Re: equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
A.T.Hofkamp wrote: Hmm, maybe numbers in sets are broken then? a = 12345 b = 12345 a == b True a is b False set([a,b]) set([12345]) Numbers and my Car2 objects behave the same w.r.t. '==' and 'is', yet I get a set with 1 number, and a set with 2 cars. Something is wrong here imho. The point I intended to make was that having a default __hash__ method on objects give weird results that not everybody may be aware of. In addition, to get useful behavior of objects in sets one should override __hash__ anyway, so what is the point of having a default object.__hash__ ? The point is: let us have good default behavior. Generally, two equal numbers are two conceptual references to the same thing. (Say, the Platonic form of the number.) So it is good that the hash value is determined by the number. Similarly for strings. Two equal numbers or strings are **also** identical, in the sense of having the same conceptual reference. In contrast, two equal cars are generally not identical in this sense. Of course you can make them so if you wish, but it is odd. So *nothing* is wrong here, imo. Btw: a = 12 b = 12 a == b True a is b True Cheers, Alan Isaac -- http://mail.python.org/mailman/listinfo/python-list
Re: equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
On 2007-06-29, Steve Holden [EMAIL PROTECTED] wrote: Just the same there are sound reasons for it, so I'd prefer to see you using counterintuitive or difficult to fathom rather than broken and wrong. You are quite correct, in the heat of typing an answer, my wording was too strong, I am sorry. Albert -- http://mail.python.org/mailman/listinfo/python-list
Re: equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
On 2007-06-28, Roy Smith [EMAIL PROTECTED] wrote: In article [EMAIL PROTECTED], A.T.Hofkamp [EMAIL PROTECTED] wrote: In object oriented programming, objects are representations of values, and the system shouldn't care about how many instances there are of some value, just like numbers in math. Every instance with a certain value is the same as every other instance with the same value. Whether two things are equal depends on the context. Is one $10 note equal to another? It depends. If the context is a bank teller making change, then yes, they are equal. What's more, there are lots of sets of smaller notes which would be equally fungible. If the context is a district attorney showing a specific $10 note to a jury as evidence in a drug buy-and-bust case, they're not. It's got to be exactly that note, as proven by a recorded serial number. In object oriented programming, objects are representations of the real world. In one case, the $10 note represents some monetary value. In another, it represents a piece of physical evidence in a criminal trial. Without knowing the context of how the objects are going to be used, it's really not possible to know how __eq__() should be defined. I can see your point, but am not sure I agree. The problem is that OO uses models tailored to an application, ie the model changes with each application. In a bank teller application, one would probably not model the serial number, just the notion of $10 notes would be enough, as in Note(value). The contents of a cash register would then for example be a dictionary of Note() objects to a count. You can merge two of such dictionaries, where the 'value' data of the Note objects would be the equivalence notion. In an evidence application one **would** record the serial number, since it is a relevant distinguishing feature between notes, ie one would model Note(value, serialnumber). In this application the combination of value and serial number together defines equivalence. However, also in this situation we use values of the model for equivalence. If we have a data base that relates evidence to storage location, and we would like to know where a particular note was stored, we would compare Note objects with each other based in the combination of value and serial number, not on their id()'s. You tell me. This is really just the does 1 == (1 + 0j) question in disguise. There's reasonable arguments to be made on both sides, but there is no one true answer. It depends on what you're doing. While we don't agree on how OO programming handles equality (and it may well be that there are multiple interpretations possible), wouldn't your argument also not lead to the conclusion that it is better not to have a pre-defined __eq__ method? Albert -- http://mail.python.org/mailman/listinfo/python-list
Re: equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
A.T.Hofkamp wrote: On 2007-06-29, Steve Holden [EMAIL PROTECTED] wrote: Just the same there are sound reasons for it, so I'd prefer to see you using counterintuitive or difficult to fathom rather than broken and wrong. You are quite correct, in the heat of typing an answer, my wording was too strong, I am sorry. No problem, I do the same thing myself ... regards Steve -- Steve Holden+1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden --- Asciimercial -- Get on the web: Blog, lens and tag the Internet Many services currently offer free registration --- Thank You for Reading - -- http://mail.python.org/mailman/listinfo/python-list
Re: equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
On 2007-06-27, Alex Martelli [EMAIL PROTECTED] wrote: A.T.Hofkamp [EMAIL PROTECTED] wrote: I think that again now with the default implementation of the object.__eq__ and object.__hash__ methods. I believe these methods should not exist until the programmer explicitly defines them with a suitable notion of equivalence. Anybody have a good argument against that? :-) It's very common and practical (though not ideologically pure!) to want each instance of a class to stand for itself, be equal only to itself: this lets me place instances in a set, etc, without fuss. Convenience is the big counter argument, and I have thought about that. I concluded that the convenience advantage is not big enough, and the problem seems to be what itself exactly means. In object oriented programming, objects are representations of values, and the system shouldn't care about how many instances there are of some value, just like numbers in math. Every instance with a certain value is the same as every other instance with the same value. You can also see this in the singleton concept. The fact that it is a pattern implies that it is special, something not delivered by default in object oriented programming. This object-oriented notion of itself is not what Python delivers. Python 2.4.4 (#1, Dec 15 2006, 13:51:44) [GCC 3.4.4 20050721 (Red Hat 3.4.4-2)] on linux2 Type help, copyright, credits or license for more information. class Car(object): ... def __init__(self, number): ... self.number = number ... def __repr__(self): ... return Car(%r) % self.number ... 12345 == 12345 True Car(123) == Car(123) False So in Python, the default equivalence notion for numbers is based on values, and the default equivalence notion for objects assumes singleton objects which is weird from an object oriented point of view. Therefore, I concluded that we are better off without a default __eq__ . The default existence of __hash__ gives other nasty surprises: class Car2(object): ...def __init__(self, number): ... self.number = number ...def __repr__(self): ... return Car2(%r) % self.number ...def __eq__(self, other): ... return self.number == other.number ... Above I have fixed Car to use value equivalence (albeit not very robust). Now if I throw these objects naively in a set: a = Car2(123) b = Car2(123) a == b True set([a,b]) set([Car2(123), Car2(123)]) I get a set with two equal cars, something that never happens with a set my math teacher once told me. Of course, I should have defined an appropiate __hash__ method together with the __eq__ method. Unfortunately, not every Python programmer has always had enough coffee to think about that when he is programming a class. Even worse, I may get a class such as the above from somebody else and decide that I need a set of such objects, something the original designer never intended. The problem is then that something like set([Car2(123), Car2(124)]) does the right thing for the wrong reason without telling me. Without a default __hash__ I'd get at least an error that I cannot put Car2 objects in a set. In that setup, I can still construct a broken set, but I'd have to write a broken __hash__ function explicitly rather than implicitly inheriting it from object. I don't want, in order to get that often-useful behavior, to have to code a lot of boilerplate such as def __hash__(self): return hash(id(self)) and the like -- so, I like the fact that object does it for me. I'd I understand that you'd like to have less typing to do. I'd like that too if only it would work without major accidents by simple omission such as demonstrated in the set example. Another question can be whether your coding style would be correct here. Since you apparently want to have singleton objects (since that is what you get and you are happy with them), shouldn't you be using is rather than ==? Then you get the equivalence notion you want, you don't need __eq__, and you write explicitly that you have singleton objects. In the same way, sets have very little value for singleton objects, you may as well use lists instead of sets since duplicate **values** are not filtered. For lists, you don't need __hash__ either. The only exception would be to filter multiple inclusions of the same object (that is what sets are doing by default). I don't know whether that would be really important for singleton objects **in general**. (ie wouldn't it be better to explicitly write a __hash__ based on identity for those cases?) have no objection if there were two variants of object (object itself and politically_correct_object), inheriting from each other either way 'round, one of which kept the current practical approach while the other made __hash__ and comparisons abstract. Or you define your own base object class class Myobject(object) and add a default __eq__ and __hash__ method. This at least gives an explicit definition of the equivalence
Re: equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
A.T.Hofkamp wrote: a = Car2(123) b = Car2(123) a == b True set([a,b]) set([Car2(123), Car2(123)]) I get a set with two equal cars, something that never happens with a set my math teacher once told me. Then your math teacher misspoke. You have two different cars in the set, just as expected. Use `is`. http://docs.python.org/ref/comparisons.html This is good behavior. Cheers, Alan Isaac -- http://mail.python.org/mailman/listinfo/python-list
Re: equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
On 2007-06-28, Alan Isaac [EMAIL PROTECTED] wrote: A.T.Hofkamp wrote: a = Car2(123) b = Car2(123) a == b True set([a,b]) set([Car2(123), Car2(123)]) I get a set with two equal cars, something that never happens with a set my math teacher once told me. Then your math teacher misspoke. You have two different cars in the set, just as expected. Use `is`. http://docs.python.org/ref/comparisons.html This is good behavior. Hmm, maybe numbers in sets are broken then? a = 12345 b = 12345 a == b True a is b False set([a,b]) set([12345]) Numbers and my Car2 objects behave the same w.r.t. '==' and 'is', yet I get a set with 1 number, and a set with 2 cars. Something is wrong here imho. The point I intended to make was that having a default __hash__ method on objects give weird results that not everybody may be aware of. In addition, to get useful behavior of objects in sets one should override __hash__ anyway, so what is the point of having a default object.__hash__ ? The one should override __hash__ anyway argument is being discussed in my previous post. Albert -- http://mail.python.org/mailman/listinfo/python-list
Re: equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
In article [EMAIL PROTECTED], A.T.Hofkamp [EMAIL PROTECTED] wrote: In object oriented programming, objects are representations of values, and the system shouldn't care about how many instances there are of some value, just like numbers in math. Every instance with a certain value is the same as every other instance with the same value. Whether two things are equal depends on the context. Is one $10 note equal to another? It depends. If the context is a bank teller making change, then yes, they are equal. What's more, there are lots of sets of smaller notes which would be equally fungible. If the context is a district attorney showing a specific $10 note to a jury as evidence in a drug buy-and-bust case, they're not. It's got to be exactly that note, as proven by a recorded serial number. In object oriented programming, objects are representations of the real world. In one case, the $10 note represents some monetary value. In another, it represents a piece of physical evidence in a criminal trial. Without knowing the context of how the objects are going to be used, it's really not possible to know how __eq__() should be defined. Let me give you a more realistic example. I've been doing a lot of network programming lately. We've got a class to represent an IP address, and a class to represent an address-port pair (a sockaddr). Should you be able to compare an address to a sockaddr? Does 192.168.10.1 == 192.168.10.1:0? You tell me. This is really just the does 1 == (1 + 0j) question in disguise. There's reasonable arguments to be made on both sides, but there is no one true answer. It depends on what you're doing. -- http://mail.python.org/mailman/listinfo/python-list
Re: equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
A.T.Hofkamp wrote: On 2007-06-28, Alan Isaac [EMAIL PROTECTED] wrote: A.T.Hofkamp wrote: a = Car2(123) b = Car2(123) a == b True set([a,b]) set([Car2(123), Car2(123)]) I get a set with two equal cars, something that never happens with a set my math teacher once told me. Then your math teacher misspoke. You have two different cars in the set, just as expected. Use `is`. http://docs.python.org/ref/comparisons.html This is good behavior. Hmm, maybe numbers in sets are broken then? a = 12345 b = 12345 a == b True a is b False set([a,b]) set([12345]) Numbers and my Car2 objects behave the same w.r.t. '==' and 'is', yet I get a set with 1 number, and a set with 2 cars. Something is wrong here imho. The point I intended to make was that having a default __hash__ method on objects give weird results that not everybody may be aware of. In addition, to get useful behavior of objects in sets one should override __hash__ anyway, so what is the point of having a default object.__hash__ ? The one should override __hash__ anyway argument is being discussed in my previous post. Hmm, I suspect you'll like this even less: set((1.0, 1, 1+0j)) set([1.0]) Just the same there are sound reasons for it, so I'd prefer to see you using counterintuitive or difficult to fathom rather than broken and wrong. Such language implies you have thought about this more deeply than the developers (which I frankly doubt) and that they made an inappropriate decision (which is less unlikely, but which in the case you mention I also rather doubt). regards Steve -- Steve Holden+1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden --- Asciimercial -- Get on the web: Blog, lens and tag the Internet Many services currently offer free registration --- Thank You for Reading - -- http://mail.python.org/mailman/listinfo/python-list
Re: equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
En Thu, 28 Jun 2007 11:38:56 -0300, A.T.Hofkamp [EMAIL PROTECTED] escribió: The point I intended to make was that having a default __hash__ method on objects give weird results that not everybody may be aware of. In addition, to get useful behavior of objects in sets one should override __hash__ anyway, so what is the point of having a default object.__hash__ ? __hash__ and equality tests are used by the dictionary implementation, and the default implementation is OK for immutable objects. I like the fact that I can use almost anything as dictionary keys without much coding. This must always be true: (a==b) = (hash(a)==hash(b)), and the documentation for __hash__ and __cmp__ warns about the requisites (but __eq__ and the other rich-comparison methods are lacking the warning). -- Gabriel Genellina -- http://mail.python.org/mailman/listinfo/python-list
Re: equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
On Fri, 29 Jun 2007 00:47:16 -0300 Gabriel Genellina [EMAIL PROTECTED] wrote: __hash__ and equality tests are used by the dictionary implementation, and the default implementation is OK for immutable objects. That is probably why inf == inf yields True. In this unique case, I do not like the default implementation. Martin -- http://mail.python.org/mailman/listinfo/python-list
equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
A.T.Hofkamp [EMAIL PROTECTED] wrote: I think that again now with the default implementation of the object.__eq__ and object.__hash__ methods. I believe these methods should not exist until the programmer explicitly defines them with a suitable notion of equivalence. Anybody have a good argument against that? :-) It's very common and practical (though not ideologically pure!) to want each instance of a class to stand for itself, be equal only to itself: this lets me place instances in a set, etc, without fuss. I don't want, in order to get that often-useful behavior, to have to code a lot of boilerplate such as def __hash__(self): return hash(id(self)) and the like -- so, I like the fact that object does it for me. I'd have no objection if there were two variants of object (object itself and politically_correct_object), inheriting from each other either way 'round, one of which kept the current practical approach while the other made __hash__ and comparisons abstract. In Python 3000, ordering comparisons will not exist by default (sigh, a modest loss of practicality on the altar of purity -- ah well, saw it coming, ever since complex numbers lost ordering comparisons), but equality and hashing should remain just like now (yay!). Alex -- http://mail.python.org/mailman/listinfo/python-list
Re: equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
In article [EMAIL PROTECTED], Alex Martelli [EMAIL PROTECTED] wrote: In Python 3000, ordering comparisons will not exist by default (sigh, a modest loss of practicality on the altar of purity -- ah well, saw it coming, ever since complex numbers lost ordering comparisons), but equality and hashing should remain just like now (yay!). While emotionally I agree with you, in practice I have come to agree with the POV that allowing default ordering comparisons between disjoint types causes subtle bugs that are more difficult to fix than the small amount of boilerplate needed to force comparisons when desired. -- Aahz ([EMAIL PROTECTED]) * http://www.pythoncraft.com/ as long as we like the same operating system, things are cool. --piranha -- http://mail.python.org/mailman/listinfo/python-list
Re: equality comparison by default (was Re: Too many 'self' in python.That's a big flaw in this language.)
Aahz wrote: In article [EMAIL PROTECTED], Alex Martelli [EMAIL PROTECTED] wrote: In Python 3000, ordering comparisons will not exist by default (sigh, a modest loss of practicality on the altar of purity -- ah well, saw it coming, ever since complex numbers lost ordering comparisons), but equality and hashing should remain just like now (yay!). While emotionally I agree with you, in practice I have come to agree with the POV that allowing default ordering comparisons between disjoint types causes subtle bugs that are more difficult to fix than the small amount of boilerplate needed to force comparisons when desired. I agree. It makes more sense to have to specify an ordering rather than assume an arbitrary one that may or may not have any relation to what you're interested in. I always did think that the inability to compare complex numbers was a bit of a wart -- not because it's not mathematically correct, since it is, but rather since everything else was comparable, even between distinct types -- but I think the more sensible approach is to allow equality by default (which defaults to identity), and only support comparisons when the user defines what it is he wants them to mean. -- Erik Max Francis [EMAIL PROTECTED] http://www.alcyone.com/max/ San Jose, CA, USA 37 20 N 121 53 W AIM, Y!M erikmaxfrancis Water which is too pure has no fish. -- Ts'ai Ken T'an -- http://mail.python.org/mailman/listinfo/python-list
