, text)
self.send(:puts_override, master_puts, "")
}
}
# Readline loop and command parsing here...
end
This works well as long as only the Shell class is outputting any text, but as soon as a command class tries to puts I get this:
NoMethodError: undefined method `puts_override' for #<AddressKit::CLI::Interactiv
e::Commands::LoadTable:0x000000027735b0>
I thought that the print and puts blocks I wrote above would stay in this scope, not execute in which ever scope they happened to be called from. Is it possible to fix that?
Bonus question: How do I put the original print functions back? I had planned this:
Kernel.module_exec {
define_method(:print, &master_print)
define_method(:puts, &master_puts)
undef_method(:puts_padded)
}
But I haven't tried it yet, and I don't know if that will leave Kernel exactly as I found it.

Answer 1

While blocks do preserve scope, self is special and refers to the instance running the method (I assume because this is more useful).
An alternative strategy would be to use the fact that Kernel#puts just does $stdout.puts, so instead of overriding puts, set $stdout to one of your classes, that can massage the values before passing them to puts on the former $stdout.
When you're done, restore $stdout to its original value.

Answer 2

Well, I think I fixed the problem, but I'm not sure why it works. I'm guessing that the blocks are in both the scope where it was called from, as well as in the scope they were defined in, but with higher priority on the former. That would mean that I can't use self and expect it to point to the shell. Instead I did this:
shell = self
Kernel.module_exec {
define_method(:print) { |text = ""|
shell.send(:print_override, master_print, text)
}
define_method(:puts) { |text = ""|
shell.send(:puts_override, master_puts, text)
}
define_method(:puts_padded) { |text = ""|
shell.send(:puts_override, master_puts, "")
shell.send(:puts_override, master_puts, text)
shell.send(:puts_override, master_puts, "")
}
}
Which works perfectly but (if I'm right) would break as soon as the variable shell exists in the same scope as print or puts is called from. But it doesn't break, I tested it! I really have no idea what's going on and I'd be very grateful if somebody could explain it :-)