why not "'in' in 'in'"?

Grant Griffin not.this at seebelow.org
Thu Jun 13 07:15:14 CEST 2002


Erik Max Francis wrote:
> 
> Grant Griffin wrote:
> 
> > So why is the "in" operator limited to single characters when used as
> > a
> > membership operator?  This peculiar lack of generality seems a bit
> > (dare I say
> > it?) "non-Pythonic".

> 
> On the contrary, it is exquisitely Pythonic.

(I sure hope my "enlightenment" light will go on one day.  Until then, I
guess I'll just have to bask in the warm glow of others <wink>.)

> You used the magic word
> yourself:  _membership_.

In that case, perhaps I should have thought of a better one <wink>.  In
any event, in the case of strings, I think one might stretch the concept
of "membership" to include substrings.  In fact, note that since Python
has no actual character type (because a character is adequately
represented as a string of length one) the "in" operator actually _does_
test membership of strings in strings.  But for some reason, it's
limited to testing strings of length one.  Go figure!

> A string is a sequence of characters, so when
> testing membership with the `in' operator, the left-hand side of the
> expression should be a character (really a string of length one).  `in'
> tests membership, not for the existence of subsequences.  Would you have
> expected
> 
>         [1, 2] in [1, 2, 3, 4]
> 
> to evaluate to true?

Honest answer: although I can't see much use for that construct, I can't
see much harm in it either.  So although it's probably not worth adding
to Python, I'm sure _somebody_ would find a use for it.

But anyway, let's "just say no" to the tuple case rather than confuse
ourselves; let's just make the string version of "in" a special case. 
Then, we can rest easy knowing that:

   7.  Readability counts. 
   8.  Special cases aren't special enough to break the rules. 
   9.  Although practicality beats purity.

(Emphasis on the last one.)

see-what-i-mean-about-"concrete"-principles?-<wink>-ly y'rs,

=g2
-- 
_____________________________________________________________________

Grant R. Griffin                                       g2 at dspguru.com
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