[Python-Dev] Re: Generalizing "in" to pairs of sequences

[Guido van Rossum]

> On Sun, 26 Aug 2001, Eric S. Raymond wrote:
> > You're right.  But since a string can't be an element of a string, the
> > case I'm really interested in would still work.
> 
> It's arguable.  A character can indeed be an element of a string, and
> in Python characters are one-character strings.  So this violates
> 
>     x in s   <=>  there exists i such that s[i] == x
> 
> You could argue that strings should have special behaviour for "in"
> to support the common case of finding a substring; then you would have
> to argue on grounds of "practicality beats purity".
> 
> I might support the argument on grounds of practicality beating purity,
> but i'd have to think carefully about violating the above definition.
> 
> I can see that "substring in string" would be very convenient, but then
> 
>     if char in '0123456789': ...
> 
> would suddenly have a very different meaning -- it would succeed for
> char = '12'.

We *could* allow this for strings, but not for other sequences.
For strings, type(s[0]) == type(s), and this avoids the ambiguity.
The fact that currently ``noncharacter in string'' raises an exception
makes it relatively painess to add semantics for it.  But I'll have to
think about why I ruled it out originall.  I'll track down Eric here
at LinuxWorld and discuss it with him, if we find time.

--Guido van Rossum (home page: http://www.python.org/~guido/)