Is it really good?

[Bengt Richter]

On Tue, 07 Jan 2003 11:12:47 +0200, Nadav Horesh <NadavH at envision.co.il> wrote:

>Can someone explain why python does not raise NameError in the first 
>test line blow.
>I get similar results with Python 2.3a1, wich (obviously) answer False 
>instead of 0.
>
>===============================================================
>Python 2.2.2 (#5, Dec 19 2002, 10:39:16)
>[GCC 3.2.1] on linux2
>Type "copyright", "credits" or "license" for more information.
>IDLE 0.8 -- press F1 for help
>
> >>> 2 == 3 is good
>0     # ????????????????????????
>
> >>> (2 == 3) is good
>Traceback (most recent call last):
>  File "<pyshell#1>", line 1, in ?
>    (2 == 3) is good
>NameError: name 'good' is not defined
> >>> 2 == (3 is good)
>Traceback (most recent call last):
>  File "<pyshell#2>", line 1, in ?
>    2 == (3 is good)
>NameError: name 'good' is not defined
> >>>
>
>===============================================================
[16:38] C:\pywk\clp>ppcomp "2==3 is good"
Module(
  None,
  Stmt(
    [
      Discard(
        Compare(
          Const(
          | 2),
          [
            (
            | '==',
            | Const(
            |   3)),
            (
              'is',
              Name(
                'good'))]))]))

I see what is happening, I think. It is treating 'is' as just
one more compare operator in a chain, and 2==3 short-cuts to false,
so the next thing doesn't get evaluated. E.g.,

 >>> 2==3 is good
 0
 >>> 2<=3 is good
 Traceback (most recent call last):
   File "<stdin>", line 1, in ?
 NameError: name 'good' is not defined
 >>> 2<=3 is 4 is good
 0
 >>> 2<=3 is 3 is good
 Traceback (most recent call last):
   File "<stdin>", line 1, in ?
 NameError: name 'good' is not defined

Regards,
Bengt Richter