[Tutor] Limit raw_input to hundredth decimal point

[Steven D'Aprano]

Ryan Kirk wrote:
> Is there a way to limit raw_input to the hundredth decimal point?

No. raw_input is a tool that does one thing: it collects input from the 
user. It doesn't understand numbers, check for decimal places, check the 
input for spelling errors, or anything else. It's a hammer, not a 
combination hammer-screwdriver-wrench-drill-saw-axe :)

One solution is to build a new tool that checks for decimal places:


def check(text):
     try:
         x = float(text)
     except ValueError:
         print "please enter a number"
         return None
     y = x*100
     if y - int(y) != 0:
         print "please enter only two decimal places"
         return None
     return x


def get_number(prompt):
     answer = None
     while answer is None:
         text = raw_input(prompt)
         answer = check(text)
     return answer


At first, this seems to work well:

 >>> get_number("Please enter a number with two decimal places: ")
Please enter a number with two decimal places: 77.25
77.25
 >>>

but there's a fundamental problem. The user is entering numbers in 
decimal (base 10), but Python does calculations in binary (base 2), and 
something that has two decimal places may not be exact in binary:

 >>> get_number("Please enter a number with two decimal places: ")
Please enter a number with two decimal places: 77.21
please enter only two decimal places

Huh? 77.21 does have two decimal places. But the closest float to 77.21 
is in fact 77.209999999999994. No computer on Earth can store 77.21 
*exactly* as a binary float, no matter how hard you try!

So, what to do...? You can:

(1) Give up on forcing the user to only enter two decimal places, and 
instead use the round() function to round to two places:

 >>> round(77.2123456, 2)
77.209999999999994

This is still not two decimal places, but it is the closest possible 
float to 7.21, so you can't do any better.

(2) Or give up on using float, and use the decimal module instead. 
(However decimals are slower and less convenient than floats.)

 >>> from decimal import Decimal
 >>> x = Decimal("77.21")
 >>> x
Decimal("77.21")


If you are working with currency, then you should use decimal, and not 
floats.



Good luck!



-- 
Steven