Still walking the old and used paths of rspec 1 and Rails 2.3 I've now battled with helpers ...
Turns out using ActionView helpers from a spec is not directly possible. If you look around stack overflow etc you'll find a bunch of threads talking about and how helper.some_helper will do the trick.
Alas it's not that simple.
Someone did publish something far more useful and precise : Roger Campos from Itnig : How to get access to rails view helpers from rspec support files .
Here is the trick : you need to define that helper method and include the helpers modules you want to use. Yep.
So in your spec (or the spec_helper) you need to add this :
def helper Helper.instance end class Helper include Singleton include ActionView::Helpers::NumberHelper end
In this case we only include the ActionView::Helpers::NumberHelper helper, but you can add more.
Then to use an helper from there you just have to like this :
my_value.should == helper.number_to_human_size(10000)
That's it !