[Python-Dev] Dict constructor

[Raymond Hettinger]

From: "Tim Peters" <tim@zope.com>
> -1 because of ambiguity.  Is this trying to build a set with the single
> element (42, 666), or a mapping of 42 to 666?
>
>     dict([(42, 666)]}

I've been thinking about this and the unabiguous explicit solution is to 
specify a value argument like dict.get().

>>> dict([(42, 666)])           # current behavior unchanged
{42: 666}

>>> dict([(42, 666)], True)
{(42, 666): True}

>>> dict( '0123456789abcdef', True)
{'a': True, 'c': True, 'b': True, 'e': True, 'd': True, 'f': True, '1':
True, '0': True, '3': True, '2': True, '5': True, '4': True, 7': True, '6':
True, '9': True, '8': True}

>>> dict('0123456789abcdef')    # current behavior unchanged
ValueError: dictionary update sequence element #0 has length 1; 2 is
required



The goal is not to provide full set behavior but to facilitate the common
task of building dictionaries with a constant value.  It comes up in
membership testing and in uniquifying sequences.  The task of dict() is to
construct dictionaries and this is a reasonably common construction.


Raymond Hettinger