Dictionary from list?

[Tim Peters]

[Terry Reedy]
> Fact: a Python dictionary literal is a sequence of key:value pairs of
> literals, with the first becoming a hashable object, and only such a
> sequence.

Sounds more like a definition than a fact <wink>.

> Observation: ':' is analogous to '( , )', which could have been the
> syntax chosen (though I'm glad it wasn't).
>
> Therefore:
>
> Proposed rule 1: the dictionary() constructor should accept a sequence
> of pairs of objects, with the first being keyable (hashable).  In
> particular, it should invert dict.items.

In current CVS, it does.  In 2.2-speak, it accepts an iterable object
producing iterable objects producing exactly 2 objects.  It also accepts a
mapping object (as it did in 2.2b1), and also accepts nothing (ditto -- it
returns {} then, much as list() returns [] and tuple() returns ()).

> Note: by type, 'pair' might mean duple only; by interface, it would
> mean an object reporting a length of two and yielding objects with
> indexes 0 and 1.

"By interface" is important, but the details there are off for 2.2 --
iterable objects don't have to support len or indexing.  For example,

    class AddressBookEntry:
        # with .firstname, .lastname attributes
        ...
        def __iter__(self):
            return iter((self.firstname, self.lastname))

A sequence of AddressBookEntry instances is OK to pass to dictionary(),
despite that an AddressBookEntry defines neither __len__ nor __getitem__.  A
generator yielding instances of AddressBookEntry is also fine; etc.

> Comment: once pair is defined, this rule gives a uniform target for
> conversion from other formats, including those generated by other
> software systems.  I currently vote for the latter.

I didn't understand "the latter" here, unless it's a vote for "other
software systems", in which case I'm keen to see the patch <wink>.

> Proposed rule 2: dictionary() should reject any other sequence, just
> as does the internal constructor-from-literals.

"Sequence" is a slippery word.  dictionary() continues to accept a mapping
object too.  Of course, "mapping object" is also a slippery phrase.

> Paraphrase: conversions from the many other possible formats should be
> handled externally from dictionary().

Indeed, we're going to cheerfully endure abuse for sticking to that.

> Opinion 3: Given the fact and comments above, these two rules should
> be easy to understand and teach.

Yup.