loops

Steven D'Aprano steve+comp.lang.python at pearwood.info
Sun Dec 2 22:51:21 CET 2012


On Sun, 02 Dec 2012 16:39:07 -0500, Verde Denim wrote:

> I'm just getting into py coding, and have come across an oddity in a py
> book - while loops that don't work as expected...

This error has nothing to do with the while loop. Read the error message 
that Python gives you:

> Traceback (most recent call last):
>   File "dice_roll.py", line 17, in <module>
>     main()
>   File "dice_roll.py", line 15, in main
>     again = input('Roll again? (y = yes): ')
>   File "<string>", line 1, in <module>
> NameError: name 'y' is not defined


In Python 2, there is a serious design flaw with the "input" function, 
fortunately corrected in Python 3. The flaw is that input automatically 
evaluates whatever you type as Python code. So when you type "y", the 
input function tries to evaluate the name y, which doesn't exist.

No while loop required:

py> again = input('Roll again? (y = yes): ')
Roll again? (y = yes): y
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1, in <module>
NameError: name 'y' is not defined


The solution is either to use Python 3, where this is no longer an issue, 
or to replace "input" with "raw_input":

py> again = raw_input('Roll again? (y = yes): ')
Roll again? (y = yes): y
py> again
'y'




> This same loop structure appears in many places in this book "Starting
> out with Python, 2nd ed, Tony Gaddis), and they all yield the same
> error. Is there something I'm missing here?
> 
> Thanks for the input...

I see what you did there... *wink*



-- 
Steven



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