ValueError Still Present with Except Statement

Question

I am trying to create a program for that explores the collatz sequence. It works until I try to add try and except statements to ensure the user enters a number and not text. Below is the code:

def collatz(number):
    try:
        if number % 2 == 0:
            print(number // 2)
            return number // 2
        elif number % 2 == 1:
            result = 3 * number + 1
            print(result)
            return result
    except ValueError:
        print('That is not a number')

print('Enter a number')
n = int(input())

while n != 1:
    n = collatz(int(n))

I can run it with no problems if I enter numbers. However, when I enter the word "puppy" I encounter this:

Traceback (most recent call last):
  File "C:/Users/kredeker/Desktop/python/collatz2.py", line 14, in <module>
    n = int(input())

ValueError: invalid literal for int() with base 10: 'puppy'

I thought I was accounting for the ValueError with this:

except ValueError:
    print('That is not a number')

The exception handling is done in the function. `n = int(input())` is outside the function. You can drop the int call `n = input()` — Moses Koledoye, Jun 21 '17 at 16:39

cosinepenguin · Accepted Answer · 2017-06-21T16:46:59.257

2

You are getting the error at n = int(input())!

So try doing

try:
  n = int(input())
except ValueError:
  print('That is not a number')

The full code (so that you don't get an error with 'n'):

def collatz(number):
    if number % 2 == 0:
        print(number // 2)
        return number // 2
    elif number % 2 == 1:
        result = 3 * number + 1
        print(result)
        return result

print('Enter a number')

try:
  n = int(input())
  while n != 1:
    n = collatz(n)
except ValueError:
  print('That is not a number')

edited Jun 21 '17 at 16:46

answered Jun 21 '17 at 16:39

cosinepenguin

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1

None of the code in the `collatz()` function will throw a `ValueError` exception, there is no need to keep the `try....except` in that function. – Martijn Pieters Jun 21 '17 at 16:42
1

You have a redundant call to `int()` still: `n = collatz(n)` will do. – Martijn Pieters Jun 21 '17 at 16:45

score 2 · Answer 2 · answered Jun 21 '17 at 16:40

The try...except applies to only code that is run between these words, including function calls. As the traceback shows, your exception occurs on the line 14:

n = int(input())  # <---

which is caused by the string that you entered (puppy) not being interpretable as an integer in base 10.

You need to wrap this within a try ... except; but a better option would be to adopt - and adapt - the functions from this excellent answer.

ValueError Still Present with Except Statement

2 Answers2