[Tutor] ValueError

[Peter Otten]

Johnson Tran wrote:

> Thanks for the replies..so I added the "try" block but it still does not
> seem to be outputting my default error message:
> 
> def Conversion():
>     try:
>         
>         print "This program converts the first value from inches to
>         centimeters and second value centimeters to inches." print "(1
>         inch = 2.54 centimeters)" inches = input("Enter length in inches:
>         ") centimeters = 2.54 * float(inches)
>         print "That is", centimeters, "centimeters."
>         centimeters = input("Enter length in centimeters: ")
>         inch = float(centimeters) / 2.54
>         print "That is", inch, "inches."
> 
>     except ValueError:
>          print "Invalid digit, please try again."
> Conversion()
> 
> Error message:
> 
> Traceback (most recent call last):
>   File "/Users/JT/Desktop/hw#2.py", line 16, in <module>
>     Conversion()
>   File "/Users/JT/Desktop/hw#2.py", line 9, in Conversion
>     centimeters = input("Enter length in centimeters: ")
>   File "<string>", line 1, in <module>
> NameError: name 'fs' is not defined

input() in Python 2.x tries to evaluate your input as a Python expression, 
so if you enter "2*2" it gives you 4, and when you enter "fs" it tries to 
look up the value of a global variable "fs" in your python script. You don't 
have such a variable in your script, so it complains with a NameError.

The best way to avoid such puzzling behaviour is to use raw_input() instead 
of input().

Also you should make the try...except as narrow as possible

try:
    centimeters = float(centimeters)
except ValueError as e:
    print e

is likely to catch the float conversion while with many statements in the 
try-suite you are more likely to hide a problem that is unrelated to that 
conversion.

PS: In Python 3.x raw_input() is gone, but input() behaves like raw_input() 
in 2.x