"in"consistency?

Terry Reedy tjreedy at udel.edu
Tue Jul 8 20:52:52 CEST 2008



David C. Ullrich wrote:
> In article <mailman.137.1215480143.20628.python-list at python.org>,
>  Terry Reedy <tjreedy at udel.edu> wrote:

>>> Is there a reason for the inconsistency? I would
>>> have thought "in" would check for elements of a
>>> sequence, regardless of what sort of sequence it was...
>> It is not an inconsistency but an extension corresponding to the 
>> limitation of what an string element can be.
> 
> It's an inconsistency. That doesn't mean it's a bad thing or that
> I want my money back. It may well be a reasonable inconsistency -
> strings _can_ work that way while it's clear lists had better not.
> But it's an inconsistency.

To decisively argue 'inconsistency' as factual or not, versus us having 
divergent opinions, you would have to supply a technical definition ;-) 
  The math definition of 'leading to a contradiction' in the sense of 
being able to prove False is True, does not seem to apply here.

However,
a) In common English, 'in' and 'contains', applied to strings of 
characters (text), is understood as applying to substrings that appear 
in the text.  This is also true of many other programming languages. 
'Dictionary' contains 'diction'.  This is even the basis of various word 
games.
b) Python otherwise allows operators to vary in meaning for different 
classes.

In any case, back to your original question: the extension of meaning, 
'inconsistent' or not, was deliberated and adopted on the basis that the 
usefulness of the extension would outweigh the confusion wrought by the 
class-specific nature of the extension.  (In other words, threads such 
as this *were* anticipated ;-)

Terry Jan Reedy




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