Do you realize that 2**54-1 isn't exactly representable as a float?  It
requires 54 bits of precision, but the Python float format only has 53
bits available (on all popular boxes).

>>> 2**54-1
18014398509481983L
>>> float(_)  # rounds to closest representable float
18014398509481984.0

In fact, x//y == x//float(y) for only a small subset of integer x and y
values, mostly because only a small subset of integer x and y values are
exactly representable as floats (given that Python integers enjoy
unbounded precision, but floats only retain 53 bits).