Converting input containing special character to float

Question

This is the beginning of my program that calculates simple interest. Interest rate will have the following format : 0.97 , 0.67 , 0.17 etc. They won't be bigger than 1. So if the user enter 9 for the interest, program will convert it to 0.09 (by dividing it by 100) . Also user can enter input using '/'. So program will convert input like 97/100 to 0.97.

I wrote the code below. It works but it seems to me that there might be a easier and more elegant solution to this. Maybe using more build-in functions etc. If you help me with that I would be very appreciated.

def toNum(interest):
 if '/' not in interest:
 if float(interest) > 1:
 return float(interest)/100
 else:
 return float(interest)
 else:
 l= []
 n = 0
 count = 1
 list_interest=[]
 for e in interest:
 list_interest.append(e)
 for e in list_interest:
 if count == 1 or count == 3:
 l.append(e)
 count = count +1
 continue
 if e == '/':
 n = n + 1
 count = count +1
 else:
 l[n] = l[n] + e
 return int(l[0]) / int(l[1])
interest = input("Interest rate: ")
interest = toNum(interest)
print(interest)

Answer 1

For reading a fraction such as "97/100", you can use the fractions library.

For example:

from fractions import Fraction
f = Fraction("97/100")
print(float(f)) # prints 0.97

And because the constructor also takes a float, we can remove the check for /. Therefore, the final code is:

from fractions import Fraction
def toNum(interest):
 f = Fraction(interest)
 f = float(f)
 if f > 1:
 f /= 100
 return f
print(toNum("97/100")) # prints 0.97
print(toNum(0.97)) # prints 0.97
print(toNum(9)) # prints 0.09

• 301_Moved_Permanently: Why convert to float at all and not keep a fractions.Fraction object all along?
• esote: @MathiasEttinger That's also a good option, just depends on preference.

Answer 2

While the answer by @esote is correct and I would also recommend using the fractions module, you should also work on your text parsing. In this case you could have used a simple str.split and map to parse the string containing a /:

if "/" in interest:
 numerator, denominator = map(int, interest.split("/"))
 return numerator / denominator

Note that int ignores whitespace, so this works with both "97/100", "97 / 100" and any combination thereof.

Note also that using sensible names makes it immediately obvious what this code does.