Re: Pure virtual functions in Python?
On Feb 21, 11:21 am, Lie Ryan lie.1...@gmail.com wrote: On 02/21/10 19:27,lallouswrote: snip If the base defines the method and it was empty, then my C++ code would still call the function. This is not optimal because I don't want to go from C++ to Python if the _derived_ class does not implement the cb. That sounds like a microoptimization; have you profiled your code and determined that calling empty function causes a bottleneck? I doubt it. Now the base class should define it so that doc parsers properly describe the base class. The recipe suggested is not worth the trouble. Unfortunately I cannot use abc module since I use Python 2.5 Because nobody here could have guessed that your dispatcher was written in C++; your problem is near trivial if your dispatcher is a pure-python code. You are right. I haven't checked how much it costs to continuously call an empty function, but why do it if I know (during initialization from my C++ dispatcher code) that certain Python object should not have certain methods called. I still prefer not to call at all, even if it was an empty function. Regards, Elias -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
On Feb 22, 12:42 am, Gregory Ewing greg.ew...@canterbury.ac.nz wrote: lallouswrote: If the base defines the method and it was empty, then my C++ code would still call the function. This is not optimal because I don't want to go from C++ to Python if the _derived_ class does not implement the cb. I would simply not implement the method at all in the base class. Then the C++ code can do an attribute lookup for the method, and if it's not found, do nothing. -- Greg That is what I currently do. But if I comment out the implementations (empty ones) then the documentation generation tool will not document the callbacks. -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
lallous wrote: I still prefer not to call at all, even if it was an empty function. Regards, Elias Is there any way we could convince you that there is no point caring about this ? Even if you were trying to optimize speed, it would still require proof that an empty function is part of the problem. It sounds like someone stating I prefer to write difficult-to-read code, because in 1978, code size used to matter. JM -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
In article 38ddd614-583c-430d-b998-214bd6360...@b2g2000yqi.googlegroups.com, lallous elias.bachaal...@gmail.com wrote: On Feb 22, 12:42=A0am, Gregory Ewing greg.ew...@canterbury.ac.nz wrote: lallouswrote: If the base defines the method and it was empty, then my C++ code would still call the function. This is not optimal because I don't want to go from C++ to Python if the _derived_ class does not implement the cb. I would simply not implement the method at all in the base class. Then the C++ code can do an attribute lookup for the method, and if it's not found, do nothing. That is what I currently do. But if I comment out the implementations (empty ones) then the documentation generation tool will not document the callbacks. Maybe deleting the method after the class would work. -- Aahz (a...@pythoncraft.com) * http://www.pythoncraft.com/ Many customs in this life persist because they ease friction and promote productivity as a result of universal agreement, and whether they are precisely the optimal choices is much less important. --Henry Spencer -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
Arnaud Delobelle wrote: lallous elias.bachaal...@gmail.com writes: Hello How can I do something similar to pure virtual functions in C++ ? Let us consider this: class C1: # Pure virtual def cb(self, param1, param2): This is a callback @param param1: ... @param param2: ... raise NotImplementedError, Implement me Why define it if it is virtual? Slightly off topic but this is often useful when writing interfaces. You can then properly document what should any subclass (interface implemention) be doing. The thing is that in case of virtual methods, you *do want* to raise the notImplemented exceptions, meaning you've failed to implement all the required methods. Strange thing that the OP want to silently call nothing at all when calling a virtual method, he looses all the benefits from a virtual design. Anyway, I don't deal into code optimization, this is not healthy :-) JM -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
On Feb 20, 6:08 pm, Martin v. Loewis mar...@v.loewis.de wrote: class C1: # Pure virtual def cb(self, param1, param2): This is a callback �...@param param1: ... �...@param param2: ... raise NotImplementedError, Implement me # Dispatcher function that calls 'cb' only if 'cb' is implemented in child classes def dispatcher(c): if hasattr(c, 'cb'): c.cb(Hello, World) dispatcher(C2()) dispatcher(C3()) What I want is the ability to have the dispatcher() not to call 'cb' if it was not implemented in one of the child classes. Please advise. There is nothing more beyond that what you already did. You can raise a NotImplementedError for classes that don't implement the method. That's it. That's not true. Currently, the hasattr() call would report that cb is available, when it is actually not implemented. It would be possible to do something like if hasattr(c, 'cb') and not is_pure(c.cb): c.cb(Hello, World) is_pure could, for example, look at a function attribute of the callback. You'd write something like @pure_virtual def cb(self, param1, param2): not_implemented Regards, Martin Hello Martine, Can you elaborate more on how to use the mechanism you described? Thanks, Elias -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
Thanks everyone for the answers. The dispatcher() is actually sits in C++ code. So my code receives an object that is an instance of the base class, it PyObject_GetAttrString(py_obj, 'funcname'). If the attribute exists I will call PyObject_CallMethod on it. If the base defines the method and it was empty, then my C++ code would still call the function. This is not optimal because I don't want to go from C++ to Python if the _derived_ class does not implement the cb. Now the base class should define it so that doc parsers properly describe the base class. The recipe suggested is not worth the trouble. Unfortunately I cannot use abc module since I use Python 2.5 -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
That's not true. Currently, the hasattr() call would report that cb is available, when it is actually not implemented. It would be possible to do something like if hasattr(c, 'cb') and not is_pure(c.cb): c.cb(Hello, World) is_pure could, for example, look at a function attribute of the callback. You'd write something like @pure_virtual def cb(self, param1, param2): not_implemented Regards, Martin Hello Martine, Can you elaborate more on how to use the mechanism you described? There are various ways to do it; the one I had in mind uses function attributes: def pure_virtual(func): func.pure_virtual = True # only presence of attribute matters, # not value return func def is_pure(method): # method might be either a method or a function try: func = method.im_func except AttributeError: func = method return hasattr(func, 'pure_virtual') not_implemented = object() # could also write pass instead, or raise HTH, Martin -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
On 02/21/10 19:27, lallous wrote: snip If the base defines the method and it was empty, then my C++ code would still call the function. This is not optimal because I don't want to go from C++ to Python if the _derived_ class does not implement the cb. That sounds like a microoptimization; have you profiled your code and determined that calling empty function causes a bottleneck? I doubt it. Now the base class should define it so that doc parsers properly describe the base class. The recipe suggested is not worth the trouble. Unfortunately I cannot use abc module since I use Python 2.5 Because nobody here could have guessed that your dispatcher was written in C++; your problem is near trivial if your dispatcher is a pure-python code. -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
lallous wrote: If the base defines the method and it was empty, then my C++ code would still call the function. This is not optimal because I don't want to go from C++ to Python if the _derived_ class does not implement the cb. I would simply not implement the method at all in the base class. Then the C++ code can do an attribute lookup for the method, and if it's not found, do nothing. -- Greg -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
lallous wrote: Hello How can I do something similar to pure virtual functions in C++ ? See, for example http://code.activestate.com/recipes/266468/ Regards, Martin -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
Am 20.02.10 17:12, schrieb lallous: Hello How can I do something similar to pure virtual functions in C++ ? Let us consider this: class C1: # Pure virtual def cb(self, param1, param2): This is a callback @param param1: ... @param param2: ... raise NotImplementedError, Implement me # Implementation w/o a 'cb', thus 'cb' should not be used class C2(C1): def __init__(self): pass # Implementation w/ 'cb', thus 'cb' can be used class C3(C1): def __init__(self): pass def cb(self, param1, param2): print i am c3 cb # Dispatcher function that calls 'cb' only if 'cb' is implemented in child classes def dispatcher(c): if hasattr(c, 'cb'): c.cb(Hello, World) dispatcher(C2()) dispatcher(C3()) What I want is the ability to have the dispatcher() not to call 'cb' if it was not implemented in one of the child classes. Please advise. There is nothing more beyond that what you already did. You can raise a NotImplementedError for classes that don't implement the method. That's it. Diez -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
class C1: # Pure virtual def cb(self, param1, param2): This is a callback @param param1: ... @param param2: ... raise NotImplementedError, Implement me # Dispatcher function that calls 'cb' only if 'cb' is implemented in child classes def dispatcher(c): if hasattr(c, 'cb'): c.cb(Hello, World) dispatcher(C2()) dispatcher(C3()) What I want is the ability to have the dispatcher() not to call 'cb' if it was not implemented in one of the child classes. Please advise. There is nothing more beyond that what you already did. You can raise a NotImplementedError for classes that don't implement the method. That's it. That's not true. Currently, the hasattr() call would report that cb is available, when it is actually not implemented. It would be possible to do something like if hasattr(c, 'cb') and not is_pure(c.cb): c.cb(Hello, World) is_pure could, for example, look at a function attribute of the callback. You'd write something like @pure_virtual def cb(self, param1, param2): not_implemented Regards, Martin -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
Sorry, I totally mis-read the OP, too tired. You are right of course. Diez -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
On Saturday 20 February 2010 11:46:42 Diez B. Roggisch wrote: Am 20.02.10 17:12, schrieb lallous: Hello How can I do something similar to pure virtual functions in C++ ? Let us consider this: class C1: # Pure virtual def cb(self, param1, param2): This is a callback @param param1: ... @param param2: ... raise NotImplementedError, Implement me # Implementation w/o a 'cb', thus 'cb' should not be used class C2(C1): def __init__(self): pass # Implementation w/ 'cb', thus 'cb' can be used class C3(C1): def __init__(self): pass def cb(self, param1, param2): print i am c3 cb # Dispatcher function that calls 'cb' only if 'cb' is implemented in child classes def dispatcher(c): if hasattr(c, 'cb'): c.cb(Hello, World) dispatcher(C2()) dispatcher(C3()) What I want is the ability to have the dispatcher() not to call 'cb' if it was not implemented in one of the child classes. Please advise. There is nothing more beyond that what you already did. You can raise a NotImplementedError for classes that don't implement the method. That's it. Diez Perhaps you could use an easier-to-ask-forgiveness-than-permission idiom? def dispatcher(c): try: c.cb(Hello, World) except NotImplementedError: pass Rami Chowdhury Passion is inversely proportional to the amount of real information available. -- Benford's Law of Controversy 408-597-7068 (US) / 07875-841-046 (UK) / 01819-245544 (BD) -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
lallous wrote: How can I do something similar to pure virtual functions in C++ ? http://docs.python.org/library/abc.html#abc.abstractmethod Peter -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
On Sat, 20 Feb 2010 08:12:01 -0800, lallous wrote: How can I do something similar to pure virtual functions in C++ ? From what you want, it seems like you want cb() to not be called if it isn't implemented in the derived class; this isn't really what pure virtual functions in C++ do - pure virtual functions enforce, at compile time, that the derived class implements the method. If you have a situation when you want to either call a derived class's version of cb(), or do nothing, can you not just have an implementation of cb() in the base class that does nothing, i.e. class C1(object): def cb(self, param1, param2): pass -- http://mail.python.org/mailman/listinfo/python-list
Re: Pure virtual functions in Python?
lallous elias.bachaal...@gmail.com writes: Hello How can I do something similar to pure virtual functions in C++ ? Let us consider this: class C1: # Pure virtual def cb(self, param1, param2): This is a callback @param param1: ... @param param2: ... raise NotImplementedError, Implement me Why define it if it is virtual? # Implementation w/o a 'cb', thus 'cb' should not be used class C2(C1): def __init__(self): pass # Implementation w/ 'cb', thus 'cb' can be used class C3(C1): def __init__(self): pass def cb(self, param1, param2): print i am c3 cb # Dispatcher function that calls 'cb' only if 'cb' is implemented in child classes def dispatcher(c): if hasattr(c, 'cb'): c.cb(Hello, World) dispatcher(C2()) dispatcher(C3()) What I want is the ability to have the dispatcher() not to call 'cb' if it was not implemented in one of the child classes. If you don't define cb in the parent class, it'll work. -- Arnaud -- http://mail.python.org/mailman/listinfo/python-list
