On Mon, Jul 2, 2018 at 2:32 AM Nicolas Rolin <nicolas.rolin@tiime.fr> wrote:
For example the default could be such that grouping unpack tuples (key, value) from the iterator and do what's expected with it (group value by key). It is quite reasonable, and you have one example with (key, value) in your example, and no example with the current default.

On Mon, Jul 2, 2018 at 3:22 AM Nicolas Rolin <nicolas.rolin@tiime.fr> wrote:
My question would be: does it have to be a key function? Can't we just remove the "key" argument?

In the examples or from the parameters? A key function is necessary to support a wide variety of uses.

Because for pretty much all the given examples, I would find my default as readable and nearly as short as the "key" syntax :

> grouping(words, key=len)
grouping((len(word), word for word in words))

I think the fact that you misplaced a closing parenthesis demonstrates how the key-function pattern can be more clear.

The code is slightly more verbose, but it is akin to filter(iterable, function) vs (i for i in iterable if function(i)).

Sometimes I prefer ``map`` and sometimes I prefer a list comprehension. It usually hinges on whether I think the reader might get confused over what one of the elements is. If so, I like to write out the comprehension to provide that extra variable name for clarity.

I'd write: 
    map(len, words)

But I'd also write
    [len(fullname) for fullname in contacts]

I appreciate that defaulting the grouping key-function to ``itemgetter(0)`` would enable a pleasant flexibility for people to make that same choice for each use. I haven't fully come around to that, yet, because so many other tools use the equality function as the default.

On Mon, Jul 2, 2018 at 3:48 AM Steven D'Aprano <steve@pearwood.info> wrote:
On Mon, Jul 02, 2018 at 02:52:03AM -0700, Michael Selik wrote:
> Third, some classes might have a rich equality method that allows many
> interesting values to all wind up in the same group even if using the
> default "identity" key-function.

I would expect an identity key function to group by *identity* (is), not 
equality. But I would expect the default grouper to group by *equality*.

Yep, I should have been saying "equality function" instead of "identity function." Thanks for the clarification.