From: Philip Mak <pmak@...>
Date: 2002-10-17T10:23:59+09:00
Subject: How to tell if a method accepts this many arguments?

I have this bit of code:

    m = board.method(cmd)
    begin
        m.call(p, *args)
    rescue ArgumentError => err
        p.notify "Too many arguments to command '#{cmd}'."
    end

The "rescue ArgumentError" is meant to catch it if I tried to call the
method with too many arguments, because *args is user input that may
be invalid.

However, if m.call(p, *args) succeeds, but there's a bug *inside* that
method that generates an ArgumentError, then the rescue also catches
it. (I don't want to catch it; I want it to crash so that I'll see
where the bug is. Dead Programs Tell No Lies.)

Would it be possible to:

1. Check how many arguments the method wants without actually calling
it,

or

2. Make the rescue only apply to the m.call.

I don't think #1 is possible; there is a Method#arity method, but if
the method takes a variable number of arguments, Method#arity only
tells me the minimum number of arguments, not the maximum number. For
example, take this function:

irb(main):004:0> def something(arg1, arg2, arg3=nil, arg4=nil); end
nil
irb(main):005:0> method(:something).arity                          
-3

This tells me the method needs at least 3 arguments. But it doesn't
tell me that the method will not accept more than 4 arguments.

As for #2, I think I might be able to look at err.backtrace[0] and
make sure that it mentions the name of the method that I just called.
That seems like a dirty way of doing it, though (but I can't think of
anything else).