python 2.7 - Making ArcPy interactive?

I'm developing an interface with arcgis and python.

When I have only python, I can make my script interactive using:

S = float(raw_input( 'Please enter the value: '))

I can open and run my script at the python window of the ArcGIS, and the script run, but when I try become this interactive, asking the value, ArcGIS gives me an error:

Runtime error 
Traceback (most recent call last):
  File "", line 769, in 
EOFError: EOF when reading a line