`groupby` Update and Use Case

groupby seems to still produce an itertools._grouper object, which appears to be a type in process to deprecate by 2.0.

I also wonder how the keyfunc parameter works and if it’s scoped for pipe end users. I tried to pass something to it, and I got a multiple values error. Does it, by chance, allow for type recasting of the object returned? If not, a feature like that would be wonderful so as to quickly generate outputs of pipe operations (maybe pipes have something like that already?)

Finally, in the documentation x%2 and "Even" will produce unexpected results 😃

[JulienPalard]

Wait, so the lambda is the key function?

Yes!

It’s used in itertools.groupby and in sorted, whose keyfunc I thought had to be something that told sorted how some elements should be sorted relative to one another (like a before b, b before c, whatever).

Yes, the same key function are used to both sort then group. Read like like “sort by group before groupping”.

But it seems the pipe makes the first argument (the lambda) the second and passes the iterable prior to the pipe to the first?

Exactly \o/

If that’s true, I think I get it now. Just surprised that the keyfunc works for sorted as well as for the itertools.groupby.

Python use keyfuncs in other places too like max and min. As I said a few line before, sorting by the same keyfunc is a “sort by groups”, so for example feeding:

A A B B A A

won’t give three groups, one of two A’s, one of two B’s and one of two A’s, but the sort will first reorganise as:

A A A A B B

then the itertools.groupby will give only two groups, one of 4 A’s and one of two B’s.

It’s probably a bold move to stuff a sorted inside a groupby, it distances the Pipe semantic from the expected itertools semantic, which is probably not that good, but I wrote this like 10+ years ago, and I don’t think changing it now is any better.

Anyway, if you really need a groupby which does not uses sort, feel free to implement it yourself, it should be as simple as:

@Pipe
def groupby(iterable, keyfunc):
    return itertools.groupby(iterable, keyfunc)

(nothing enforces using only pipes declared on pipe.py).

[Servinjesus1]

Wonderful, thank you for walking me through this. I know this was only a very random example of what Pipe can do, but it’s the first time I’ve seen something like this, so I appreciate your patience.