How to limit the amount of numbers given in a float input? (Python)

Question

Novice Pythoner here. I am trying to finish my first program (a tip calculator) and I have one last piece of code to write. Here is the part of code I need to add on to:

bill_amt = True
while bill_amt:
    try:
        bill_amt = float(input('First, what was the price of your meal?:'))
    except:
        print('Please enter a number only.')
        continue
    if bill_amt <= 0:
        print('Your meal wasn\'t $',bill_amt,'! Please try again.')
        bill_amt = True
    else:
        x = float(bill_amt)
        bill_amt = False

What I want to do is add a command that will limit the amount of numbers you can input when the code asks how much your meal was so user can't type in 4511511513545513513518451.32. I've tried using len(bill_amt) > 8, but I get an error that floats don't have strings. How do I get around this? Thanks, sorry if it's a duplicate! -Pottsy

@jonrsharpe 0.00000000000000000000000001 > 99999999 evaluates to False — ElmoVanKielmo
– ElmoVanKielmo, Commented May 7, 2014 at 22:45

Ryan Haining · Accepted Answer · 2014-05-07 23:49:15Z

1

use regex matching, this will also prevent the user from typing in something like "12.123456"

import re
# ...

while True:
    inp = input('First, what was the price of your meal?:'))
    if bool(re.match(r"^\d{1,8}\.\d\d$", inp)):
        return float(inp)
    else:
        print('invalid entry')

\d means digit, {1,8} means allow anywhere from 1 to 8 digits. \d\d looks for two digits after the ., so this regex will match 1-8 digits, followed by a dot, followed by two more digits.

Note that if you are dealing with money, you don't generally want to use floats, but rather decimal.Decimals. Try doing

decimal.Decimal(inp)

at the end instead.

edited May 7, 2014 at 23:49

answered May 7, 2014 at 22:43

Ryan Haining

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Comments

sabbahillel · Accepted Answer · 2014-05-07 22:47:11Z

0

In order to get the length use str(bill_amount)

len(str(123.53)) is 6

Note that even though Python will allow you to use a float value and then change it to the boolean, it would be better to make it a different name rather than have both the float and the boolean as bill_amt .

answered May 7, 2014 at 22:47

sabbahillel

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Comments

ElmoVanKielmo · Accepted Answer · 2014-05-07 23:00:26Z

0

Float doesn't have length but str does:

bill_amt_str = input('First, what was the price of your meal?:')
if len(bill_amt_str.replace(".", "")) > 8:
    print("No more than 8 digits please.")
    continue
bill_amt = float(bill_amt_str)

edited May 7, 2014 at 23:00

answered May 7, 2014 at 22:44

ElmoVanKielmo

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7 Comments

Gabriel M Over a year ago

Or, use in-place conversion without adding extra variable; having bill_amt as it was, just check if len(str(bill_amt))>8:

ElmoVanKielmo Over a year ago

@GMGray garbage collector will wipe out this tiny little string anyway.

Ryan Haining Over a year ago

@GMGray there's an extra name but no more space allocated for the variable

Gabriel M Over a year ago

@Ryan Haining - of course, it's just a matter of personal coding preferences.

Gabriel M Over a year ago

@ElmoVanKielmo - could you explain where are potential problems with this shortcut? I'd have to get rid of bad habits then..

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