Re: Scope and classes
Chris Rebert a écrit : (snip) To access class-level variables from within instance methods of the class, you have 2 options: A. Use the class name, i.e. Abc.message B. Reference the class indirectly, i.e. self.__class__.message Or even simpler - *if* there's no synonym instance attribute: = self.message Attribute lookup will try to resolve the attribute name on the class (and it's parent classes) if it's not found on the instance. -- http://mail.python.org/mailman/listinfo/python-list
Re: Scope and classes
David a écrit : (snip) Out of 'Abc.message' and 'self.message', which is the favoured convention? It would be very easy to accidentally override 'self.messages' with an instance attribute! Only use 'Abc.message' if you want to make sure you get the Abc class 'message' attribute - that is, if you want to skip possible overrides in subclasses. As far as I'm concerned, I'd tend to consider this bad style unless it's a very very specific implementation point of an abstract class (in which case it would probably be named '__message' to prevent accidental override) - but YMMV of course. Use 'self.__class__.message' (or 'type(self).message') if you want to make sure you get the class 'message' attribute - that is, if you want to honor possible overrides in subclasses, but not per-instance override. But then - at least in your example code - I'd use a classmethod: class Abc: message = 'Hello World' @classmethod def print_message(cls): print cls.message Now the most common idiom - that is, outside classmethods - is to just use 'self.message'. Someone (even you) might have a very valid reason to override the 'message' attribute on a per-instance basis. FWIW, if you start to worry about possible accidental overrides here, then Python might not be the right language for you - nothing prevents accidental overrides of method and even the class (the '__class__' attribute) itself - yes, they are all attributes, and you can dynamically override them !-) -- http://mail.python.org/mailman/listinfo/python-list
Scope and classes
Hi all, I'm trying to understand how scopes work within a class definition. I'll quickly illustrate with an example. Say I had the following class definition: class Abc: message = 'Hello World' def print_message(self): print message instance = Abc() instance.print_message() NameError: global name 'message' not defined My question is, why? message is not defined in print_message, but it is defined in the enclosing scope (the class)? Thanks! -- http://mail.python.org/mailman/listinfo/python-list
Re: Scope and classes
On Tue, Aug 18, 2009 at 3:47 PM, Daviddavidsh...@googlemail.com wrote: Hi all, I'm trying to understand how scopes work within a class definition. I'll quickly illustrate with an example. Say I had the following class definition: class Abc: message = 'Hello World' def print_message(self): print message instance = Abc() instance.print_message() NameError: global name 'message' not defined My question is, why? message is not defined in print_message, but it is defined in the enclosing scope (the class)? A class' scope is never consulted when resolving variable names in its methods. The scopes consulted are roughly (and in this order): 1. Local variable scope (i.e. of the current function/method) 2. Enclosing functions (if you have functions nested inside other functions) 3. Globals (i.e. module-level variables) 3. Builtins (i.e. the built-in functions and methods, such as len()) To access class-level variables from within instance methods of the class, you have 2 options: A. Use the class name, i.e. Abc.message B. Reference the class indirectly, i.e. self.__class__.message Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
Re: Scope and classes
On Aug 19, 12:16 am, Chris Rebert c...@rebertia.com wrote: On Tue, Aug 18, 2009 at 3:47 PM, Daviddavidsh...@googlemail.com wrote: Hi all, I'm trying to understand how scopes work within a class definition. I'll quickly illustrate with an example. Say I had the following class definition: class Abc: message = 'Hello World' def print_message(self): print message instance = Abc() instance.print_message() NameError: global name 'message' not defined My question is, why? message is not defined in print_message, but it is defined in the enclosing scope (the class)? A class' scope is never consulted when resolving variable names in its methods. The scopes consulted are roughly (and in this order): 1. Local variable scope (i.e. of the current function/method) 2. Enclosing functions (if you have functions nested inside other functions) 3. Globals (i.e. module-level variables) 3. Builtins (i.e. the built-in functions and methods, such as len()) To access class-level variables from within instance methods of the class, you have 2 options: A. Use the class name, i.e. Abc.message B. Reference the class indirectly, i.e. self.__class__.message Cheers, Chris --http://blog.rebertia.com Ah, thanks! -- http://mail.python.org/mailman/listinfo/python-list
Re: Scope and classes
On Tue, 18 Aug 2009 15:47:09 -0700, David wrote: Hi all, I'm trying to understand how scopes work within a class definition. I'll quickly illustrate with an example. Say I had the following class definition: class Abc: message = 'Hello World' def print_message(self): print message instance = Abc() instance.print_message() NameError: global name 'message' not defined My question is, why? message is not defined in print_message, but it is defined in the enclosing scope (the class)? It's a deliberate design decision. I don't recall the reasons for it, but the class namespace is deliberately excluded from the method scope. The correct way of writing the above is: class Abc: message = 'Hello World' def print_message(self): print self.message self.message will first search the instance namespace for an attribute `message`, then search the class, then any superclasses. If you specifically want the class attribute, even if the instance masks it with an instance attribute of the same name, you do this instead: print self.__class__.message -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Scope and classes
19-08-2009 o 00:47:09 David davidsh...@googlemail.com wrote: Hi all, I'm trying to understand how scopes work within a class definition. I'll quickly illustrate with an example. Say I had the following class definition: class Abc: message = 'Hello World' def print_message(self): print message instance = Abc() instance.print_message() NameError: global name 'message' not defined My question is, why? message is not defined in print_message, but it is defined in the enclosing scope (the class)? Note that when *running* the code that is *inside* print_message() method called within the global scope, the outer (enclosing) scope is indeed the global scope (the scope in which e.g. 'instance' name exists; note that e.g. 'print instance' in that method would be ok). As Chris wrote, class' scope (contrary to outer function scope) is not considered as the 'enclosing' scope. The only ways to reach Abc's attribute 'message' from that method are: * 'Abc.message' * 'self.__class__.message' * 'self.message' (unless there is an instance attribute 'message' which overrides the class attribute). Cheers, *j -- Jan Kaliszewski (zuo) z...@chopin.edu.pl -- http://mail.python.org/mailman/listinfo/python-list
Re: Scope and classes
19-08-2009 o 02:10:58 Jan Kaliszewski z...@chopin.edu.pl wrote: The only ways to reach Abc's attribute 'message' from that method are: * 'Abc.message' * 'self.__class__.message' * 'self.message' (unless there is an instance attribute 'message' which overrides the class attribute). And of course getattr(Abc), getattr(self.__class__) etc. :-) -- Jan Kaliszewski (zuo) z...@chopin.edu.pl -- http://mail.python.org/mailman/listinfo/python-list
Re: Scope and classes
On Aug 19, 1:17 am, Jan Kaliszewski z...@chopin.edu.pl wrote: 19-08-2009 o 02:10:58 Jan Kaliszewski z...@chopin.edu.pl wrote: The only ways to reach Abc's attribute 'message' from that method are: * 'Abc.message' * 'self.__class__.message' * 'self.message' (unless there is an instance attribute 'message' which overrides the class attribute). And of course getattr(Abc), getattr(self.__class__) etc. :-) -- Jan Kaliszewski (zuo) z...@chopin.edu.pl Out of 'Abc.message' and 'self.message', which is the favoured convention? It would be very easy to accidentally override 'self.messages' with an instance attribute! -- http://mail.python.org/mailman/listinfo/python-list
Re: Scope and classes
Ben Finney ben+pyt...@benfinney.id.au writes: Right. So you use ‘Abc.message’ when you specifically want a class independent of any instance That's “a class attribute independent of any instance”, of course. -- \“Humanity has advanced, when it has advanced, not because it | `\ has been sober, responsible, and cautious, but because it has | _o__)been playful, rebellious, and immature.” —Tom Robbins | Ben Finney -- http://mail.python.org/mailman/listinfo/python-list
