Check user input is valid for decimal?

Question

I want to make sure the input will be number. I have tried testing with signs and letters but the shell just throws an error saying "Invalid literal for Decimal". I'm working on a calculator so thought the decimal module would be best suited. Thanks in advance.

This is my code:

import decimal

while True:
 userInput = (raw_input("Enter number:"))
 try:
  userInput = decimal.Decimal(userInput)
  break
 except ValueError:
  print ("Number please")

Using Python 2.7.6

Your code seems to do what you ask for - it will detect an error in in input and ask again. — Jan Vlcinsky, Jun 04 '14 at 14:10
It seems a different exception than `ValueError` is thrown. Try just `except:`, i.e. catch-all, or better: `except decimal.InvalidOperation:` — tobias_k, Jun 04 '14 at 14:13
@tobias_k: No! Please don't advocate Pokemon exception handling. You can catch multiple explicit exception types instead. Also see [Why is "except: pass" a bad programming practice?](http://stackoverflow.com/q/21553327) — Martijn Pieters, Jun 04 '14 at 14:13

score 9 · Accepted Answer · answered Jun 04 '14 at 14:15

9

Catch decimal.InvalidOperation

>>> a = 's'
>>> try:
...     decimal.Decimal(a)
... except decimal.InvalidOperation:
...     print 'fds'
... 
fds

answered Jun 04 '14 at 14:15

pynovice

7,424
25
69
109

score 3 · Answer 2 · answered Jun 04 '14 at 14:14

3

Instead of catching a ValueError, catch a decimal.InvalidOperation error. This error is thrown when invalid data is passed to the decimal.Decimal constructor.

answered Jun 04 '14 at 14:14

BeetDemGuise

954
7
11

score 1 · Answer 3 · answered Sep 26 '18 at 12:35

The correct way to check if a value is a valid input for Decimal is:

from decimal import Decimal, DecimalException

try:
    Decimal(input_value)
except DecimalException:
    pass

https://docs.python.org/2/library/decimal.html#decimal.DecimalException

score 0 · Answer 4 · answered Jun 04 '14 at 14:17

You are catching the wrong Exception. You are catching a ValueError, but the code throws decimal.InvalidOperation for various inputs that are not valid decimal values.

>python test.py
Enter number:10

>python test.py
Enter number:10.2

>python test.py
Enter number:asdf
Traceback (most recent call last):
  File "test.py", line 6, in <module>
    userInput = decimal.Decimal(userInput)
  File "C:\Python27\lib\decimal.py", line 548, in __new__
    "Invalid literal for Decimal: %r" % value)
  File "C:\Python27\lib\decimal.py", line 3872, in _raise_error
    raise error(explanation)
decimal.InvalidOperation: Invalid literal for Decimal: 'asdf'

>python test.py
Enter number:10.23.23
Traceback (most recent call last):
  File "test.py", line 6, in <module>
    userInput = decimal.Decimal(userInput)
  File "C:\Python27\lib\decimal.py", line 548, in __new__
    "Invalid literal for Decimal: %r" % value)
  File "C:\Python27\lib\decimal.py", line 3872, in _raise_error
    raise error(explanation)
decimal.InvalidOperation: Invalid literal for Decimal: '10.23.23'

Change your except line to except decimal.InvalidOperation:

Check user input is valid for decimal?

4 Answers4