/ vs // in python3

Code speaks:

>>> a=b=5
>>> a,b
(5, 5)
>>> type(a), type(b)
(<class 'int'>, <class 'int'>)
>>> a/=2
>>> b//=2
>>> a,b
(2.5, 2)
>>> type(a), type(b)
(<class 'float'>, <class 'int'>)
>>> alist = list(range(10))
>>> alist
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> alist[:a]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: slice indices must be integers or None or have an __index__ method
>>> alist[:b]
[0, 1]

Note if you are trying calculating the indices with '/' you would get trouble as showed above.