Python : Limiting input as numbers and numbers with decimal places

Question

How to limit input to only integers or numbers with two decimal places, else (alphabet or other symbols) it will display 'invalid' and allow the user to enter again.

code:

counter = 100
income = input("Enter income: ")

while counter >0:
    try:
       val = int(income)
       print ("YES")
       income = input("Enter money: ")
       counter = counter - 1
    except ValueError:
       print("NO")
       income = input("Enter money: ")
       counter = counter - 1

I've taken the val = int (???) from another question and it works if it's limiting to integer input only, since my program involves money I need it to go up to two decimal places but it does not accept decimals. (The counter are only for me to test the program)

Should the money amount be stored as cents/pence? It is not a good idea to store money amounts as float. — cdarke, May 04 '15 at 16:10
[It's a much better idea to use Decimal](http://stackoverflow.com/a/1406800/344286) or something to that effect. — Wayne Werner, May 04 '15 at 17:13

score 1 · Answer 1 · answered May 04 '15 at 16:17

1

You could define your own input function

def input_number(msg):
    while True:
        try:
            res = round(float(input(msg)), 2)
            break
        except:
            print("Invalid")
    return res

income = input_number("Enter money:\n")

answered May 04 '15 at 16:17

Yoann Quenach de Quivillic

5,206
26
41

score 1 · Answer 2 · answered May 04 '15 at 16:30

You could use regexp :

import re
is_number=re.compile('^\d+\.?\d{,2}$')

>>>is_number.match('3.14') is not None
True
>>>is_number.match('32222') is not None
True
>>> is_number.match('32A') is not None
False
>>> is_number.match('3.1445') is not None
False

score 1 · Answer 3 · answered May 04 '15 at 16:31

In my opinion you need a regular expression for the features you need, in particular ensuring exactly two digits after the decimal point, while making that optional.

I have imposed the income as pence or cents. This is because you may get rounding issues using float.

import re

# income as pence/cents
income = None

while income is None:
    instr = input("Enter money: ")
    m = re.fullmatch(r'(\d+)(?:\.(\d{2}))?', instr)
    if m:
        full, cents =  m.groups()
        if cents == '' or cents is None: 
            cents = '0'
        income = (int(full) * 100) + int(cents)
    else:
        print("NO")

print("Income is %d.%02d" % (income/100, income % 100))

jwilner · Answer 4 · 2015-05-04T16:38:19.413

1

With this many constraints on the pattern, I really think a regex is more declarative and readable.

import re 
NUM_REGEX = re.compile(r"^\d+(?:\.\d{,2})?$")

input_str = input("give me a number") 

while NUM_REGEX.match(input_str) is None:
    input_str = input("Nope, not valid. Give me another.")

return float(input_str)

edited May 04 '15 at 16:38

answered May 04 '15 at 16:36

jwilner

6,348
6
35
47

Python : Limiting input as numbers and numbers with decimal places

4 Answers4