RE: data attributes override method attributes?
Terry Reedy wrote: On 9/25/2012 4:07 PM, Ian Kelly wrote: On Tue, Sep 25, 2012 at 1:58 PM, Terry Reedy tjre...@udel.edu wrote: On 9/25/2012 11:03 AM, Chris Angelico wrote: Instance attributes override (shadow) class attributes. except for (some? all?) special methods Those names are shadowed too. If you call foo.__len__() and the name is bound on the instance, it will call that function preferentially. It's just that when the special Python machinery calls the method, it skips the instance and goes straight to the class. I added Ian Kelly reminds me that instance.__xxx__ is only skipped by the internal machinery and not by direct accesses in user code. In the other hand, docs, official or otherwise, are filled with things like 'len(a) calls a.__len__', so I think something should be said that giving instances special method attributes does not have the effect one might expect. to the issue. Just to make sure I am following, if you call foo.__len__() it goes to the instance code while if you do len(foo) it will go to class.__len__()? If so, why? This email is confidential and subject to important disclaimers and conditions including on offers for the purchase or sale of securities, accuracy and completeness of information, viruses, confidentiality, legal privilege, and legal entity disclaimers, available at http://www.jpmorgan.com/pages/disclosures/email. -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
On Fri, Sep 28, 2012 at 12:02 PM, Prasad, Ramit ramit.pra...@jpmorgan.com wrote: Just to make sure I am following, if you call foo.__len__() it goes to the instance code while if you do len(foo) it will go to class.__len__()? Yes: class Foo(object): ... def __len__(self): ... return 42 ... foo = Foo() foo.__len__ = lambda: 43 foo.__len__() 43 len(foo) 42 If so, why? In the first case, foo.__len__ just does the normal attribute lookup for the class. Instance attributes shadow class attributes, so the instance attribute is returned and then called. In the second case, len(foo) is implemented by a method in a prescribed location: foo.__class__.__len__. It only looks in the class for efficiency and because that is what the class object is for: to define how its instances behave. -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
On 9/28/2012 2:02 PM, Prasad, Ramit wrote: Just to make sure I am following, if you call foo.__len__() it goes to the instance code while if you do len(foo) it will go to class.__len__()? len(foo) calls someclass.__len__(foo) where someclass is foo.__class__or some superclass. If so, why? Efficiency and perhaps simpler implementation, especially for binary ops. -- Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
On Fri, 28 Sep 2012 18:02:04 +, Prasad, Ramit wrote: Just to make sure I am following, if you call foo.__len__() it goes to the instance code while if you do len(foo) it will go to class.__len__()? If you call foo.__len__, the attribute lookup of __len__ will use the exact same search path as foo.spam would use: 1) does __getattribute__ exist and intercept the call? 2) if not, does a instance attribute exist? 3) if not, does a class attribute exist? 4) if not, does a superclass attribute exist? 5) if not, does __getattr__ exist and intercept the call? Using len(foo) bypasses steps 1) and 2) as a speed optimization. For the common case where an instance's class directly defines a __len__ method, that saves about 10-15% of the overhead of calling a special method, compared to old-style classes. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
On Sep 25, 11:41 pm, Jayden jayden.s...@gmail.com wrote: Dear All, In the Python Tutorial, Section 9.4, it is said that Data attributes override method attributes with the same name. But in my testing as follows: #Begin class A: i = 10 def i(): print 'i' A.i unbound method A.i #End I think A.i should be the number 10 but it is the method. There must be something I misunderstand. Would you please tell me why? What the tutorial is referring to is this, I think: class A(object): def i(self): print 'i' a = A() a.i = 10 a.i 10 That is, you can create a data attribute on an object even if a method attribute of the same name exists. -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
Jayden wrote: In the Python Tutorial, Section 9.4, it is said that Data attributes override method attributes with the same name. The tutorial is wrong here. That should be Instance attributes override class attributes with the same name. As methods are usually defined in the class and data attributes are usually set in the instance it will look like data override method attributes. What the author had in mind: class A: ... def i(self): print method ... a = A() a.i() method a.i = 42 # this could also happen in a method with self.i = 42 a.i() Traceback (most recent call last): File stdin, line 1, in module TypeError: 'int' object is not callable a.i 42 But in my testing as follows: #Begin class A: i = 10 def i(): print 'i' A.i unbound method A.i #End but class A: def i(self): print i i = 42 print A().i # 42 If two objects are assigned to the same name the last assignment always wins. I think A.i should be the number 10 but it is the method. There must be something I misunderstand. Would you please tell me why? No, you're right. Please file a bug report at http://bugs.python.org -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
On Sep 26, 12:08 am, Peter Otten __pete...@web.de wrote: Jayden wrote: In the Python Tutorial, Section 9.4, it is said that Data attributes override method attributes with the same name. The tutorial is wrong here. That should be Instance attributes override class attributes with the same name. As methods are usually defined in the class and data attributes are usually set in the instance it will look like data override method attributes. But you can assign attributes on the class, which has the same impact, so the tutorial is correct. No, you're right. Please file a bug report athttp://bugs.python.org Didn't you just demonstrate the behaviour you're now saying is a bug? -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
On Tue, 25 Sep 2012 06:41:43 -0700, Jayden wrote: Dear All, In the Python Tutorial, Section 9.4, it is said that Data attributes override method attributes with the same name. But in my testing as follows: #Begin class A: i = 10 def i(): print 'i' A.i unbound method A.i #End I think A.i should be the number 10 but it is the method. There must be something I misunderstand. Would you please tell me why? When you create the class, two things happen: first you define a class- level attribute i, then you define a method i. Since you can only have a single object with the same name in the same place, the method replaces the attribute. In this case, classes and methods are irrelevant. It is exactly the same behaviour as this: i = 10 i = 20 # i now equals 20, not 10 except that instead of 20, you use a function object: i = 10 def i(): return something # i is now a function object, not 10 What the manual refers to is the use of attributes on an instance: py class Test(object): ... def f(self): ... return something ... py t = Test() py t.f = 20 py t.f 20 In this case, there is an attribute called f (a method) which lives in the class and is shared by all instances, and another attribute called f which lives in the instance t, is not shared, and has the value 20. This instance attribute masks the method with the same name. We can see that it is only hidden, not gone, by creating a new instance: py u = Test() py u.f bound method Test.f of __main__.Test object at 0x871390c -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
alex23 wrote: On Sep 26, 12:08 am, Peter Otten __pete...@web.de wrote: Jayden wrote: In the Python Tutorial, Section 9.4, it is said that Data attributes override method attributes with the same name. The tutorial is wrong here. That should be Instance attributes override class attributes with the same name. As methods are usually defined in the class and data attributes are usually set in the instance it will look like data override method attributes. But you can assign attributes on the class, which has the same impact, so the tutorial is correct. No, you're right. Please file a bug report athttp://bugs.python.org Didn't you just demonstrate the behaviour you're now saying is a bug? To me Data attributes override method attributes with the same name implies that data attributes take precedence over method attributes, not that they replace them only when there is an assignment of data after the method definition. With your interpretation (if I understand you correctly) Method attributes override data attributes with the same name is equally correct, and therefore I think it is misleading to focus on the type of the attributes at all. I would even consider replacing the whole paragraph Data attributes override method attributes with the same name; to avoid accidental name conflicts, which may cause hard-to-find bugs in large programs, it is wise to use some kind of convention that minimizes the chance of conflicts. Possible conventions include capitalizing method names, prefixing data attribute names with a small unique string (perhaps just an underscore), or using verbs for methods and nouns for data attributes. http://docs.python.org/dev/py3k/tutorial/classes.html with something like Data attributes and method attributes share the same namespace. To avoid name conflicts consider using verbs for methods and nouns for data attributes -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
On Wed, Sep 26, 2012 at 12:54 AM, Peter Otten __pete...@web.de wrote: To me Data attributes override method attributes with the same name implies that data attributes take precedence over method attributes, not that they replace them only when there is an assignment of data after the method definition. I would even consider replacing the whole paragraph with something like Data attributes and method attributes share the same namespace. To avoid name conflicts consider using verbs for methods and nouns for data attributes Instance attributes override (shadow) class attributes. Since methods tend to be on the class and data tends to be on the instance, the original sentence does make some sense. The section is talking about conventions, so it's not inherently wrong, but perhaps just needs a comment about methods not usually being attached to the instance. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
Am 25.09.2012 16:11, schrieb alex23: On Sep 26, 12:08 am, Peter Otten __pete...@web.de wrote: Jayden wrote: In the Python Tutorial, Section 9.4, it is said that Data attributes override method attributes with the same name. The tutorial is wrong here. That should be Instance attributes override class attributes with the same name. As methods are usually defined in the class and data attributes are usually set in the instance it will look like data override method attributes. But you can assign attributes on the class, which has the same impact, so the tutorial is correct. You can assign attributes of the class or the instance, and you can assign with functions or data (actually, both functions and data are objects, Python doesn't make a distinction there). The important thing is that lookup first looks in the instance (where data attributes are usually set) before looking in the class (where method attributes are usually set). Observing typical use and deriving a rule from this is misleading though. No, you're right. Please file a bug report athttp://bugs.python.org Didn't you just demonstrate the behaviour you're now saying is a bug? I think he meant a bug in the tutorial, not in the implementation of Python. Uli -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
On 9/25/2012 11:03 AM, Chris Angelico wrote: On Wed, Sep 26, 2012 at 12:54 AM, Peter Otten __pete...@web.de wrote: To me Data attributes override method attributes with the same name implies that data attributes take precedence over method attributes, not that they replace them only when there is an assignment of data after the method definition. I would even consider replacing the whole paragraph with something like Data attributes and method attributes share the same namespace. To avoid name conflicts consider using verbs for methods and nouns for data attributes Instance attributes override (shadow) class attributes. except for (some? all?) special methods Since methods tend to be on the class and data tends to be on the instance, the original sentence does make some sense. but it *is* wrong The section is talking about conventions, so it's not inherently wrong, The suggestion to Capitalize method names and prefix data names with '_' are wrong with respect to the style guide. but perhaps just needs a comment about methods not usually being attached to the instance. ChrisA -- Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
Am 25.09.2012 16:08 schrieb Peter Otten: Jayden wrote: In the Python Tutorial, Section 9.4, it is said that Data attributes override method attributes with the same name. The tutorial is wrong here. That should be Instance attributes override class attributes with the same name. I jump in here: THere is one point to consider: if you work with descriptors, it makes a difference if they are data descriptors (define __set__ and/or __delete__) or non-data descriptors (define neither). As http://docs.python.org/reference/datamodel.html#invoking-descriptors tells us, methods are non-data descriptors, so they can be overridden by instances. OTOH, properties are data descriptors which cannot be overridden by the instance. So, to stick to the original example: class TestDesc(object): def a(self): pass @property def b(self): print trying to get value - return None; return None @b.setter def b(self, v): print value, v, ignored. @b.deleter def b(self): print delete called and ignored and now t=TestDesc() t.a bound method TestDesc.a of __main__.TestDesc object at 0xb7387ccc t.b trying to get value - return None t.a=12 t.b=12 value 12 ignored. t.a 12 t.b trying to get value - return None del t.a del t.b delete called and ignored t.a bound method TestDesc.a of __main__.TestDesc object at 0xb7387ccc t.b trying to get value - return None Thomas -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
On Tue, Sep 25, 2012 at 1:58 PM, Terry Reedy tjre...@udel.edu wrote: On 9/25/2012 11:03 AM, Chris Angelico wrote: Instance attributes override (shadow) class attributes. except for (some? all?) special methods Those names are shadowed too. If you call foo.__len__() and the name is bound on the instance, it will call that function preferentially. It's just that when the special Python machinery calls the method, it skips the instance and goes straight to the class. -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
On 9/25/2012 10:54 AM, Peter Otten wrote: alex23 wrote: On Sep 26, 12:08 am, Peter Otten __pete...@web.de wrote: Jayden wrote: In the Python Tutorial, Section 9.4, it is said that Data attributes override method attributes with the same name. The tutorial is wrong here. That should be Instance attributes override class attributes with the same name. Except for special methods. I would even consider replacing the whole paragraph I agree Data attributes override method attributes with the same name; to avoid accidental name conflicts, which may cause hard-to-find bugs in large programs, it is wise to use some kind of convention that minimizes the chance of conflicts. Possible conventions include capitalizing method names, prefixing data attribute names with a small unique string (perhaps just an underscore), or using verbs for methods and nouns for data attributes. http://docs.python.org/dev/py3k/tutorial/classes.html with something like Data attributes and method attributes share the same namespace. and instance attributes usually override class attributes To avoid name conflicts consider using verbs for methods and nouns for data attributes This applies within and between. I opened http://bugs.python.org/issue16048 -- Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: data attributes override method attributes?
On 9/25/2012 4:07 PM, Ian Kelly wrote: On Tue, Sep 25, 2012 at 1:58 PM, Terry Reedy tjre...@udel.edu wrote: On 9/25/2012 11:03 AM, Chris Angelico wrote: Instance attributes override (shadow) class attributes. except for (some? all?) special methods Those names are shadowed too. If you call foo.__len__() and the name is bound on the instance, it will call that function preferentially. It's just that when the special Python machinery calls the method, it skips the instance and goes straight to the class. I added Ian Kelly reminds me that instance.__xxx__ is only skipped by the internal machinery and not by direct accesses in user code. In the other hand, docs, official or otherwise, are filled with things like 'len(a) calls a.__len__', so I think something should be said that giving instances special method attributes does not have the effect one might expect. to the issue. -- Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
