How to run IRB.start in context of current class

Question

I've been just going through PragProg Continuous Testing With Ruby, where they talk about invoking IRB in context of current class to inspect the code manually.

However, they quote that if you invoke IRB.start in a class, self is predefined, and refers to the object we were in when start was called which isn't true in my case.

Even for very simple example like

a = "hello"
require 'irb'
ARGV.clear # otherwise all script parameters get passed to IRB
IRB.start

When I try to access the a variable, I get the obvious

NameError: undefined local variable or method `a' for main:Object

It works only when I change a to global variable

$a = "hello"
require 'irb'
ARGV.clear # otherwise all script parameters get passed to IRB
IRB.start

then I can access it

irb(main):001:0> $a
=> 1

Is there any way around this to access local and instance variables in the current class?

Answer 1

As you've already discovered, self does not refer to the object where IRB was started, but to the TOPLEVEL_BINDING, which seems to be an instance of the Object class itself.

You can still run an IRB session with a specific class or object as the context, but it's not as simple as just starting IRB.

If you care about is starting IRB with a specific context, then it's really easy to do when you're starting IRB manually. Just start IRB normally and then call the irb method, passing it the object/class you want as the context.

$ irb
irb(main):002:0> require 'myclass'
=> true
irb(main):003:0> irb MyClass
irb#1(MyClass):001:0> self
=> MyClass

You can also start an IRB session programmatically and specify the context, but it's not nearly as easy as it should be because you have to reproduce some of IRB's start-up code. After a lot of experimenting and digging around in the IRB source code, I was able to come up with something that works:

require 'irb'
IRB.setup nil
IRB.conf[:MAIN_CONTEXT] = IRB::Irb.new.context
require 'irb/ext/multi-irb'
IRB.irb nil, self