Useful decorator

[Paul Rubin]

In the course of writing a bunch of generator-oriented code I kept
wanting to temporarily truncate the output of generators for debugging,
i.e. I might have a function like

    def generate_bazillion_items():
       for line in bazillion_line_file:
          yield itemify(line)

where I wanted to test the program on just the first 20 lines of the
file.  After a number of rounds of editing the program to wrap an
xrange(20) loop I went to using islice(bazillion_line_file, 20), but
even more handy was to write this decorator:

    def truncate(n): 
         from itertools import islice
         def trunc(gen):  # truncate generator to n items
            return lambda *a,**kw: islice(gen(*a,**kw), n)
         return trunc

Then to chop bazillion_items to 20 items, I just write:

    @truncate(20)
    def generate_bazillion_items():
       for line in bazillion_line_file:
          yield itemify(line)

When I want to run the whole file, I comment out the @truncate line,
and if I want to debug again, I just uncomment it.