[Including replies to On 05/25/2016 05:29 PM, Michael Selik wrote:
On Wed, May 25, 2016 at 7:41 PM Ethan Furman wrote:
a, b = mapping a, b, **_ = mapping # when you don't care about the rest
I'd grudgingly accept this as a second-best syntax. It doesn't support keys that aren't strs of valid identifiers, but I suppose ``def foo(**kwargs)`` doesn't either. I frequently have keys that are strs with punctuation, keys that are tuples of ints, etc. Being restricted to identifiers feels cramped.
Doesn't have to be the only syntax, just the easiest. [from other email] ------------------
Breaks backwards compatibility.
py> a, b = {'a': 1, 'b': 2} py> a, b ('b', 'a')
Ah. That is a very good point. I'll meet you halfway: {a, b} = some_dict which is currently a SyntaxError, so little backwards compatibility concerns, plus it clearly state the mapping should have two elements, similarly to [a] = some_iterable and (b) = some_iterable both clearly state that a one-element iterable is being unpacked.
I think that most Pythonistas would say: a, b = 1, 2 # look ma! no round brackets!
You need brackets for nested/recursive destructuring.
py> a, b, (c, d) = 1, 2, (3, 4)
Sure, but that isn't the simple case.
py> {'a': x, 'b': {'c': y, 'd': z} = {'a': 1, 'b': {'c': 2, 'd': 3}}
With the above syntax: {a, b {c, d}} = a_mapping
Yep. I just want to look ahead a bit. While you're considering dict unpacking syntax options, keep in mind that tuple matching will need to parallel dict matching. Also, our future selves will want one-way-to-do-it when considering matching syntaxes.
py> a, b, c, 0 = 1, 2, 3, 4 ValueError: index 3 does not match value 0 py> {'x': a, 'y': 0} = {'x': 1, 'y': 2} ValueError: key 'y' does not match value 0
The adage is actually one /obvious/ way to do it -- which can change depending on the circumstances. On 05/25/2016 07:16 PM, David Mertz wrote:
I can see how this spelling might be intuitive at first brush. But the more I think about it, the more I recoil against the violation of a relatively uniform semantic principle in Python.
In no other case in Python, does the RHS of an assignment "probe into" the LHS to figure out how to determine its value. Moreover, the idea that variable names are not just bindings, but also pseudo- literals, or maybe something akin to a Lisp 'symbol', feels enormously unpythonic to me.
On 05/25/2016 07:18 PM, Guido van Rossum wrote:
I have to warn here. This looks cool but it does something that AFAIK no other Python syntax uses -- it takes variable names and does something to those variables but *also* uses their actual names as string literals. I agree that the use cases for this seem pretty sweet, but perhaps we should give it a somewhat different syntax just so it's clear that the names on the LHS matter. The precedent that the targets must be actual names rather than anything you can assign to is also kind of scary.
Very good points. I'm glad we had the discussion, though, since it elicited Random's contribution -- which is what I will use from now on for dict unpacking. :) -- ~Ethan~