Just some quick notes on this. The decorator should become:

  def catchwarning(*, record=False, using=warnings)

Name change is to follow the naming convention in 'warnings' (even
though I prefer underscores). The 'using' argument is what module is
being used as the 'warnings' module (mostly just for testing
warings/_warnings). And they are both keyword-only (or at least in
spirit in 2.6) so that the potential to support properly dealing with
__warningregistry__ in the future can be added and the *args arguments
can be those modules to watch.