You’re using Python 2.x, where integer divisions will truncate instead of becoming a floating point number.
>>> 1 / 2
0
You should make one of them a float
:
>>> float(10 - 20) / (100 - 10)
-0.1111111111111111
or from __future__ import division
, which the forces /
to adopt Python 3.x’s behavior that always returns a float.
>>> from __future__ import division
>>> (10 - 20) / (100 - 10)
-0.1111111111111111