str(float) isn't a good way of converting a float to a string with all the precision
Eg
>>> f = os.path.getmtime("z")
>>> f
1347717491.343554
>>> str(f)
'1347717491.34'
>>> "%f" % f
'1347717491.343554'
Change-Id: I6eb02f7f6730eff814c348d1039eae4606642b52