I don't pretend to fully understand the proposal and how it would be implemented, but IMO adding an overhead (not to mention more complicated semantics) to *every* chained attribute lookup...
It's a good thing that isn't the case then :p
Sorry, couldn't resist that quip. But more seriously, this would not be an issue. Normal attribute lookups would remain wholly unchanged. Because attribute lookups on namespace proxies *are* normal attribute lookups. That's the entire reason I've proposed it the way I have (well, that and the fact that it allows methods within namespaces to still bind to the parent class basically 'for free' without requiring any magical behaviour). Because aside from the magic behaviour of the namespace block (the way it binds names to its parent scope) it does not require *any* changes to the current attribute lookup semantics. In fact, it is built entirely on them.
The only *possible* difference would be that perhaps namespace proxies could be special-cased if any room for optimization could be found at the implementation stage (purely hypothetical) to attempt to make them more performant than the rough python pseudocode sketch of their behaviour I posted yesterday in response to Paul. So fear not, no current semantics would be harmed in the making of this film keyword.
So, is it a prerequisite that whatever object in which I'm trying to
establish a namespace must support getattr/setattr?
I'd imagine so, yes. At the very least, any namespaceable (new word!) object would have to have some sort of __dict__ or equivalent (the built-in `vars` would have to work on them), because otherwise I don't see how they could store attributes.
Also, it occurs to me that if I can declare a namespace in any object, I
might be tempted to (or might inadvertently) monkey patch external objects with it. Any thoughts on guarding against that, or is this "we're adults here" case?
Nothing stops you from monkey-patching objects right now. Declaring namespaces on arbitrary live objects you've obtained from who-knows-where would have all the same benefits to code conciseness and clarity as declaring them on `self` within a method, versus conventional monkey-patching. You could, for instance, group all the attributes you're monkey-patching in under a single namespace with a descriptive name (namespace custom? namespace extension? namespace monkey???) to keep track of what belongs to the original object and what doesn't, more easily.
Monkey-patching is often considered an anti-pattern, but sometimes there's a library that does 99% of what you need but without that last 1% it's worthless for your use-case. Rather than forking and rewriting the entire library yourself, sometimes monkey-patching is a better solution. Practicality over purity, right?
So yes, this definitely falls under the "consenting adults" category.
A problem I sense here is the fact that the interpreter would always need
to attempt to resolve "A.B.C" as getattr(getattr(A, "B"), "C") and getattr(A, "B.C"). Since the proxy would be there to serve up the namespace's attributes, why not just let it and do away with "B.C" in A.__dict__? What will the performance costs be of attempting to get an attribute in two calls instead of one?
Well, the performance would be, at worst, identical to a set of chained attribute lookups currently, which also have to be done in multiple steps, one at a time. We want to keep to existing python semantics as much as possible so that the change is minimally invasive (As Rob Cliffe pointed out, this is a proposal for syntactic sugar, so it probably doesn't warrant changes to the semantics of something.so fundamental to the language as attribute lookups). Buy hey, if there's some room for optimization under-the-hood that would allow nested namespace lookups to be done in a single step in certain situations that would be undetectable to an end-user of the language, then that would be fine too, assuming it didn't add too much burden of maintenance.
I somewhat doubt it would be possible to do that though, because the namespace proxies would be python objects like any other. You can pass references to them around.
So in the case of:
namespace A: namespace B: C = "foo"
You could just grab a reference to B:
ref = A.B
and then later on look up C on the reference:
ref.C # this looks up vars(sys.modules[__name__])['A.B.C']
The attribute lookup behavior behaves exactly like you would expect any similar set of chained attribute lookups to behave. No different than:
class A: class B: C = "foo"
So, if in a nested scenario, A.B.C.D, I'm trying to understand the
combination of getattr calls to resolve D. Would it just still be two attempts, getattr(A, "B.C.D") and getattr(getattr(getattr(A, "B"), "C"), "D")? If it were to become a Cartesian product of calls, there'll likely be a problem. 🤔️
The only way you could ever resolve D in a single attribute lookup like getattr(A, "B.C.D") is if you literally typed that statement out verbatim:
That would actually work, because `namespace A` would prepend its name to the attribute lookup and forward it on to its parent scope, serving up (sys.modules[__name__])['A.B.C.D'], which is where the key under which D is stored in the module globals.
But if you just look it up using normal attribute access syntax, like you would 99% of the time:
Then it would be done in 3 separate lookups, one for each dot, exactly the way you described: getattr(getattr(getattr(A, "B"), "C"), "D")
Looking B up on `namespace A` gets you `namespace A.B`, looking up C on `namespace A.B` gets you `namespace A.B.C`, and looking up D on `namespace A.B.C` finally gets you a reference to whatever D contains.
This isn't a cartesian product though. It's just one attribute lookup per level of reference nesting, just the same as any other form of attribute access.
As a side-note to everyone who has asked questions and participated in the discussion so far, thank you for at least entertaining my proposal. I'm under no delusions about the incredibly high bar that adding a new keyword to the language represents. But I still think discussions like this are important! Even if only one in every hundred such ideas actually has potential, if we don't brainstorm them up and discuss them as a community we'd never end up finding the ones that are valuable enough to seriously consider taking forward to a PEP.
I've found that on this list there are a few people who fancy themselves gatekeepers and will dismiss virtually any new idea that gets posted here out-of-hand in a way that is often pretty unkind, and without really offering up much of a rationale. Maybe it's just a fundamental difference of mindset, but to me, the idea that python is 'finished' and should remain largely untouched forever is kind of horrifying. Programming languages that don't develop and evolve just become obsolete. It seems to me that with literally every recent new enhancement to the language that I've been incredibly excited about in the last few years (f-strings, assignment expressions, pattern matching) there's been a huge number of people loudly complaining that 'we don't really need this' and it just baffles me.
Of course we don't *need* syntactic sugar. We could all write code like this:
3 + 4*2
evens =  for num in range(1, 11): if not num % 2: evens.append(num)
[num for num in range(1, 11) if not num % 2]
But I think it's hard to argue that a language without any syntactic sugar is better, or for that matter is something anyone would actually enjoy using.
So yeah, to make this point explicit. *This is a proposal for syntactic sugar*. It doesn't really add anything you can't do currently, although many of the things that would be idiomatic and trivial with `namespace` would be horrible antipatterns without it (and equivalent implementations available currently are much more verbose and less performant). but it should hopefully allow for more organized and DRYer code, and more easy usage of namespacing in situations where it makes sense and would be otherwise non-trivial to add them in (there are several examples of this in this thread and in the doc).
Arguing that this is pointless because it doesn't add completely new functionality misses the mark in the same way that it's not particularly helpful to argue that listcomps are pointless because for-loops exist. When they were first introduced many people hated them, and yet they (and other types of comprehensions) are one of the most beloved features of the language nowadays.
I can respect that many people will feel that the upsides of this proposal don't justify the growth in complexity of the language. I obviously disagree (or else I wouldn't have written all this up!) and am using this thread to try to convince you that it is. But even if I'm unsuccessful maybe this will put the idea on someone else's radar who might come up with a better and more compelling suggestion down the road.
Maybe something like how people wanted switch-cases and they were judged to be not sufficiently worth it, but then led to the match statement later on (basically switch-cases on steroids that also do your laundry and walk your dog).
So cheers, everyone.
On Tue, May 4, 2021 at 6:53 PM Rob Cliffe via Python-ideas < email@example.com> wrote:
On 04/05/2021 14:39, Paul Bryan wrote:
A problem I sense here is the fact that the interpreter would always need to attempt to resolve "A.B.C" as getattr(getattr(A, "B"), "C") and getattr(A, "B.C"). Since the proxy would be there to serve up the namespace's attributes, why not just let it and do away with "B.C" in A.__dict__? What will the performance costs be of attempting to get an attribute in two calls instead of one?
[snip] So, if in a nested scenario, A.B.C.D, I'm trying to understand the combination of getattr calls to resolve D. Would it just still be two attempts, getattr(A, "B.C.D") and getattr(getattr(getattr(A, "B"), "C"), "D")? If it were to become a Cartesian product of calls, there'll likely be a problem. 🤔️
I don't pretend to fully understand the proposal and how it would be implemented, but IMO adding an overhead (not to mention more complicated semantics) to *every* chained attribute lookup is enough to kill the proposal, given that it seems to have relatively slight benefits. Best wishes Rob Cliffe
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