[Tutor] Convert String to Int

[Marc Tompkins]

On Sun, Jan 18, 2009 at 5:15 PM, Ian Egland <echolynx at gmail.com> wrote:

> I know that, should you want to get an int from the user, you use
> int(input("Question!")). However, what if the user wasn't that savvy and
> didn't realize he/she HAD to enter a number? Program crash. Is there a way
> that I can get the input as a string, check and see if it's convertible to
> an integer, convert it if it is and ask them to try again if it's not?
>
>
You have two options: to test the input ahead of time, or to catch the
crash.  Believe it or not, number two is usually the preferred method
(there's a programming axiom "It's better to ask forgiveness than
permission")  In Python, you put the operations that have a potential for
disaster inside a try/except block, like so:

try:
>     int(input("Question!"))
>     do whatever...
> except:
>     ask again...
> do the next thing
>

When Python "crashes", an Exception is generated, which carries all sorts of
useful information about just what went wrong.  It's good form to try to
guess what the error might be before catching it (how many times have you
been frustrated by some commercial program that says "Out of memory" {or
some other catch-all} any time ANYTHING bad happens?  It's because the
programmer assumed that the only errors that would ever come up would be
memory-related.  Don't be that guy.)  For example, do a test run like so:

> >>> int('hello')
> Traceback (most recent call last):
>   File "<input>", line 1, in <module>
> ValueError: invalid literal for int() with base 10: 'hello'
>

Now you know that invalid input would result in a ValueError, so change my
above code to:

> try:
>     int(input("Question!"))
>     do whatever...
> except ValueError:
>     ask again...
> do the next thing
>

Someone else will no doubt already have posted how to test your input ahead
of time - I thought I'd chime in with the other method...


-- 
www.fsrtechnologies.com
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