Fix grad assignment and `grad` property read

[Hofer-Julian]

Double assignment seems to do something different than expected in Python Changing it to two statements fixes it. This also reveals that the grad property is accessed at the wrong value.

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[PiotrCzapla]

The issue is with map being a generator/iterator that you can use only once

>>> m = map(lambda x:x*10, [1,2,3])
>>> a,b,c = m
>>> test = m
>>> a, b, c
(10, 20, 30)
>>> test
<map object at 0x1046689a0>
>>> list(test)
[]

you can solve this by adding tuple in front of map

>>> test = a,b,c = tuple(map(str, [1,2,3]))
>>> test
('1', '2', '3')
>>> a, b, c
('1', '2', '3') 

But it is good catch, the double assignment is indeed quite problematic, as it doesn't work for some getitem implementations.

you can simplify the code a bit by saving to grads first, like so

ptgrads = w12,w22,b12,b22,xt2 = map(mkgrad, chks)

becomes:

ptgrads = tuple(map(mkgrad, chks))
w12,w22,b12,b22,xt2 = ptgrads

[jph00]

Many thanks! I'll update it to the tuple version.