[Python-Dev] Dict constructor

[Raymond Hettinger]

From: "Jeremy Hylton" <jeremy@zope.com>
>   RH> Second wild idea of the day: The dict constructor currently
>   RH> accepts sequences where each element has length 2, interpreted
>   RH> as a key-value pair.
> 
>   RH> Let's have it also accept sequences with elements of length 1,
>   RH> interpreted as a key:None pair.
> 
> That seems a little too magical to me.

Fair enough.

> 
>   RH> Raymond Hettinger 'regnitteh dnomyar'[::-1]
> Then again it seems like you like magic!

While I'm a fan of performance magic, a la the Magic Castle,
the root of this suggestion is more mundane.  There are too
many pieces of code that test membership with 'if elem in container'
where the container is not a dictionary.  This results in O(n) 
performance rather than O(1).  To fix it, I found myself 
writing the same code over and over again:

  def _toset(container):
       return dict([(elem, True) for elem in container])

This repeated dictionary construction exercise occurs in so many
guises that it would be worthwhile to provide a fast, less magical
looking approach.  Being able to construct dictionaries with
default values isn't exactly the most exotic idea ever proposed.

IMO, it's clearer, faster, commonly needed, and easy to implement.

'nuff said,


Raymond Hettinger