[Python-ideas] How assignment should work with generators?

C Anthony Risinger c at anthonyrisinger.com
Mon Nov 27 12:48:35 EST 2017

This proposal resonates with me. I've definitely wanted to use unpacking to
crank an iterator a couple times and move on without exhausting the
iterator. It's a very natural and intuitive meaning for unpacking as it
relates to iterators.

In my mind, this ask is aligned with, and has similar motivation to, lazy
zip(), map(), and keys() in Python 3. Syntax support for unpacking as it
stands today is not very conducive to iterables and conflicts with a
widespread desire to use more of them.

Couple thoughts:

* Perhaps existence of `__len__` should influence unpacking? There is a
semantic difference (and typically a visual one too) between 1-to-1
matching a fixed-width sequence/container on the RHS to identifiers on the
LHS, even if they look similar (ie. "if RHS has a length make it fit,
otherwise don't").

* (related to above) Perhaps the "rigidity"(?) of both RHS and LHS should
influence unpacking? If they are both fixed-width, expect exact match. If
either is variable-width, then lazily unravel until a different problem
happens (eg. LHS is fixed-width but RHS ran out of values... basically we
always unravel lazily, but keep track of when LHS or RHS become variable,
and avoid checking length if they do).

* Python 4 change as the language moves towards lazy iterators everywhere?
If `__len__` influenced behavior like mentioned above, then a mechanical
fix to code would simply be `LHS = tuple(*RHS)`, similar to keys().

While I like the original proposal, adding basic slice support to iterables
is also a nice idea. Both are independently useful, eg.
`gen.__getitem__(slice())` delegates to islice(). This achieves the goal of
allowing meaningful unpacking of an iterator window, using normal syntax,
without breaking existing code.

The fixed/variable-width idea makes the most sense to me though. This
enables things like:

>>> a, b, c = (1, *range(2, 100), 3)
(1, 2, 3)

Since both sides are not explicitly sized unpacking is not forcibly sized

Expecting LHS/RHS to exactly match 100% of the time is the special case
here today, not the proposed general unpacking rules that will work well
with iterators. This change also has a Lua-like multiple return value feel
to it that appeals to me.


On Nov 27, 2017 10:23 AM, "Paul Moore" <p.f.moore at gmail.com> wrote:

> On 27 November 2017 at 16:05, Chris Angelico <rosuav at gmail.com> wrote:
> > On Tue, Nov 28, 2017 at 2:35 AM, Steven D'Aprano <steve at pearwood.info>
> wrote:
> >> In this case, there is a small but real benefit to counting the
> >> assignment targets and being explicit about the number of items to
> >> slice. Consider an extension to this "non-consuming" unpacking that
> >> allowed syntax like this to pass silently:
> >>
> >>     a, b = x, y, z
> >>
> >> That ought to be a clear error, right? I would hope you don't think that
> >> Python should let that through. Okay, now we put x, y, z into a list,
> >> then unpack the list:
> >>
> >>     L = [x, y, z]
> >>     a, b = L
> >>
> >> That ought to still be an error, unless we explicity silence it. One way
> >> to do so is with an explicit slice:
> >>
> >>     a, b = L[:2]
> >
> > I absolutely agree with this for the default case. That's why I am
> > ONLY in favour of the explicit options. So, for instance:
> >
> > a, b = x, y, z # error
> > a, b, ... = x, y, z # valid (evaluates and ignores z)
> Agreed, only explicit options are even worth considering (because of
> backward compatibility if for no other reason). However, the unpacking
> syntax is already complex, and hard to search for. Making it more
> complex needs a strong justification. And good luck in doing a google
> search for "..." if you saw that code in a project you had to
> maintain. Seriously, has anyone done a proper investigation into how
> much benefit this proposal would provide? It should be reasonably easy
> to do a code search for something like "=.*islice", to find code
> that's a candidate for using the proposed syntax. I suspect there's
> very little code like that.
> I'm -1 on this proposal without a much better justification of the
> benefits it will bring.
> Paul
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