Try catch for validating an input in Python

Question

I'm trying to validate an input in Python that needs to be both ten digits long and a non-negative number. I thought I'd use a couple try-catch statements to do so, but even when the output fits an exception, the exception ain't being triggered

i = input("Please input a value: ")
try:
    (len(str(i)) == 10) == True
except:
    print("False")
try:
    (float(i) > 0) == True
except:
    print("False")

I'm expecting it to simply return "False" when I insert something like "123" or "-1029347365", but there's no output

Why would that data cause an exception to be thrown? Did you mean to use `if` instead? Also, `== True` is unnecessary. — Carcigenicate, Oct 04 '19 at 12:16
This is not how you raise an exception - just returning ```False``` won't do. You need to ```raise``` it: https://stackoverflow.com/questions/2052390/manually-raising-throwing-an-exception-in-python However from what you share ```if``` statement seems to be way more suitable for your purpose — Grzegorz Skibinski, Oct 04 '19 at 12:19

score 1 · Answer 1 · answered Oct 04 '19 at 12:33

1

I agree with the previous answer, this will do the trick:

assert (((len(str(i)) == 10) == True) and ((float(i) > 0) == True)), "False"

answered Oct 04 '19 at 12:33

im2527

391
3
4

score 1 · Answer 2 · answered Oct 04 '19 at 12:36

If you're set on using a try catch, which can come in handy. You could always create your own exception classes.

What your looking to do can be accomplished with a simple if statement such as

if not (( len(str(i)) == 10) and (float(i) > 0)):
    #do something

but I'm assuming you might have a reason for using Try catch. Here's a link that might be helpful for you. https://www.programiz.com/python-programming/user-defined-exception

score 1 · Accepted Answer · answered Oct 04 '19 at 12:44

1

i = input()
if len(str(i)) == 10:
  print("true")
else:
  print("False")

if float(i) > 0:
    print("true")
else:
    print("False")

simple enough... hope it helps

if you want to raise the exceptions you can opt the previous answers

answered Oct 04 '19 at 12:44

Alex ÁñTø ÄçhäDäñ

72
5

score 0 · Answer 4 · answered Oct 04 '19 at 12:22

0

try ... except only works, when your code produce an exception. In your case, you only compare two statements and the result might be True or False. To make it work, you could put it in:

if (len(str(i)) == 10) == False :
    raise

(to manually raise an exception if your statement is False), or better use assert:

# assert <condition>,<error message>
assert (float(i) > 0) == True, "False"

answered Oct 04 '19 at 12:22

natter1

354
2
10

Also, as carcigenicate mentioned, `== True ` is unneccessary. Also, instead of `== False` it might be better to use ìf not ...`. Both would increase readability. – natter1 Oct 04 '19 at 12:43

Try catch for validating an input in Python

4 Answers4