Why str[:len(str) / 2] but not str[:len(str) * 0.5]?

Question

I'm playing on coding bat and

return str[:len(str) / 2]

works fine but

return str[:len(str) * 0.5]

returns

Error:slice indices must be integers or None or have an __index__ method

and although I can kind of speculate as to why this is I am very interested in hearing a better informed explanation. Thank you!

Bonus Points: Offer a tag I can place on this post so that we can categorize it well!

Answer 1

In Python 2:

Dividing two integers results in an integer.

Multiplying an integer and a float results in a float.

Slices must be integer.

Answer 2

In python, slices must be integers ( 5.0 is wrong, 5 is good).

In python 2

len(str) / 5 # => integer
len(str) * 0.5 # => float

len(str) / 5 # => float
len(str) // 5 # => integer (equivalent of math.floor(len(str) / 5))