Re: [Tutor] Keeping a list of attributes of a certain type
Thanks everybody for all the suggestions and ideas. I still did not have time to look them all over, but I will. Thanks again! Best, Guilherme On Thu, Jan 14, 2010 at 4:52 AM, Alan Gauld alan.ga...@btinternet.com wrote: Guilherme P. de Freitas guilhe...@gpfreitas.com wrote Here is my problem. I have two classes, 'Body' and 'Member', and some attributes of 'Body' can be of type 'Member', but some may not. The precise attributes that 'Body' has depend from instance to instance, and they can be added or deleted. I need any instance of 'Body' to keep an up-to-date list of all its attributes that belong to the class 'Member'. How do I do this? Check the type of the attribute? Using isinstance()? I'm not sure of the rationale - why it needs to know the type etc But isinstance is the most obvious way that I can think of. Alan G. ___ Tutor maillist - tu...@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor -- Guilherme P. de Freitas http://www.gpfreitas.com ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Keeping a list of attributes of a certain type
Ok, I checked the answers you all gave me, and the suggestions seem to be 2: a. Compute the list of members on the fly, with a list comprehension that looks up stuff on self.__dict___; b. Stay with my solution, but substituting the unnecessary if-else by a simple if. In sum, maintain a list of members that is updated whenever a member is added or removed. Which one is better? I am new to all this, so please correct me if I am wrong. It seems that (a) is better if you do a lot of adding/removing members operations compared to the number of lookups to the members list. Otherwise, (b) is better. Right? In my application, I will do many more lookups on the members list than adding/removing members (this sounds weird, feels like I'm Dr. Frankenstein), so I will go with (b). Thanks again for all the suggestions, Guilherme -- Guilherme P. de Freitas http://www.gpfreitas.com ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Keeping a list of attributes of a certain type
On Fri, Jan 15, 2010 at 3:43 PM, Guilherme P. de Freitas guilhe...@gpfreitas.com wrote: Ok, I checked the answers you all gave me, and the suggestions seem to be 2: a. Compute the list of members on the fly, with a list comprehension that looks up stuff on self.__dict___; b. Stay with my solution, but substituting the unnecessary if-else by a simple if. In sum, maintain a list of members that is updated whenever a member is added or removed. Which one is better? I am new to all this, so please correct me if I am wrong. It seems that (a) is better if you do a lot of adding/removing members operations compared to the number of lookups to the members list. Otherwise, (b) is better. Right? Personally (b) feels more direct and robust. There is no guarantee that attributes are stored in self.__dict__, they can be in __slots__ or delegated to some other container. For example, here is a class with no __dict__: In [1]: class Foo(object): ...: __slots__ = ['bar'] ...: def __setattr__(self, name, value): ...: print 'set', name, 'to', value ...: object.__setattr__(self, name, value) In [2]: f=Foo() In [3]: f.bar = 'baz' set bar to baz In [4]: f.__dict__ --- AttributeErrorTraceback (most recent call last) C:\Project\(misc)\MangoDB\ipython console in module() AttributeError: 'Foo' object has no attribute '__dict__' Granted if you have control over all the classes this won't happen but still...relying on __dict__ makes me nervous. Kent ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Keeping a list of attributes of a certain type
(You top-posted, which puts your two comments out of order. Now the solution comes before the problem statement) Guilherme P. de Freitas wrote: Ok, I got something that seems to work for me. Any comments are welcome. class Member(object): def __init__(self): pass class Body(object): def __init__(self): self.members = [] def __setattr__(self, obj, value): if isinstance(value, Member): self.members.append(obj) object.__setattr__(self, obj, value) else: object.__setattr__(self, obj, value) def __delattr__(self, obj): if isinstance(getattr(self, obj), Member): self.members.remove(obj) object.__delattr__(self, obj) else: object.__delattr__(self, obj) john = Body() john.arm = Member() print(john.members) del john.arm print(john.members) On Wed, Jan 13, 2010 at 6:24 PM, Guilherme P. de Freitas guilhe...@gpfreitas.com wrote: Hi everybody, Here is my problem. I have two classes, 'Body' and 'Member', and some attributes of 'Body' can be of type 'Member', but some may not. The precise attributes that 'Body' has depend from instance to instance, and they can be added or deleted. I need any instance of 'Body' to keep an up-to-date list of all its attributes that belong to the class 'Member'. How do I do this? Best, Guilherme -- Guilherme P. de Freitas http://www.gpfreitas.com If this is a class assignment, you've probably got the desired answer. But if it's a real-world problem, there are tradeoffs, and until those are known, the simplest solution is usually the best. Usually, what's desired is that the object behaves as if it has an up-to-date list of... If order of the list doesn't matter, I'd consider simply writing a single method, called 'members' which does a list comprehension of the object's attributes, calculating the list when needed. Then use a decorator to make this method look like a read-only data attribute (untested): class Body (object): @property def members(self): return [obj for . if ] DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Keeping a list of attributes of a certain type
On Wed, Jan 13, 2010 at 9:24 PM, Guilherme P. de Freitas guilhe...@gpfreitas.com wrote: Hi everybody, Here is my problem. I have two classes, 'Body' and 'Member', and some attributes of 'Body' can be of type 'Member', but some may not. The precise attributes that 'Body' has depend from instance to instance, and they can be added or deleted. I need any instance of 'Body' to keep an up-to-date list of all its attributes that belong to the class 'Member'. How do I do this? If you want to keep track of attributes as they are added and deleted, you should override __setattr__() and __delattr__() in your Body class. http://docs.python.org/reference/datamodel.html#customizing-attribute-access Here is a simple example: In [4]: class LogAttributes(object): ...: def __setattr__(self, name, value): ...: print 'setting', name, 'to', value, type(value) ...: object.__setattr__(self, name, value) ...: def __delattr__(self, name): ...: print 'removing', name ...: object.__delattr__(self, name) In [5]: l = LogAttributes() In [6]: l.foo = 'bar' setting foo to bar type 'str' In [7]: l.foo Out[7]: 'bar' In [8]: del l.foo removing foo Kent ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Keeping a list of attributes of a certain type
On Wed, Jan 13, 2010 at 11:15 PM, Guilherme P. de Freitas guilhe...@gpfreitas.com wrote: Ok, I got something that seems to work for me. Any comments are welcome. class Member(object): def __init__(self): pass class Body(object): def __init__(self): self.members = [] def __setattr__(self, obj, value): if isinstance(value, Member): self.members.append(obj) object.__setattr__(self, obj, value) else: object.__setattr__(self, obj, value) That's fine but there is no need to duplicate the object.__setattr__() call: def __setattr__(self, obj, value): if isinstance(value, Member): self.members.append(obj) object.__setattr__(self, obj, value) def __delattr__(self, obj): if isinstance(getattr(self, obj), Member): self.members.remove(obj) object.__delattr__(self, obj) else: object.__delattr__(self, obj) Same here. Kent ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Keeping a list of attributes of a certain type
On Wed, 13 Jan 2010 20:15:21 -0800 Guilherme P. de Freitas guilhe...@gpfreitas.com wrote: Ok, I got something that seems to work for me. Any comments are welcome. class Member(object): def __init__(self): pass class Body(object): def __init__(self): self.members = [] def __setattr__(self, obj, value): if isinstance(value, Member): self.members.append(obj) object.__setattr__(self, obj, value) else: object.__setattr__(self, obj, value) def __delattr__(self, obj): if isinstance(getattr(self, obj), Member): self.members.remove(obj) object.__delattr__(self, obj) else: object.__delattr__(self, obj) Seems perfectly ok to me, except for if...else constructs in place of simple if's: an optional statement is added in case obj is of type Member; this is not an alternative. if isinstance(value, Member): self.members.append(obj) object.__setattr__(self, obj, value)# in both cases Same for __delattr__, indeed. Denis la vita e estrany http://spir.wikidot.com/ ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Keeping a list of attributes of a certain type
Guilherme P. de Freitas guilhe...@gpfreitas.com wrote Here is my problem. I have two classes, 'Body' and 'Member', and some attributes of 'Body' can be of type 'Member', but some may not. The precise attributes that 'Body' has depend from instance to instance, and they can be added or deleted. I need any instance of 'Body' to keep an up-to-date list of all its attributes that belong to the class 'Member'. How do I do this? Check the type of the attribute? Using isinstance()? I'm not sure of the rationale - why it needs to know the type etc But isinstance is the most obvious way that I can think of. Alan G. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] Keeping a list of attributes of a certain type
Hi everybody, Here is my problem. I have two classes, 'Body' and 'Member', and some attributes of 'Body' can be of type 'Member', but some may not. The precise attributes that 'Body' has depend from instance to instance, and they can be added or deleted. I need any instance of 'Body' to keep an up-to-date list of all its attributes that belong to the class 'Member'. How do I do this? Best, Guilherme -- Guilherme P. de Freitas http://www.gpfreitas.com ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Keeping a list of attributes of a certain type
Ok, I got something that seems to work for me. Any comments are welcome. class Member(object): def __init__(self): pass class Body(object): def __init__(self): self.members = [] def __setattr__(self, obj, value): if isinstance(value, Member): self.members.append(obj) object.__setattr__(self, obj, value) else: object.__setattr__(self, obj, value) def __delattr__(self, obj): if isinstance(getattr(self, obj), Member): self.members.remove(obj) object.__delattr__(self, obj) else: object.__delattr__(self, obj) john = Body() john.arm = Member() print(john.members) del john.arm print(john.members) On Wed, Jan 13, 2010 at 6:24 PM, Guilherme P. de Freitas guilhe...@gpfreitas.com wrote: Hi everybody, Here is my problem. I have two classes, 'Body' and 'Member', and some attributes of 'Body' can be of type 'Member', but some may not. The precise attributes that 'Body' has depend from instance to instance, and they can be added or deleted. I need any instance of 'Body' to keep an up-to-date list of all its attributes that belong to the class 'Member'. How do I do this? Best, Guilherme -- Guilherme P. de Freitas http://www.gpfreitas.com -- Guilherme P. de Freitas http://www.gpfreitas.com ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
