On Fri, Mar 8, 2019 at 1:52 AM Jonathan Fine <jfine2358@gmail.com> wrote:

I've just learnt something new. Look at

>>> from operator import iadd
>>> lst = [1, 2, 3]
>>> iadd(lst, 'hi')
[1, 2, 3, 'h', 'i']
>>> lst
[1, 2, 3, 'h', 'i']

This shows that the proposals dict.flow_update and dict.__iadd__ are
basically the same. (I think this is quite important for understanding
the attraction of fluent programming. We ALREADY like and use it, in
the form of augmented assignment of mutables.)

well, no -- the fact that __iadd__ acts like it does is essentially an accident of implementation, and calling __iadd__ directly is frowned upon (and I'm not totally sure if it is guaranteed to keep working that way by the language spec). And, in fact, it DOESN'T act like flow_merge method -- as it both mutates the original object, and returns itself -- which I think is a no-no in fluent programming, yes? (certainly in functional programming)

In [10]: list1 = [1,2,3]

In [11]: list2 = [4,5,6]

In [12]: list3 = list1.__iadd__(list2)

In [13]: list3
Out[13]: [1, 2, 3, 4, 5, 6]

In [14]: list1
Out[14]: [1, 2, 3, 4, 5, 6]

In [15]: list1 is list3
Out[15]: True

This also shows that
combined = defaults.copy()
combined.update(options)
could, if the proposal is accepted, be written as
defaults.copy().__iadd__(options)

did you mean:

combined = defaults.copy().__iadd__(options)

because the way you wrote it, you are making a copy, mutating it, and then throwing it away...

in which case, yes it could, but it would not be recommended, and I can't see the advantage of it over:

combined = defaults + options

or even, if you REALLY want to use __ methods:

combined = defaults.__add__(options)

In [17]: list3 = list1.__add__(list2)

In [18]: list1
Out[18]: [1, 2, 3]

In [19]: list3
Out[19]: [1, 2, 3, 4, 5, 6]

I got the idea from the withdrawn PEP (thank you, Nick Coghlan, for writing it):
PEP 577 -- Augmented Assignment Expressions
https://www.python.org/dev/peps/pep-0577/

Interestingly (to me) it was withdrawn for different reasons than what I would think -- mutating and assigning at once is dangerous.

-CHB

Christopher Barker, PhD

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