On Sat, Dec 01, 2018 at 11:07:53AM -0500, Paul Svensson wrote: [...] > >Here's a map object I prepared earlier: > > > >from itertools import islice > >mo = map(lambda x: x, "aardvark") > >list(islice(mo, 3)) > > > >If I now pass you the map object, mo, what should len(mo) return? Five > >or eight? > > mo = "aardvark" > list(islice(mo, 3)) > > By what magic would the length change? > Per the proposal, it can only be eight. > Of course, that means mo can't, in this case, be an iterator. > That's what the proposal would change.
I already discussed that: map is not currently a sequence, and just giving it a __len__ is not going to make it one. Making it a sequence, or a view of a sequence, is a bigger change, but worth considering, as I already said in part of my post you deleted. However, it is also a backwards incompatible change. In case its not obvious from my example above, I'll be explicit: # current behaviour mo = map(lambda x: x, "aardvark") list(islice(mo, 3)) # discard the first three items assert ''.join(mo) == 'dvark' => passes # future behaviour, with your proposal mo = map(lambda x: x, "aardvark") list(islice(mo, 3)) # discard the first three items assert ''.join(mo) == 'dvark' => fails with AssertionError Given the certainty that this change will break code (I know it will break *my* code, as I often rely on map() being an iterator not a sequence) it might be better to introduce a new "mapview" type rather than change the behaviour of map() itself. On the other hand, since the fix is simple enough: mo = iter(mo) perhaps all we need is a depreciation period of at least one full release before changing the behaviour. Either way, this isn't a simple or obvious change, and will probably need a PEP to nut out all the fine details. -- Steve _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
