Trace local variable

Ruby
1

Hi !

I’ve searched a lot to get an answer but I can’t figured it out. I wish
to trace local variables in a method. Here is an example:

def test
a = 1
b = 2
end

Returns:
Variable a gets the value 1 in test
Variable b gets the value 2 in test

So, I’d like this output without adding some extra stuff in my method
test. I need a callback or thing like that but I can’t find it.

I’m aware of the built-in method trace_var, but unfortunately, it’s all
about global variables. So, I successed to trace attribute calls,
because it is possible to define methods like this:

def attribute=(value)
…
end

I also successed to trace method calls.

I’ve seen function like set_trace_func but it’s not really convenient to
deal with my particular problem. And it isn’t possible to redefine the
assignment operator = on its own.

Any ideas to trace local variables ?
Thanks !

2

2010/2/23 Onionwushu O. [email protected]:

Variable b gets the value 2 in test
end

I also successed to trace method calls.

I’ve seen function like set_trace_func but it’s not really convenient to
deal with my particular problem. And it isn’t possible to redefine the
assignment operator = on its own.

Any ideas to trace local variables ?

I don’t think there is an easy way with MRI and standard tools. You
could debug the code of course. Other than that, these are things
that I can imagine to work:

  • patch interpreter to generate output for variable access
  • use set_trace_func along with a parsed version of the source (e.g.
    ParseTree)
  • patch interpreter even more to allow for a similar mechanism like
    set_trace_func (aka “set_trace_local”).

Btw, why do you need that feature? If your methods are so complex
that you need tracing of individual variables maybe your methods are
too long.

Kind regards

robert

3

Thank you for your proposals. I’ll prefer to avoid patching the
interpreter for now :stuck_out_tongue:
I need this feature because I’d like to know each time a local variable
is updated. So, I found a way using set_trace_func and the binding
attribute, here it is:

set_trace_func lambda { |event, file, line, id, binding, classname|
if event == “line” or event == “return”
local = eval(“local_variables”, binding)
local.each do |l|
puts “#{l} : #{eval(l, binding)}”
end
puts “======”
end
}

I suppose this is not really nice about performance, but it’s working :stuck_out_tongue:

4

But that looks ugly :frowning:

I can not believe ruby could make it so difficult just to be able to
trace local variables? It’s a lot easier with global variables, but I
usually don’t use many global variables …