Created on 2013-06-17 17:34 by ecatmur, last changed 2013-07-02 09:13 by lukasz.langa. This issue is now closed.
|singledispatch-mro-18244.patch||ecatmur, 2013-06-17 18:31||singledispatch-mro-18244.patch||review|
|singledispatch-mro-composition.patch||ecatmur, 2013-06-19 08:12||review|
|issue18244.diff||lukasz.langa, 2013-06-20 19:30||C3-based approach||review|
|issue18244.diff||lukasz.langa, 2013-06-27 14:21||C3-based approach after GvR review||review|
|issue18244.diff||lukasz.langa, 2013-06-30 13:39||C3-based approach favoring explicit ABCs||review|
|issue18244.diff||lukasz.langa, 2013-06-30 16:40||C3-based approach favoring explicit ABCs (improved)||review|
|issue18244.diff||lukasz.langa, 2013-07-01 10:26||C3-based approach favoring explicit ABCs after GvR review||review|
|Repositories containing patches|
|msg191350 - (view)||Author: Edward Catmur (ecatmur)||Date: 2013-06-17 17:34|
Suppose we have a class C with MRO (C, B, A, object). C virtual-inherits an ABC V, while B virtual-inherits an unrelated ABC W: object / | \ A W | | .` / B` V | .` C` Recalling that per PEP 443 singledispatch prefers concrete bases to virtual bases, we would expect the following composed MRO: C, B, V, A, W, object However what actually happens is the composed MRO depends on the order of the haystack; if W is processed first the result is correct, but if V is processed first then (because V does not subclass W) W is inserted in the MRO *before* V: C, B, A, object C, B, V, A, object C, B, W, V, A, object This results in ambiguity between V and W. Suggested fix is a slight change to the MRO composition algorithm, considering whether the items already placed in the MRO are concrete base classes.
|msg191351 - (view)||Author: Edward Catmur (ecatmur)||Date: 2013-06-17 17:37|
Apologies, the linked repository is for the 2.x backport of singledispatch. I'll replace it with a proper Python repo.
|msg191355 - (view)||Author: Edward Catmur (ecatmur)||Date: 2013-06-17 18:35|
See attachment for patch and test. Note that reproducing the issue without access to singledispatch internals depends on iteration order of a dict of types and is thus intermittent/environment dependent.
|msg191424 - (view)||Author: Łukasz Langa (lukasz.langa) *||Date: 2013-06-18 18:49|
Hello, Edward. First of all, thank you for correctly pointing out the problem. We'll integrate your patch but first I'd like to have a unit test that specifically shows why the secondonary `for` loop you introduced is necessary. Currently if we change this: for index, base in enumerate(mro[i + 1:], i + 1): if not issubclass(base, needle): break to this: index = i + 1 the tests still pass.
|msg191455 - (view)||Author: Edward Catmur (ecatmur)||Date: 2013-06-19 08:12|
Łukasz, thanks. When the most-derived class virtual-inherits two related ABCs U, V: object / | \ A W V | .` .` B` U` | .` C` The secondary `for` loop is necessary to ensure U and V are ordered correctly. I'll upload a patch with an improved test that covers this case.
|msg191535 - (view)||Author: Łukasz Langa (lukasz.langa) *||Date: 2013-06-20 19:30|
I meditated on the nature of the problem. A big issue that Edward pointed out is that at times the computed MRO depends on haystack ordering. We should fix that. However, my conclusion is that whatever home-grown algorithm we come up with, it will still be unintuitive for the end user. We should rather adopt C3 linearisation, which Python uses for calculating the method resolution order. Unfortunately, it doesn't specify what to do with virtual bases (either registered or implied). To solve that we need to extend the algorithm to insert the virtual bases in a "correct" place. We have two issues here: 1. How can we determine where this "correct" place is? 2. How can we determine which order unrelated abstract classes should be put in? The correct place for a virtual base class to appear in the MRO is on the level where the functionality it describes first appears. For example: >>> from collections import * >>> class A(object): pass ... >>> class B(A): ... def __len__(self): ... return 0 # implies Sized ... >>> @Container.register ... class C(object): pass ... >>> class D(object): pass # unrelated ... >>> class X(D, C, B): ... def __call__(self): ... return 0 # implies Callable ... If we calculate the C3 MRO for strict bases only, we get the obvious: >>> from functools import _c3_mro >>> _c3_mro(X) [<class '__main__.X'>, <class '__main__.D'>, <class '__main__.C'>, <class '__main__.B'>, <class '__main__.A'>, <class 'object'>] If we want to insert related abstract base classes, they consistently appear right after the class where functionality has been added (note that the order of the `abcs` here doesn't matter): >>> _c3_mro(X, abcs=[Sized, Callable, Container]) [<class '__main__.X'>, <class 'collections.abc.Callable'>, <class '__main__.D'>, <class '__main__.C'>, <class 'collections.abc.Container'>, <class '__main__.B'>, <class 'collections.abc.Sized'>, <class '__main__.A'>, <class 'object'>] This linearisation might be somewhat surprising but I believe it's the correct way. In fact it's only surprising because it exposes ambiguous dispatch when a class virtually subclasses multiple ABCs. For example, having a simple generic function: >>> from functools import singledispatch >>> from collections import * >>> @singledispatch ... def f(arg): ... return "base" ... >>> f.register(Iterable, lambda arg: "iterable") >>> f.register(Sized, lambda arg: "sized") >>> f.register(Set, lambda arg: "set") this class evaluates reasonably: >>> class C(Sized): ... def __len__(self): ... return 0 ... >>> f(C()) 'sized' However, when we register it for another ABC, there has to be a conflict: >>> Iterable.register(C) <class '__main__.C'> >>> f(C()) Traceback (most recent call last): ... RuntimeError: Ambiguous dispatch: <class 'collections.abc.Iterable'> or <class 'collections.abc.Sized'> It doesn't matter that Sized appears explicitly in the MRO. Class C is-a Sized just as well as it is-a Iterable. However, if we also register it to be a Set as well (which subclasses both Iterable and Sized), this solves the conflict: >>> Set.register(C) <class '__main__.C'> >>> f(C()) 'set' Same goes for the case described in PEP 443 where both ABCs appear explicitly in the MRO. No conflict here becase MRO orders both bases unambiguously: >>> class D(Sized, Iterable): ... def __len__(self): ... return 0 ... def __iter__(self): ... return iter() ... >>> f(D()) 'sized' The answer to the second question ("How can we determine in which order to put unrelated abstract classes?") is that we can't in general. There's fortunately a common special case that can help here: ABCs often have subclasses which are also bases of the type we're building the MRO for. For example: when inserting Iterable and Sized to the MRO of dict we can derive the correct order from the fact that both Iterable and Sized have a subclass called Mapping which also happens to be a base of dict. This trick cannot be used to fix every case but it's good enough to handle many. Summing up, the new algorithm is an extension of C3, which makes it much more predictable in edge cases. ABCs are introduced in a way that is much more deterministic (and when it depends on haystack ordering it always ends up as a RuntimeError anyway). My only real gripe with the new algorithm is that it's much larger codewise. On the other hand the ABC-aware C3 implementation could be useful for other purposes as well. Nick, Guido, this needs a serious review. Took me a while to get it right.
|msg191921 - (view)||Author: Guido van Rossum (gvanrossum) *||Date: 2013-06-26 22:43|
Wow. This is heady stuff. I can't say I totally get every detail, so I'll just pick on some things you wrote. In particular, I'm not sure I agree that there should be a conflict when there are two applicable base classes with a different dispatch if one of them is explicit in the MRO and the other isn't. After all, it both were explicit in the MRO the first one there would be picked, right? And virtual base classes are considered to appear after explicit base classes, right? (Or not?) I do think the new approach is better (the extra code doesn't bother me), and I may be convinced yet that you made the right choice in the above case. I have some trivial review comments on your patch to, will send those via Rietveld.
|msg191947 - (view)||Author: Łukasz Langa (lukasz.langa) *||Date: 2013-06-27 14:21|
The reason why I think it's wrong to special-case ABCs that are explicitly in the MRO is that it's only one of four methods of virtual subclassing: 1. Explicit MRO; 2. Abstract functionality implicitly implemented without any form of registration; 3. abc.register(); 4. One of the above on a base of the type in question. This creates more possibilities for conflicts than just the example described in my last message. For instance, what's the preferred ABC here, Iterable or Sized? >>> class E(Sized): ... def __len__(self): ... return 0 ... def __iter__(self): ... for i in : ... yield i My answer is: neither. E equally is-a Sized and is-a Iterable. If the dispatcher favors one over the other, you will get people upset about the decision, no matter which one it is. Note that the conflict arises only for multiple ABCs which end up *on the same level* of the MRO. For instance in the example below the dispatch always chooses the Iterable implementation (the functionality appears directly on F, whereas Sized appears on a base): >>> class HasSize(Sized): ... def __len__(self): ... return 0 ... >>> class F(HasSize): ... def __iter__(self): ... for i in : ... yield i If we wanted to favor the ABCs that are explicitly in the MRO, what should we choose in this case? If we say "Sized", then it breaks the whole idea of arranging the ABCs next to the class that first implements them in the MRO. But it gets better! Suppose you have a generic function with implementations for Sized and Callable. Which implementation should we choose for class G? >>> class G(MutableMapping): ... def __call__(self): ... return None ... # the rest of the MutableMapping implementation Seems like Sized because it is explicitly in the MRO, right? What about H then? >>> class H(dict): ... def __call__(self): ... return None Well, now it's suddenly Callable because Sized is "only" a virtual base class. I don't think we should let that happen. It all comes down to the question whether you consider ABCs to be bases FOR REAL or only sort-of-but-not-really. I believe they're real bases regardless of the method of registration. Implicit implementation and abc.register() doesn't make the base any less real. All in all, the user will ask: "Hey, it's true, I have a tricky type that subclasses both an ABC1 and an ABC2 and singledispatch raises a RuntimeError. How do I make this work?" The answer is simple: just register a more specific implementation on the generic function, even if it simply selects one of the existing ones: generic_func.register(TrickyType, generic_func.dispatch(ABC2)) Explicit is better than implicit.
|msg191956 - (view)||Author: Guido van Rossum (gvanrossum) *||Date: 2013-06-27 16:51|
Hm. I interpret "explicit is better than implicit" very differently. I see a strict priority ordering from better to worse, in cases that would otherwise be ambiguous: 1. explicit base class (ABC or otherwise) 2. ABC explicitly registered 3. ABC implicitly inferred from presence of special method I'm all for using all the other heuristics and rules you describe: inferred ABCs occur at the level where they are introduced, for example, and if two different ABCs are both inferred at the same level or both registered at the same level, that should be considered ambiguous. But if one ABC is listed as an explicit base at some level and another is registered or implicit at the same level, the explicit base should just win -- just as if both ABCs were explicitly listed, the one listed first wins. So the rule should be that registered and inferred bases are only considered after explicit bases at the same level. (If you want the registered class to be preferred, you should add it as an explicit base instead -- and if you don't own the code, you should respect the choice of its author, or do something using subclassing.) If it were me, explicitly registered ABCs would also trump inferred ABCs -- after all an inferred ABC is far from obvious to most readers and might even be an accident, and the situation can be rectified by explicit registration. IOW, I disagree with your claim that "Class C is-a Sized just as well as it is-a Iterable." C is a Sized *more* than an Iterable because Size is an explicitly listed base class and Iterable is added through registration. Same if Iterable were inferred. (Just to be clear, this only applies if the ambiguity occurs at a single level. ABCs explicitly listed, registered, or inferred at some base class have explicitly lower priority.)
|msg192040 - (view)||Author: Łukasz Langa (lukasz.langa) *||Date: 2013-06-29 17:24|
Looks like the priority ordering you mention is not yet documented anywhere. It definitely makes sense but I'd like to take a step back for a moment to consider the following questions: 1. What additional functionality do our users get with this ordering? In other words, what purpose does this new ordering have? Apart from solving the conflict we're discussing, I can't see any. 2. What disadvantages does this ordering bring to the table? I think that simplicity is a feature. This ordering creates additional complexity in the language. Firstly, there is currently no obvious way for users to distinguish between implicit subclassing (via implementation) or subclassing by `abc.register()`. This creates the dangerous situation where backwards incompatibility introduced by switching between those ABC registration mechanism is nearly impossible to debug. Consider an example: version A of a library has a type which only implicitly subclasses an ABC. User code with singledispatch is created that works with the current state of things. Version B of the same library uses `ABC.register(Type)` and suddenly the dispatch changes without any way for the user to see what's going on. A similar example with explicit subclassing and a different form of registration is easier to debug, but not much, really. Secondly, it creates this awkward situation where dispatch for any custom `MutableMapping` can be different from the dispatch for `dict`. Although the latter is a `MutableMapping` "only" by means of `MutableMapping.register(dict)`, in reality the whole definition of what a mutable mapping is comes from the behaviour found in `dict`. Following your point of view, dict is less of a mutable mapping than a custom type subclassing the ABC explicitly. You're saying the user should "respect the choice of its author" but it's clearly suboptimal here. I strongly believe I should be able to swap a mutable mapping implementation with any other and get consistent dispatch. Thirdly, while I can't support this with any concrete examples, I have a gut feeling that considering all three ABC subclassing mechanisms to be equally binding will end up as a toolkit with better composability. The priority ordering on the other hand designs `abc.register()` and implicit subclassing as inferior MRO wannabes. Last but not least, the priority ordering will further complicate the implementation of `functools._compose_mro()` and friends. While the complexity of this code is not the reason of my stance on the matter, I think it nicely shows how much the user has to keep in her head to really know what's going on. Especially that we only consider this ordering to be decisive on a single interitance level. 3. Why is the ordering MRO -> register -> implicit? The reason I'm asking is that the whole existence of `abc.register()` seems to come from the following: * we want types that predate the creation of an ABC to be considered its subclasses; * we can't use implicit subclassing because either the existence of methods in question is not enough (e.g. Mapping versus Sequence); or the methods are added at runtime and don't appear in __dict__. Considering the above, one might argue that the following order is just as well justified: MRO -> implicit -> register. I'm aware that the decision to put register first is because if the user is unhappy with the dispatch, she can override the ordering by registering types which were implicit before. But, while I can register third-party types, I can't unregister any. In other words, I find this ordering arbitrary. I hope you don't perceive my position as stubborn, I just care enough to insist on this piece of machinery to be clearly defined and as simple as possible (but not simpler, sure).
|msg192060 - (view)||Author: Guido van Rossum (gvanrossum) *||Date: 2013-06-30 00:12|
> Looks like the priority ordering you mention is not yet documented > anywhere. Because up till now it has not been needed -- all you can do with ABCs is use isinstance/issubclass. > It definitely makes sense but I'd like to take a step back for > a moment to consider the following questions: > > 1. What additional functionality do our users get with this ordering? In > other words, what purpose does this new ordering have? > > Apart from solving the conflict we're discussing, I can't see any. There doesn't have to be any other functionality. We're just trying to address how ABCs should be ordered relative to classes explicitly in the MRO for the purpose of @singledispatch. > 2. What disadvantages does this ordering bring to the table? > > I think that simplicity is a feature. This ordering creates > additional complexity in the language. But so does not ordering. The underlying question is how to dispatch when two or more classes in an object's class hierarchy have a different dispatch rule. This is fundamentally a question of ordering. For regular method dispatch, the ordering used is the MRO, and there is always a unique answer: the class that comes first in the MRO wins. (Ambiguities and inconsistencies in the MRO are rejected at class definition time.) This is very convenient because the issue of coming up with a total ordering of base classes is solved once and for all, and because of the total ordering we never have to reject a request to dispatch (of regular methods or attribute lookup) as ambiguous. (Note: I call it a total ordering, but the total ordering is only within a specific class's MRO. Any class's explicit bases are totally ordered in that class's MRO -- but the order of two classes may be different in a different class's MRO. This is actually relevant for the definition of C3, and we'll see this below.) For @singledispatch, we are choosing to support ABCs (because it makes sense), and so we have to think about how handle ABCs that are relevant (isinstance/issubclass returns True) but not in the MRO. Let's introduce terminology so we can talk about different cases easily. Relevant: isinstance/issubclass returns True Explicit: it's in the MRO Implicit: relevant but not explicit Registered: Implicit due to a register() call Inferred: Implicit due to a special method These categories are not always exclusive, e.g. an ABC may be registered for one class in the MRO but inferred for a different one. The registration mechanism tries to avoid outright cycles but otherwise is not as strict about rejecting ambiguities as C3 is for the MRO calculation, and this I believe is the reason we're still debating the order. A simple example of something rejected by C3: - suppose we have classes B and C derived directly from object - class X(B, C) has MRO [X, B, C, object] - class Y(C, B) has MRO [Y, C, B, object] - class Z(X, Y) is *rejected* because there is no consistent ordering for B and C in the MRO for Z However, if we construct a similar situation using implicit ABCs, e.g. by removing B and C from the list of explicit bases of X and Y, and instead registering X and Y as their virtual subclasses -- then Z(X, Y) is valid and Z is a virtual subclass of both B and C. But there is no ordering preference in this case, so I think it's good if @singledispatch refuses the temptation to guess here, in the case where there are registered dispatches for B and C but none for X, Y or Z. I believe the rest of the discussion is purely about what to do if B was explicitly listed as a base but C wasn't (or vice versa). There are still a few different cases; the simplest is this: class X(B) -- MRO is [X, B, object] class Y(B) -- MRO is [Y, B, object] C.register(X) C.register(Y) class Z(X, Y) -- MRO is [Z, X, Y, B, object] IIUC you want @singledispatch to still reject a call for a Z instance if the only relevant registered dispatches are for B and C -- because you claim that X, Y and Z are "just as much" subclasses of B as they are of C. (It doesn't actually matter whether we use X, Y or Z as an example here -- all of them have the same problem.) But I disagree. First, in the all-explicit example, we can equally say that X is "just as much" a subclass of B as it is of C. And yet, because both appear in the explicit MRO, B wins when dispatching for X -- and C wins when dispatching for Y, because the explicit base classes are ordered differently there. So the property of being "just as much" a base class isn't used -- but the order in the explicit MRO is. It is the nature and intent of ABC registration that it does not have to occur in the same file as the class definition. In particular, it is possible that the class definitions of B, C, X, Y and Z all occur together, but the C.register() calls occur in a different, unrelated module, which is imported only on the whim of the top-level application, or as a side effect of some unrelated import made by the top-level app. This, to me, is enough to consider registered ABC inheritance as a second-class citizen compared to explicit inclusion in the list of bases. Consider this: dispatch(Z) would consider only Z, X, Y, B, object if the C.register() calls were *not* made, and then it would choose B; but under your rules, if the app changed to cause the C.register() calls to be added, dispatch(Z) would complain. This sounds fragile to me, and it is the main reason why I want explicit ABCs to prevail over "merely" registered ABCs, rather than to be treated equal and possibly cause complaints based on what happened elsewhere in the program. Now let's move on to inferred ABCs. Let's assume C is actually Sized, and both X and Y implicitly subclass Sized because they implement __len__(). On the one hand, this doesn't have the problem with register() calls that may or may not be loaded -- the __len__() definitions are always there. On the other hand, to me this feels even less explicit than using register() -- while presumably everyone who uses register() knows at some basic level about ABCs, I do not make the same assumption about people who write classes that have a __len__() method. Because of Python's long history of duck typing, many __len__() methods in code that is still in use (even if it has evolved in other ways) were probably written before ABCs (or even MROs) were introduced! Now assume X's explicit base class B is actually Iterable, and X implements both __iter__() and __len__(). X's author *could* easily have made X a subclass of both Iterable and Sized, and thereby avoided the ambiguity. But instead, they explicitly inherited only Iterable, and left Sized implicit. (I guess their understanding of ABCs was limited. :-) I really do think that in this case there is a discernible difference between the author's attitude towards Iterable and Sized -- they cared enough about Iterable to inherit from it. So I think we should assume that *if* we asked them to add Sized as a base class, they would add it second, thereby resolving the ambiguity in favor of Iterable. And that's why I think that if they left Sized out, we are doing them more of a favor by preferring Iterable than by complaining. Now on to your other examples. > Firstly, [Off-topic English-major nit: it's better to write "first" instead of "firstly". English is not a very consistent language. :-) See http://www.randomhouse.com/wotd/index.pperl?date=20010629 for a nuanced view that still ends up preferring "first".] > there is currently no obvious way for users to distinguish > between implicit (which I called "inferred" above) > subclassing (via implementation) or subclassing by > `abc.register()`. What do you mean by "no obvious way to distinguish"? That you can't tell by merely looking at the MRO and calling isinstance/issubclass? But you could look in the _abc_registry. Or you could scan the source for register() calls. TBH I would be fine if these received exactly the same status -- but explicit inclusion in the MRO still ought to prevail. > This creates the dangerous situation where > backwards incompatibility introduced by switching between those ABC > registration mechanism is nearly impossible to debug. Consider an > example: version A of a library has a type which only implicitly > subclasses an ABC. User code with singledispatch is created that > works with the current state of things. Version B of the same library > uses `ABC.register(Type)` and suddenly the dispatch changes without > any way for the user to see what's going on. A similar example with > explicit subclassing and a different form of registration is easier > to debug, but not much, really. So there are an infinite number of ways to break subtle code like this by subtle changes in inheritance. I see that mostly as a fact of life, not something we must contain at all cost. I suppose even with explicit base classes it's not inconceivable that one developer's "innocent" fix of a class hierarchy breaks another developer's code. (See also http://xkcd.com/1172/ :-) So could "innocently" adding or moving a __len__() method. > Secondly, it creates this awkward situation where dispatch for any > custom `MutableMapping` can be different from the dispatch for > `dict`. Although the latter is a `MutableMapping` "only" by means of > `MutableMapping.register(dict)`, in reality the whole definition of > what a mutable mapping is comes from the behaviour found in `dict`. > Following your point of view, dict is less of a mutable mapping than > a custom type subclassing the ABC explicitly. You're saying the user > should "respect the choice of its author" but it's clearly suboptimal > here. I strongly believe I should be able to swap a mutable mapping > implementation with any other and get consistent dispatch. Hmm... I need to make this situation explicit to think about it. I think you are still looking at a case where there are exactly two possible dispatches, one for Sized and one for Iterable, right? And the issue is that if the user defines class Foo(MutableMapping), Sized and Iterable appear explicitly in Foo's MRO, in that order (because MM explicitly subclasses them in this order), so Sized wins. But, hm, because there's a call to MutableMapping.register(dict) in collections/abc.py, the way I see it, if the only dispatches possible were on Sized and Iterable, Sized should still win, because it comes first in MM's MRO. Now, if that MM.register(dict) call was absent, you might be right. But it's there (and you mention it) so I'm not sure what is going on here -- are you talking about a slightly different example, or about a different rule than I am proposing? > Thirdly, while I can't support this with any concrete examples, > I have a gut feeling that considering all three ABC subclassing > mechanisms to be equally binding will end up as a toolkit with better > composability. The priority ordering on the other hand designs > `abc.register()` and implicit subclassing as inferior MRO wannabes. Ok, when we're talking gut feelings, you're going to have a hard time convincing others that your gut is more finely tuned to digesting Python subtleties than mine. :-) Clearly my gut tells me that explicit inclusion of an ABC in the list of bases is a stronger signal than implicit subclassing; at least part of the reason why my gut feels this way is that the C3 analysis is more strict about rejecting ambiguous orderings outright than registered or inferred ABCs. (But my gut agrees with yours that there's not much to break a tie between a registered and an inferred ABC.) > Last but not least, the priority ordering will further complicate the > implementation of `functools._compose_mro()` and friends. While the > complexity of this code is not the reason of my stance on the matter, > I think it nicely shows how much the user has to keep in her head to > really know what's going on. Especially that we only consider this > ordering to be decisive on a single inheritance level. The implementation of C3 is also complex, and understanding all its rules is hard -- harder than what I had before. But it is a better rule, and that's why we use it. > 3. Why is the ordering MRO -> register -> implicit? (Note: I already relented on the latter arrow.) > The reason I'm asking is that the whole existence of `abc.register()` > seems to come from the following: > > * we want types that predate the creation of an ABC to be considered > its subclasses; We certainly want it enough for isinstance/issubclass to work and for an unambiguous dispatch to work. But we're not sure about the relative ordering in all cases, because there's no order in the registry nor for inferred ABCs, and that may affect when dispatch is considered dispatch. > * we can't use implicit subclassing because either the existence of > methods in question is not enough (e.g. Mapping versus Sequence); > or the methods are added at runtime and don't appear in __dict__. > > Considering the above, one might argue that the following order is > just as well justified: MRO -> implicit -> register. I'm aware that > the decision to put register first is because if the user is unhappy > with the dispatch, she can override the ordering by registering types > which were implicit before. But, while I can register third-party > types, I can't unregister any. In other words, I find this ordering > arbitrary. So, if you can handle "MRO is stronger than registered or inferred" we don't actually disagree on this. :-) > I hope you don't perceive my position as stubborn, I just care enough to > insist on this piece of machinery to be clearly defined and as simple as > possible (but not simpler, sure). Not at al. You may notice I enjoy the debate. :-)
|msg192069 - (view)||Author: Łukasz Langa (lukasz.langa) *||Date: 2013-06-30 13:39|
Having all information in place, I think it's acceptable for both of us to implement preference for explicit registration, leaving both ways of implicit registration as equally binding. The latter is future proof in the sense that if we change our minds later, it's going to be easier to lift the dispatch conflict than to introduce conflicts where there weren't any before. The point you're making about `Abc.register(Cls)` being external to the definition of Cls and thus somewhat dependant on import order and other machinery is what convinced me in the end. I also like the proposed terminology and think it should appear in the documentation. I created a modified patch. This wasn't as tricky as I feared but required me to formulate an explicit rule: all implicit ABCs are inserted in the MRO of a given class directly after the last explicit ABC in the MRO of said class. One open question is what to do with the algorithm described in PEP 443 which no longer describes the state of things exactly. Although the said PEP is final, some parts of the discussion on this issue just beg to be included in the "Abstract base classes" section. What do you think we should do? Answering your questions, neither scanning the source code nor using a private attribute on ABCMeta can be considered an *obvious* way to distinguish between registered and inferred ABCs. The former is static analysis which might involve opening (and understanding) a number of black boxes, the latter is fragile by definition and breaks the abstraction (again, opening a black box). This is why we introduced a public API to get the current cache token _. As for dispatch differences between MutableMapping and dict, it's in my previous message (191947) on the issue, classes G and H. On an unrelated note, thank you for correcting my English. It seems that after achieving a level of fluency that is bearable to native speakers, nobody corrects my mistakes anymore. This in turn places me in an unfortunate plateau. And yes, I'm well aware that my gut would win no popularity contest, especially when the BDFL's one is a contender :-) I just hope this doesn't automatically make the feelings of my own gut invalid. ..  http://bugs.python.org/issue16832
|msg192076 - (view)||Author: Łukasz Langa (lukasz.langa) *||Date: 2013-06-30 16:40|
Here's an improved patch which doesn't copy data between lists in `_c3_mro()` so much.
|msg192096 - (view)||Author: Guido van Rossum (gvanrossum) *||Date: 2013-07-01 00:22|
Ok, let's do it. I just sent you a review of your latest code (admitting I don't actually follow the logic in full detail, so I'm mostly harping on tests and comments...). Regarding the PEP: feel free to update this. Clearly nobody read it this carefully before... "Final" in the Python world doesn't mean "every typo remains forever" and for PEPs this young that also applies to esoteric details that were perhaps not fully, or not correctly, specified before.
|msg192128 - (view)||Author: Roundup Robot (python-dev)||Date: 2013-07-01 14:03|
New changeset a2672dc7c805 by Łukasz Langa in branch 'default': Issue #18244: Adopt C3-based linearization in functools.singledispatch for improved ABC support http://hg.python.org/cpython/rev/a2672dc7c805
|msg192132 - (view)||Author: Łukasz Langa (lukasz.langa) *||Date: 2013-07-01 14:27|
I committed the patch with minor docstring and comment fixes; one test was tweaked to be hash randomization-proof. Edward, thanks for taking the time to file the bug. Guido, thanks for a thorough and illuminating review process.
|msg192184 - (view)||Author: Łukasz Langa (lukasz.langa) *||Date: 2013-07-02 09:13|
For the record, the PEP has been updated _ and so has been the backport for 2.6 - 3.3 _. ..  http://hg.python.org/peps/rev/000a8986ef73 ..  https://pypi.python.org/pypi/singledispatch/188.8.131.52
|2013-07-02 09:13:29||lukasz.langa||set||messages: + msg192184|
|2013-07-01 14:27:05||lukasz.langa||set||status: open -> closed|
messages: + msg192132
stage: patch review -> resolved
messages: + msg192128
|2013-07-01 13:59:10||lukasz.langa||set||title: singledispatch: When virtual-inheriting ABCs at distinct points in MRO, composed MRO is dependent on haystack ordering -> Adopt C3-based linearization for improved ABC support in functools.singledispatch|
|2013-07-01 10:26:02||lukasz.langa||set||files: + issue18244.diff|
|2013-07-01 00:22:44||gvanrossum||set||messages: + msg192096|
messages: + msg192076
messages: + msg192069
|2013-06-30 00:12:19||gvanrossum||set||messages: + msg192060|
|2013-06-29 17:24:37||lukasz.langa||set||messages: + msg192040|
|2013-06-27 16:51:06||gvanrossum||set||messages: + msg191956|
messages: + msg191947
messages: + msg191921
messages: + msg191535
messages: + msg191455
|2013-06-18 18:49:36||lukasz.langa||set||messages: + msg191424|
|2013-06-17 18:41:31||lukasz.langa||set||assignee: lukasz.langa|
stage: patch review
components: + Library (Lib), - Extension Modules
|2013-06-17 18:35:05||ecatmur||set||messages: + msg191355|
|2013-06-17 18:31:57||ecatmur||set||type: behavior|
keywords: + patch
|2013-06-17 17:37:34||ecatmur||set||messages: + msg191351|