Can anyone spot my float error?

[Richard Harvey]

I'm seeing an extrememly wacky effect in 1.5.2, and the code is so simple
I'm baffled that I cannot see the error. Here is a snippet:

timer = 0.0

def testFunc():
    global timer
    timer = timer + 0.5
    if timer > 10.0:
        print "timer > 10"

If I repeatedly call testFunc in a loop, my print statement never gets
called. However, if I change the code to the following everything works
fine:

timer = 0.0

def testFunc():
    global timer
    timer = timer + 0.5
    dummy = "timer is %s" % str(timer)
    if timer > 10.0:
        print "timer > 10"

In this case, my if statement becomes true as expected. I've tried calling
isinstance(timer,FloatType), and it returns true.  Also, if I change the
dummy line to:

dummy = "timer is %f" % timer

the code no longer works; dummy does not get assigned, and my if statement
is never true. Arrrghh!!

Rich