Dictionary from list?

[Huaiyu Zhu]

On Tue, 23 Oct 2001 01:21:04 -0400, Tim Peters <tim.one at home.com> wrote:
>There's already a dictionary() constructor in 2.2, and that's not going to
>change (it would be too odd if every scalar and container builtin type had a
>builtin constructor except for dicts).  One question is what it should do;
>all it can do in 2.2b1 is basically silly, amounting to yet another way to
>spell "shallow-copy the argument".

There have been several good arguments about why [(k,v), (k,v) ...] is
better than [k, v, k, v, ...].  Here are some more observations:

- I found Perl's hash initialization quite non-intuitive when I was using
  Perl two years ago.  Most text files put k,v in the same line, and most
  regular expression extract them at once.  It takes a little bit extra work
  to put k,v pairs into a flat array.
	
- In Python the sequence-of-pairs structure is easily created by d.items(),
  re.match.groups and possibly others.  It is quite handy for sorting,
  filtering and other processings.  It is often necessary to put them into a
  dictionary afterwards, and a constructor would be helpful.

- Perhaps a pair of functions flatten(x) and collect(x, n) would be useful
  when dealing with flat sequences, even when it is not necessarily
  associated with dictionaries.

- As to the question of what the constructor should do in borderline cases, is
  there a specific reason that it should not behave exactly like this?
	
	def dict(x):
		d = {}
		for k, v in x: d[k] = v
		return d
	
  There is no need to differentiate between tuples, lists, iterators, etc.
  Any sequence whose every element is a sequence of length two should work.
  Everything else should raise an exception.

- The form of ([k, k, ...], [v, v, ...]) is less frequently used (in my case
  anyway) and can be easily converted using zip().  
  
  I keep a file utils.py around that include a many little functions like
  the above, and I find myself doing "from utils import dict" quite often.

  Another function I use is items(x) returning x.items() if x is dictionary,
  or [(i,x[i]), ...] if x is sequence.  With dict(), items() and zip() I
  could write many programs useful for both lists and dicts.
  
Huaiyu