Re: Inheritance and forward references (prototypes)
On 21 Jun., 22:51, Scott David Daniels scott.dani...@acm.org wrote: LorenzoDiGregoriowrote: On 21 Jun., 01:54, Dave Angel da...@ieee.org wrote: ... class B(object): def __init__(self,test=None): if test==None: test = A() self.obj =() return ... I had also thought of using None (or whatever else) as a marker but I was curious to find out whether there are better ways to supply an object with standard values as a default argument. In this sense, I was looking for problems ;-) Of course the observation that def is an instruction and no declaration changes the situation: I would not have a new object being constructed for every instantiation with no optional argument, because __init__ gets executed on the instantiation but test=A() gets executed on reading 'def' If what you are worrying about is having a single default object, you could do something like this: class B(object): _default = None def __init__(self, test=None): if test is None: test = self._default if test is None: B._default = test = A() ... --Scott David Daniels scott.dani...@acm.org- Zitierten Text ausblenden - - Zitierten Text anzeigen - Well, I could also declare (ups, define ;-)) __init__(self,**kwargs) and within the __init__, if kwargs['test'] exists, do test = kwargs ['test'], if it does not exist, do test = A(). The point is that it would have been cleaner to place it straight in the __init__, but due to the semantic of 'def' this does not seem possible. -- http://mail.python.org/mailman/listinfo/python-list
Re: Inheritance and forward references (prototypes)
On 21 Jun., 01:54, Dave Angel da...@ieee.org wrote: LorenzoDiGregoriowrote: On Jun 20, 8:43 pm, Dave Angel da...@ieee.org wrote: LorenzoDiGregoriowrote: Hi, I'm wondering what would be the preferred way to solve the following forward reference problem: --- class BaseA(object): def __init__(self): return class DebugA(BaseA): def __init__(self): return # here I would have a prototype of class A which is the same as class BaseA class B(object): def __init__(self): self.obj =() return if __name__ =__main__: # class A(BaseA): # Uncomment this for using BaseA objects # pass class A(DebugA): # Uncomment this for using DebugA objects pass --- I can figure out some ways to fix this but none seems satisfying. Either they are too specific or too cumbersome. A runtime redefinition of class A does not seem to work either. What would be the most pythonesque solution other than sorting out the class order? Best Regards, Lorenzo You haven't shown us any problem. class B works fine with a forward reference to A. Now if you were trying to subclass A before defining it, that'd be a problem. Or if you were trying to make an instance of B before defining A. Better put some code together with enough meat to actually show a symptom. And tell us what sys.version says. I'm testing with 2.6.2 (r262:71605, Apr 14 2009, 22:40:02) [MSC v.1500 32 bit (Intel)], running on Win XP.- Hide quoted text - - Show quoted text - Thank you for your help: I'm working on a rather large source, but I think I have isolated the problem now. This listing generates an error: --- class BaseA(object): def __init__(self): return class DebugA(BaseA): def __init__(self): return class B(object): def __init__(self,test=A()): self.obj =() return if __name__ =__main__: # class A(BaseA): # Uncomment this for using BaseA objects # pass class A(DebugA): # Uncomment this for using DebugA objects pass --- The error happens because Python apparently evaluates the named arguments before running the script. I think I have read something about this some (long) time ago but I can't find it anymore. Suggestions? BTW, my Python version is 2.6.1 (with latest PyDev). Thx! Lorenzo This error is caused because a default argument uses class A. Default arguments of class methods are evaluated during the definition of the class, and not later when the class is instantiated. Thus the problem. To work around that specific problem, you may want to use the following: class B(object): def __init__(self,test=None): if test==None: test = A() self.obj =() return This is actually different than what you had, since what you had would have used the same A() object for all instances of B that didn't supply their own test() parameter. Maybe that's what you wanted, and maybe not, but default arguments set to mutable values are frequently a bug. But I'm wondering if you're just looking for problems. Why not put the commented switch early in the file, and test for it wherever you need to use it? import x, y, z _debug = False #_debug = True then as soon as BaseA and DebugA are defined, do the following: if _debug: class A(DebugA): pass else: class A(BaseA) pass- Zitierten Text ausblenden - - Zitierten Text anzeigen - I had also thought of using None (or whatever else) as a marker but I was curious to find out whether there are better ways to supply an object with standard values as a default argument. In this sense, I was looking for problems ;-) Of course the observation that def is an instruction and no declaration changes the situation: I would not have a new object being constructed for every instantiation with no optional argument, because __init__ gets executed on the instantiation but test=A() gets executed on reading 'def'. At this point I think there is no other way than using a marker as suggested above multiple times, if I want to supply a new object with default values for non-passed arguments. Anybody with a better idea? -- http://mail.python.org/mailman/listinfo/python-list
Re: Inheritance and forward references (prototypes)
Lorenzo Di Gregorio wrote: I had also thought of using None (or whatever else) as a marker but I was curious to find out whether there are better ways to supply an object with standard values as a default argument. In this sense, I was looking for problems ;-) Of course the observation that def is an instruction and no declaration changes the situation: I would not have a new object being constructed for every instantiation with no optional argument, because __init__ gets executed on the instantiation but test=A() gets executed on reading 'def'. At this point I think there is no other way than using a marker as suggested above multiple times, if I want to supply a new object with default values for non-passed arguments. Using None as default for mutable default argument is the common idiom for the problem you're having. -- http://mail.python.org/mailman/listinfo/python-list
Re: Inheritance and forward references (prototypes)
Lorenzo Di Gregorio wrote: On 21 Jun., 01:54, Dave Angel da...@ieee.org wrote: ... class B(object): def __init__(self,test=None): if test==None: test = A() self.obj =() return ... I had also thought of using None (or whatever else) as a marker but I was curious to find out whether there are better ways to supply an object with standard values as a default argument. In this sense, I was looking for problems ;-) Of course the observation that def is an instruction and no declaration changes the situation: I would not have a new object being constructed for every instantiation with no optional argument, because __init__ gets executed on the instantiation but test=A() gets executed on reading 'def' If what you are worrying about is having a single default object, you could do something like this: class B(object): _default = None def __init__(self, test=None): if test is None: test = self._default if test is None: B._default = test = A() ... --Scott David Daniels scott.dani...@acm.org -- http://mail.python.org/mailman/listinfo/python-list
Inheritance and forward references (prototypes)
Hi, I'm wondering what would be the preferred way to solve the following forward reference problem: --- class BaseA(object): def __init__(self): return class DebugA(BaseA): def __init__(self): return # here I would have a prototype of class A which is the same as class BaseA class B(object): def __init__(self): self.obj = A() return if __name__ == __main__: #class A(BaseA): # Uncomment this for using BaseA objects # pass class A(DebugA): # Uncomment this for using DebugA objects pass --- I can figure out some ways to fix this but none seems satisfying. Either they are too specific or too cumbersome. A runtime redefinition of class A does not seem to work either. What would be the most pythonesque solution other than sorting out the class order? Best Regards, Lorenzo -- http://mail.python.org/mailman/listinfo/python-list
Re: Inheritance and forward references (prototypes)
Lorenzo Di Gregorio wrote: Hi, I'm wondering what would be the preferred way to solve the following forward reference problem: You don't actually explain what is the problem. Fortunately, I'm good at guessing, and I think I can guess what your problem is (see below): --- class BaseA(object): def __init__(self): return class DebugA(BaseA): def __init__(self): return # here I would have a prototype of class A which is the same as class BaseA class B(object): def __init__(self): self.obj = A() return if __name__ == __main__: #class A(BaseA): # Uncomment this for using BaseA objects # pass class A(DebugA): # Uncomment this for using DebugA objects pass --- Class A only gets defined if you run the module as a script. What you need is to unconditionally define class A, outside of the if __name__ block: class A(BaseA): pass # A.__base__ = DebugA ## Uncomment this line for debugging. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Inheritance and forward references (prototypes)
Lorenzo Di Gregorio wrote: Hi, I'm wondering what would be the preferred way to solve the following forward reference problem: --- class BaseA(object): def __init__(self): return class DebugA(BaseA): def __init__(self): return # here I would have a prototype of class A which is the same as class BaseA class B(object): def __init__(self): self.obj = A() return if __name__ == __main__: #class A(BaseA): # Uncomment this for using BaseA objects # pass class A(DebugA): # Uncomment this for using DebugA objects pass --- I can figure out some ways to fix this but none seems satisfying. Either they are too specific or too cumbersome. A runtime redefinition of class A does not seem to work either. What would be the most pythonesque solution other than sorting out the class order? Best Regards, Lorenzo You haven't shown us any problem. class B works fine with a forward reference to A. Now if you were trying to subclass A before defining it, that'd be a problem. Or if you were trying to make an instance of B before defining A. Better put some code together with enough meat to actually show a symptom. And tell us what sys.version says. I'm testing with 2.6.2 (r262:71605, Apr 14 2009, 22:40:02) [MSC v.1500 32 bit (Intel)], running on Win XP. -- http://mail.python.org/mailman/listinfo/python-list
Re: Inheritance and forward references (prototypes)
Steven D'Aprano st...@removethis.cybersource.com.au (SD) wrote: SD Lorenzo Di Gregorio wrote: Hi, I'm wondering what would be the preferred way to solve the following forward reference problem: SD You don't actually explain what is the problem. Fortunately, I'm good at SD guessing, and I think I can guess what your problem is (see below): --- class BaseA(object): def __init__(self): return class DebugA(BaseA): def __init__(self): return # here I would have a prototype of class A which is the same as class BaseA class B(object): def __init__(self): self.obj = A() return if __name__ == __main__: #class A(BaseA): # Uncomment this for using BaseA objects # pass class A(DebugA): # Uncomment this for using DebugA objects pass --- SD Class A only gets defined if you run the module as a script. What you need SD is to unconditionally define class A, outside of the if __name__ block: SD class A(BaseA): SD pass SD # A.__base__ = DebugA ## Uncomment this line for debugging. A.__base__ = DebugA TypeError: readonly attribute Make that: A.__bases__ = DebugA, SD -- SD Steven -- Piet van Oostrum p...@cs.uu.nl URL: http://pietvanoostrum.com [PGP 8DAE142BE17999C4] Private email: p...@vanoostrum.org -- http://mail.python.org/mailman/listinfo/python-list
Re: Inheritance and forward references (prototypes)
On Jun 20, 8:43 pm, Dave Angel da...@ieee.org wrote: Lorenzo Di Gregorio wrote: Hi, I'm wondering what would be the preferred way to solve the following forward reference problem: --- class BaseA(object): def __init__(self): return class DebugA(BaseA): def __init__(self): return # here I would have a prototype of class A which is the same as class BaseA class B(object): def __init__(self): self.obj = A() return if __name__ == __main__: # class A(BaseA): # Uncomment this for using BaseA objects # pass class A(DebugA): # Uncomment this for using DebugA objects pass --- I can figure out some ways to fix this but none seems satisfying. Either they are too specific or too cumbersome. A runtime redefinition of class A does not seem to work either. What would be the most pythonesque solution other than sorting out the class order? Best Regards, Lorenzo You haven't shown us any problem. class B works fine with a forward reference to A. Now if you were trying to subclass A before defining it, that'd be a problem. Or if you were trying to make an instance of B before defining A. Better put some code together with enough meat to actually show a symptom. And tell us what sys.version says. I'm testing with 2.6.2 (r262:71605, Apr 14 2009, 22:40:02) [MSC v.1500 32 bit (Intel)], running on Win XP.- Hide quoted text - - Show quoted text - Thank you for your help: I'm working on a rather large source, but I think I have isolated the problem now. This listing generates an error: --- class BaseA(object): def __init__(self): return class DebugA(BaseA): def __init__(self): return class B(object): def __init__(self,test=A()): self.obj = A() return if __name__ == __main__: #class A(BaseA): # Uncomment this for using BaseA objects #pass class A(DebugA): # Uncomment this for using DebugA objects pass --- The error happens because Python apparently evaluates the named arguments before running the script. I think I have read something about this some (long) time ago but I can't find it anymore. Suggestions? BTW, my Python version is 2.6.1 (with latest PyDev). Thx! Lorenzo -- http://mail.python.org/mailman/listinfo/python-list
Inheritance and forward references (prototypes)
Arg, forgot to post to the mailing list again. -_- On a smaller issue, don't you need to do: class DebugA(BaseA): def __init__(self): BaseA.__init__(self) return As in, explicitly call the __init__ function when you initalise DebugA, since DebugA extends BaseA? I'm just getting this necessary step because my books say so. If anyone has a good explanation, please do tell. Best regards, Ching-Yun Xavier Ho, Technical Artist Contact Information Mobile: (+61) 04 3335 4748 Skype ID: SpaXe85 Email: cont...@xavierho.com Website: http://xavierho.com/ On Sun, Jun 21, 2009 at 6:26 AM, Lorenzo Di Gregorio lorenzo.digrego...@gmail.com wrote: On Jun 20, 8:43 pm, Dave Angel da...@ieee.org wrote: Lorenzo Di Gregorio wrote: Hi, I'm wondering what would be the preferred way to solve the following forward reference problem: --- class BaseA(object): def __init__(self): return class DebugA(BaseA): def __init__(self): return # here I would have a prototype of class A which is the same as class BaseA class B(object): def __init__(self): self.obj = A() return if __name__ == __main__: #class A(BaseA): # Uncomment this for using BaseA objects # pass class A(DebugA): # Uncomment this for using DebugA objects pass --- I can figure out some ways to fix this but none seems satisfying. Either they are too specific or too cumbersome. A runtime redefinition of class A does not seem to work either. What would be the most pythonesque solution other than sorting out the class order? Best Regards, Lorenzo You haven't shown us any problem. class B works fine with a forward reference to A. Now if you were trying to subclass A before defining it, that'd be a problem. Or if you were trying to make an instance of B before defining A. Better put some code together with enough meat to actually show a symptom. And tell us what sys.version says. I'm testing with 2.6.2 (r262:71605, Apr 14 2009, 22:40:02) [MSC v.1500 32 bit (Intel)], running on Win XP.- Hide quoted text - - Show quoted text - Thank you for your help: I'm working on a rather large source, but I think I have isolated the problem now. This listing generates an error: --- class BaseA(object): def __init__(self): return class DebugA(BaseA): def __init__(self): return class B(object): def __init__(self,test=A()): self.obj = A() return if __name__ == __main__: #class A(BaseA): # Uncomment this for using BaseA objects #pass class A(DebugA): # Uncomment this for using DebugA objects pass --- The error happens because Python apparently evaluates the named arguments before running the script. I think I have read something about this some (long) time ago but I can't find it anymore. Suggestions? BTW, my Python version is 2.6.1 (with latest PyDev). Thx! Lorenzo -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: Inheritance and forward references (prototypes)
On Sat, 20 Jun 2009 21:26:56 +0100, Lorenzo Di Gregorio lorenzo.digrego...@gmail.com wrote: Thank you for your help: I'm working on a rather large source, but I think I have isolated the problem now. This listing generates an error: --- class BaseA(object): def __init__(self): return class DebugA(BaseA): def __init__(self): return class B(object): def __init__(self,test=A()): self.obj = A() return if __name__ == __main__: #class A(BaseA): # Uncomment this for using BaseA objects #pass class A(DebugA): # Uncomment this for using DebugA objects pass --- The error happens because Python apparently evaluates the named arguments before running the script. I think I have read something about this some (long) time ago but I can't find it anymore. I could have sworn this was in the FAQ, but apparently not. You're right, Python evaluates the default arguments when it executes the `def` instruction. (Yes, `def` is an executable instruction. It's got to create the function/method object after all!) The usual fix is not to make the default an instance of A (which will have all sorts of other side effects as well, since the same instance of A will be shared by all the Bs that don't supply a `test` parameter), but to use `None` as a marker. class B(object): def __init__(self, test=None): if test is None: test = A() self.obj = A() and so on. -- Rhodri James *-* Wildebeest Herder to the Masses -- http://mail.python.org/mailman/listinfo/python-list
