Re: epiphany
Roy Smith於 2013年4月25日星期四UTC+8上午7時50分33秒寫道： I discovered something really neat today. We've got a system with a bunch of rules. Each rule is a method which returns True or False. At some point, we need to know if all the rules are True. Complicating things, not all the rules are implemented. Those that are not implemented raise NotImplementedError. We used to have some ugly logic which kept track of which rules were active and only evaluated those. So, here's the neat thing. It turns out that bool(NotImplemented) returns True. By changing the unimplemented rules from raising NotImplementedError to returning NotImplemented, the whole thing becomes: return all(r() for r in rules) Problems of rules in Boolean algebra or the bi-level logic inference engine in AI were all solved long time ago in the text book about AI. There are some variations about the multi-level or the continuous level logic engine with some new phases in Fuzzy theory in the expert system. A dynamical typed language is better to be used in this kind of problems. -- http://mail.python.org/mailman/listinfo/python-list
Re: epiphany
In article 5178b1db$0$29977$c3e8da3$54964...@news.astraweb.com, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: The semantics of NotImplemented is that it is a signal for one object to say I don't know how to do this, let somebody else try. That's precisely the logic here. The rule says, I don't know how to tell you if this is OK or not, ask another rule. Since rules apparently take no arguments, either: 1) they rely on global state, which is a nasty design; or 2) rules actually have a fixed return result, in which case why make them functions in the first place? Yes, rules take arguments. I elided them from the original description since it wasn't germane to what I was trying to show. Since both possibilities seem stupid, and I do not believe that Roy actually is stupid, I am honored that you have such a high opinion of me :-) Here's what the docs say about NotImplemented: This type has a single value. There is a single object with this value. This object is accessed through the built-in name NotImplemented. Numeric methods and rich comparison methods may return this value if they do not implement the operation for the operands provided. (The interpreter will then try the reflected operation, or some other fallback, depending on the operator.) Its truth value is true. It gives an example of a use by numeric methods. It doesn't say that's the only thing it can be used for. It also says, Its truth value is true. Why would they document that fact if you weren't supposed to use it as a boolean operand? -- http://mail.python.org/mailman/listinfo/python-list
Re: epiphany
On Thu, 25 Apr 2013 08:36:34 -0400, Roy Smith wrote: In article 5178b1db$0$29977$c3e8da3$54964...@news.astraweb.com, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: The semantics of NotImplemented is that it is a signal for one object to say I don't know how to do this, let somebody else try. That's precisely the logic here. The rule says, I don't know how to tell you if this is OK or not, ask another rule. Sounds good to me then. It looks like your design is actually much closer to what I believe the standard Python semantics are intended to be than it appeared at first. [...] Here's what the docs say about NotImplemented: This type has a single value. There is a single object with this value. This object is accessed through the built-in name NotImplemented. Numeric methods and rich comparison methods may return this value if they do not implement the operation for the operands provided. (The interpreter will then try the reflected operation, or some other fallback, depending on the operator.) Its truth value is true. It gives an example of a use by numeric methods. It doesn't say that's the only thing it can be used for. Right. You can do a lot of things in Python, including shooting your foot off :-) but that doesn't mean you should. The further away from standard Python conventions you get, the more wary you should be. That's all. It also says, Its truth value is true. Why would they document that fact if you weren't supposed to use it as a boolean operand? You can use *anything* in Python in a boolean context. That's a language feature: all objects are either truthy or falsey. As for why it is documented for NotImplemented, I guess that's because some people might guess that it is falsey, like None. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: epiphany
In article 51792710$0$29977$c3e8da3$54964...@news.astraweb.com, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: It also says, Its truth value is true. Why would they document that fact if you weren't supposed to use it as a boolean operand? You can use *anything* in Python in a boolean context. That's a language feature: all objects are either truthy or falsey. As for why it is documented for NotImplemented, I guess that's because some people might guess that it is falsey, like None. That was part of what added the epiphanality to the experience. My first guess was exactly as you say, that bool(NotImplemented) would be false. Once I discovered that it was true, the rest immediately fell into place and many lines of code got replaced by the simple: return all(r(...) for r in rules) ^ | + stuff that I'm not showing goes here :-) -- http://mail.python.org/mailman/listinfo/python-list
epiphany
I discovered something really neat today. We've got a system with a bunch of rules. Each rule is a method which returns True or False. At some point, we need to know if all the rules are True. Complicating things, not all the rules are implemented. Those that are not implemented raise NotImplementedError. We used to have some ugly logic which kept track of which rules were active and only evaluated those. So, here's the neat thing. It turns out that bool(NotImplemented) returns True. By changing the unimplemented rules from raising NotImplementedError to returning NotImplemented, the whole thing becomes: return all(r() for r in rules) -- http://mail.python.org/mailman/listinfo/python-list
Re: epiphany
On Wed, 24 Apr 2013 19:50:33 -0400, Roy Smith wrote: I discovered something really neat today. We've got a system with a bunch of rules. Each rule is a method which returns True or False. At some point, we need to know if all the rules are True. Complicating things, not all the rules are implemented. Those that are not implemented raise NotImplementedError. NotImplementedError is intended to be raised by abstract base classes to indicate a method that must be overridden. I also use it as a place- holder for functions or methods I haven't actually written yet. I'm not sure what semantics you're giving NotImplementedError in your code, but I wonder whether a neater solution might be to just use rule = None for unimplemented rules, rather than: def unimplemented(): raise NotImplementedError rule = unimplemented Then your logic for seeing if all rules return true would become: all(r() for r in rules if r is not None) and for seeing if all rules return true or are unimplemented: all(r is None or r() for r in rules) We used to have some ugly logic which kept track of which rules were active and only evaluated those. I don't see why you would need anything like that. Reading further on, I see that you are counting unimplemented rules as true, for some reason which I don't understand. (Knowing nothing of your use-case, I would have expected intuitively that unimplemented rules count as not true.) A simple helper function will do the job: def eval(rule): try: return rule() except NotImplementedError: return True everything_is_true = all(eval(r) for r in rules) No need for complicated ugly logic keeping track of what rules are implemented. But if you're worried about the cost of catching those exceptions (you've profiled your code, right?) then that's easy with a decorator: def not_implemented(func): @functools.wraps(func) def inner(*args, **kw): raise NotImplementedError inner.ni = True return inner # Decorate only the rules you want to be unimplemented. @not_implemented def my_rule(): pass everything_is_true = all(r() for r in rules if not hasattr(r, 'ni')) Note that if you could reverse the logic so that unimplemented rules count as not true, this will also work: try: everything_is_true = all(r() for r in rules) except NotImplementedError: everything_is_true = False So, here's the neat thing. It turns out that bool(NotImplemented) returns True. By changing the unimplemented rules from raising NotImplementedError to returning NotImplemented, the whole thing becomes: return all(r() for r in rules) Objects are supposed to return NotImplemented from special dunder methods like __add__, __lt__, etc. to say I don't know how to implement this method for the given argument. Python will then try calling the other object's special method. If both objects return NotImplemented, Python falls back on whatever default behaviour is appropriate. So, knowing nothing of your application, I fear that this is an abuse of NotImplemented's semantics. If a rule returns NotImplemented, I would expect your application to fall back on a different rule. If that's not the case, you're using it in a non-standard way that will cause confusion for those with expectations of what NotImplemented means. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: epiphany
In article 5178884b$0$29977$c3e8da3$54964...@news.astraweb.com, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: I don't see why you would need anything like that. Reading further on, I see that you are counting unimplemented rules as true, for some reason which I don't understand. The top-level logic we need to enforce is this configuration doesn't violate any rules. -- http://mail.python.org/mailman/listinfo/python-list
Re: epiphany
On Thu, Apr 25, 2013 at 11:41 AM, Roy Smith r...@panix.com wrote: In article 5178884b$0$29977$c3e8da3$54964...@news.astraweb.com, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: I don't see why you would need anything like that. Reading further on, I see that you are counting unimplemented rules as true, for some reason which I don't understand. The top-level logic we need to enforce is this configuration doesn't violate any rules. Then have your unimplemented rules simply return True. Easy! ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: epiphany
In article mailman.1044.1366856455.3114.python-l...@python.org, Chris Angelico ros...@gmail.com wrote: On Thu, Apr 25, 2013 at 11:41 AM, Roy Smith r...@panix.com wrote: In article 5178884b$0$29977$c3e8da3$54964...@news.astraweb.com, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: I don't see why you would need anything like that. Reading further on, I see that you are counting unimplemented rules as true, for some reason which I don't understand. The top-level logic we need to enforce is this configuration doesn't violate any rules. Then have your unimplemented rules simply return True. Easy! ChrisA It's nice to have tri-state logic: * This rule passes * This rule fails * This rule was not evaluated What I've got now expresses that perfectly. -- http://mail.python.org/mailman/listinfo/python-list
Re: epiphany
On 04/24/2013 06:35 PM, Steven D'Aprano wrote: Objects are supposed to return NotImplemented from special dunder methods like __add__, __lt__, etc. to say I don't know how to implement this method for the given argument. Python will then try calling the other object's special method. If both objects return NotImplemented, Python falls back on whatever default behaviour is appropriate. So, knowing nothing of your application, I fear that this is an abuse of NotImplemented's semantics. If a rule returns NotImplemented, I would expect your application to fall back on a different rule. If that's not the case, you're using it in a non-standard way that will cause confusion for those with expectations of what NotImplemented means. Why would you assume some random application is going to deal with NotImplemented the same way the python interpreter does? And even the interpreter isn't consistent -- sometimes it will return false (__eq__) and sometimes it will raise an Exception (__add__). I hardly think it an abuse of NotImplemented to signal something is not implemented when NotImplemented means, um, not implemented. possibly-not-implemented-ly yours, -- ~Ethan~ -- http://mail.python.org/mailman/listinfo/python-list
Re: epiphany
On 04/24/2013 07:20 PM, Chris Angelico wrote: On Thu, Apr 25, 2013 at 11:41 AM, Roy Smith r...@panix.com wrote: In article 5178884b$0$29977$c3e8da3$54964...@news.astraweb.com, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: I don't see why you would need anything like that. Reading further on, I see that you are counting unimplemented rules as true, for some reason which I don't understand. The top-level logic we need to enforce is this configuration doesn't violate any rules. Then have your unimplemented rules simply return True. Easy! And less clear. -- ~Ethan~ -- http://mail.python.org/mailman/listinfo/python-list
Re: epiphany
On Wed, 24 Apr 2013 19:25:37 -0700, Ethan Furman wrote: On 04/24/2013 06:35 PM, Steven D'Aprano wrote: Objects are supposed to return NotImplemented from special dunder methods like __add__, __lt__, etc. to say I don't know how to implement this method for the given argument. Python will then try calling the other object's special method. If both objects return NotImplemented, Python falls back on whatever default behaviour is appropriate. So, knowing nothing of your application, I fear that this is an abuse of NotImplemented's semantics. If a rule returns NotImplemented, I would expect your application to fall back on a different rule. If that's not the case, you're using it in a non-standard way that will cause confusion for those with expectations of what NotImplemented means. Why would you assume some random application is going to deal with NotImplemented the same way the python interpreter does? Why would you assume that some random application is going to treat x==y the same way the Python interpreter does? Just because you can design your objects to do anything you want doesn't mean you should. Breaking conventions carries costs by the mere fact that you're breaking conventions. There are established semantics that an experienced Python developer will expect for NotImplemented, and doing something else risks causing confusion and mistakes. Or worse, bugs. If there is any chance that a rule might be called in a context where the Python interpreter gets to interpret the return result before you see it, then returning NotImplemented could lead to difficult to debug problems. And even the interpreter isn't consistent -- sometimes it will return false (__eq__) and sometimes it will raise an Exception (__add__). As I said: If both objects return NotImplemented, Python falls back on whatever default behaviour is appropriate. If neither object knows how to compare the other for equality, the appropriate behaviour is to treat them as unequal. If neither object knows how to add itself to the other, the appropriate behaviour is to raise an exception. I hardly think it an abuse of NotImplemented to signal something is not implemented when NotImplemented means, um, not implemented. It doesn't just mean not implemented in general, it has a specific meaning: I don't know what to do here, let the other object handle it. As I have repeatedly said, I don't know the context of the application, but from what little has been described, this part of it doesn't feel to me like a good, clean design. I might be wrong, but from the outside it feels like the API should be that rules return a three-state logic instance: True, False, Unknown where Unknown can be trivially created with Unknown = object() The semantics of NotImplementedError is that it is an *error*, and that doesn't sound appropriate given the example shown. Why would a rule that raises an *error* exception be treated as if it had passed? That's just wrong. The semantics of NotImplemented is that it is a signal for one object to say I don't know how to do this, let somebody else try. That also doesn't seem appropriate. There's no sign that Roy's application does the equivalent to this: result = rule() if result is NotImplemented: result = another_rule() if result is NotImplemented: result = some_default Since rules apparently take no arguments, either: 1) they rely on global state, which is a nasty design; or 2) rules actually have a fixed return result, in which case why make them functions in the first place? Since both possibilities seem stupid, and I do not believe that Roy actually is stupid, I suspect that his example over-simplifies the situation. But I can't comment on the infinite number of things that his code might do, I can only comment on the examples as actually given, and as given, I don't think that either NotImplementedError or NotImplemented is a clean solution to the problem. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
