On Fri, Apr 28, 2017 at 5:02 PM, Random832 <random...@fastmail.com> wrote:
> On Fri, Apr 28, 2017, at 01:30, Chris Angelico wrote:
>> Nothing whatsoever says that this is a good idea, but it's perfectly
>> legal, because the for loop is defined in terms of assignment. If this
>> were to be accepted (which, fwiw, I'm not actually advocating, but
>> IF), it would also be defined in terms of assignment. You still
>> shouldn't be assigning to arbitrary objects, especially not randomly
>> rebinding module names, but it's easy to grok the assignment
>> equivalence.
>
> What's not clear is when the left side (an object whose attribute/item
> is being assigned, and the item index) is evaluated, and why this should
> be different from when default arguments are evaluated.
Now that is a very good, dare I say it, argument. It's easy enough to
explain to an expert, but it's not clear to a beginner.
Expert explanation (omitting kwargs):
def __init__(self, self.spam=[]):
is roughly equivalent to:
_defaults = ([],)
def __init__(*args):
self, self.spam = (args + _defaults)[:2]
It's pretty straight-forward for default arguments to be evaluated at
declaration time, but assignment targets at execution time; but it
isn't clear.
I was never really in favour of the proposal, but this is a fairly big downside.
ChrisA
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