Tim Peters added the comment:
Users certainly have been fooled by this, although "unpacking" is a red
herring. I've been burned by it, and I've seen StackOverflow puzzles related
to the same thing. I think this is the heart of it: given a finite iterable
I, it usually makes no semantic difference whether a program does:
for thing in I:
or
for thing in list(I):
But when `I` is obtained from `groupby()`, `thing` contains an iterator that
shares state with `I` itself, so they're not at all the same. It's surprising
just because it's so uncommon. "unpacking" is one special case of this.
I'd like to see an attempt to invoke a sub-iterator raise an exception if the
primary iterator has moved beyond it. The behavior in that case now is
undefined, utterly accidental, and useful only for surprising people ;-)
Nobody _intends_ to do this, unless they have no idea how `groupby()` is meant
to be used.
----------
nosy: +tim.peters
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<http://bugs.python.org/issue30346>
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