[myhdl-list] strange problem

[Neal B. <ndb...@gm...>]

I'm still trying to wrap my head around myhdl.  This one is weird.

In my testbench/monitor, which was working, I just added a variable 
'input_sample_cnt':

    input_sample_cnt = 0
    
    @instance
    def stimulus():
        while (1):
            yield clock.negedge
            if (en):
                in_i.next = in_i_iter.next()
                in_q.next = in_q_iter.next()
                input_sample_cnt += 1
                
            reset.next = 0

I get:
    input_sample_cnt += 1
UnboundLocalError: local variable 'input_sample_cnt' referenced before 
assignment

But I've had this all along:
    result_fd = file ('result', 'w')
    count_fd = file ('count', 'w')
    reset_fd = file ('reset', 'w')
    x_fd = file ('x', 'w')
    en_fd = file ('en', 'w')
    oe_fd = file ('oe', 'w')
    
    @instance
    def monitor():
        while 1:
            yield clock.posedge
            ##print 'reset:', reset, 'en:', en, 'x:', x, 'count:', 
to_hex(count), 'result:', to_hex(result)
            if (oe):
                print >> result_fd, to_hex (result_i), to_hex (result_q)
            print >> count_fd, to_hex (count)
            print >> x_fd, to_hex (in_i), to_hex (in_q)
            print >> reset_fd, to_hex (reset)
            print >> en_fd, to_hex (en)
            print >> oe_fd, to_hex (oe)

This seems functionally the same thing (other than using +=).  Why does this 
latter work, but the former not?  What should I be doing instead?