[Python-Dev] Re: Sets: elt in dict, lst.include

[Tim Peters]

[Ping]
>     dict[key] = 1
>     if key in dict: ...
>     for key in dict: ...

[Guido]
> No chance of a time-machine escape, but I *can* say that I agree that
> Ping's proposal makes a lot of sense.  This is a reversal of my
> previous opinion on this matter.  (Take note -- those don't happen
> very often! :-)
>
> First to submit a working patch gets a free copy of 2.1a2 and
> subsequent releases,

Thomas since submitted a patch to do the "if key in dict" part (which I
reviewed and accepted, pending resolution of doc issues).

It does not do the "for key in dict" part.  It's not entirely clear whether
you intended to approve that part too (I've simplified away many layers of
quoting in the above <wink>).  In any case, nobody is working on that part.

WRT that part, Ping produced some stats in:

http://mail.python.org/pipermail/python-dev/2001-January/012106.html

> How often do you write 'dict.has_key(x)'?          (std lib says: 206)
> How often do you write 'for x in dict.keys()'?     (std lib says: 49)
>
> How often do you write 'x in dict.values()'?       (std lib says: 0)
> How often do you write 'for x in dict.values()'?   (std lib says: 3)

However, he did not report on occurrences of

    for k, v in dict.items()

I'm not clear exactly which files he examined in the above, or how the
counts were obtained.  So I don't know how this compares:  I counted 188
instances of the string ".items(" in 122 .py files, under the dist/ portion
of current CVS.  A number of those were assignment and return stmts, others
were dict.items() in an arglist, and at least one was in a comment.  After
weeding those out, I was left with 153 legit "for" loops iterating over
x.items().  In all:

    153 iterating over x.items()
    118     "     over x.keys()
     17     "     over x.values()

So I conclude that iterating over .values() is significantly more common
than iterating over .keys().

On c.l.py about an hour ago, Thomas complained that two (out of two) of his
coworkers guessed wrong about what

    for x in dict:

would do, but didn't say what they *did* think it would do.  Since Thomas
doesn't work with idiots, I'm guessing they *didn't* guess it would iterate
over either values or the lines of a freshly-opened file named "dict"
<wink>.

So if you did intend to approve "for x in dict" iterating over dict.keys(),
maybe you want to call me out on that "approval post" I forged under your
name.

falls-on-swords-so-often-there's-nothing-left-to-puncture<wink>-ly y'rs
    - tim