Python: except ValueError only for strings.

Question

Is there a way in Python 3.3 to only except ValueError for strings? If I type a string into k, I want "Could not convert string to float" to be printed, rather than "Cannot take the square root of a negative number."

while True:
    try:
        k = float(input("Number? "))

....

    except ValueError:
        print ("Cannot take the square root of a negative number")
        break
    except ValueError:
        print ("Could not convert string to float")
        break

You're not taking a square root anywhere in the code, so why is that first except ValueError there at all? — Brendan Long
– Brendan Long, Commented Nov 8, 2012 at 21:06
I am beginner to Python so I may not have done this correctly. I need two separate messages to be printed, one for negative numbers, and one for strings. I am not sure how to do this with one exception. — Ace
– Ace, Commented Nov 8, 2012 at 21:08
You could also just print the exception. For a string, you get ValueError: count not convert string to float: 'fdsdfsd'. For a negative number, you get ValueError: math domain error. — abarnert
– abarnert, Commented Nov 8, 2012 at 21:48

Michael Mauderer · Accepted Answer · 2012-11-08 21:16:55Z

6

If you want to handle exceptions different depending on their origin, it is best to separate the different code parts that can throw the exceptions. Then you can just put a try/except block around the respective statement that throws the exception, e.g.:

while True:
    try:
        k = float(input("Number? "))
    except ValueError:
        print ("Could not convert string to float")
        break
    try:
        s = math.sqrt(k)
    except ValueError:
        print ("Cannot take the square root of a negative number")
        break

edited Nov 8, 2012 at 21:16

answered Nov 8, 2012 at 21:07

Michael Mauderer

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Comments

Brendan Long · Accepted Answer · 2012-11-08 21:07:43Z

1

Easy, just remove your other except ValueError:

while True:
    try:
        k = float(input("Number? "))

....

    except ValueError:
        print ("Could not convert string to float")
        break

If you want to check if number is negative, just.. check if it's negative:

if k < 0:
   print("Number is negative!")

answered Nov 8, 2012 at 21:07

Brendan Long

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