Re: Creating new instances of subclasses.
On Fri, 2009-01-09 at 10:46 -0800, Dennis Lee Bieber wrote: On Wed, 07 Jan 2009 11:38:29 -0500, J. Cliff Dyer j...@unc.edu declaimed the following in comp.lang.python: I want to be able to create an object of a certain subclass, depending on the argument given to the class constructor. I have three fields, and one might need to be a StringField, one an IntegerField, and the last a ListField. But I'd like my class to delegate to the proper subclass automatically, so I can just do: f1 = Field('abc') f2 = Field('123') f3 = Field('D,E,F') And how do you differentiate a string that contains a comma from a purported list? What is expected for: ambiguous = Field('He said Blast, it won''t work') My strings don't look like that. Nor is that relevant to my question. I created a simple, uncluttered example to illustrate my issue with instantiating the proper subclass for a given argument. I did not try to make sure my example handled all edge cases properly. Cheers, Cliff -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating new instances of subclasses.
Thanks for the solutions everyone! I'm not sure which I'll end up using, but I think I've got a better grasp of the problem now. Cool stuff. Cheers, Cliff On Thu, 2009-01-08 at 06:52 -0800, Paul McGuire wrote: On Jan 7, 12:00 pm, Paul McGuire pt...@austin.rr.com wrote: On Jan 7, 10:38 am, J. Cliff Dyer j...@unc.edu wrote: I want to be able to create an object of a certain subclass, depending on the argument given to the class constructor. I have three fields, and one might need to be a StringField, one an IntegerField, and the last a ListField. But I'd like my class to delegate to the proper subclass automatically, so I can just do: f1 = Field('abc') f2 = Field('123') f3 = Field('D,E,F') O-O is not always the solution to every problem. Since inheritance is getting in your way, try using a class-level factory method. Instead of using the Field constructor, use a staticmethod of Field, something like: @staticmethod def make_Field(a) if is_list(a): return ListField(a) elif is_integer(a): return IntegerField(a) else: return StringField(a) and then get rid of all those __new__ methods, too. -- Paul After looking this over a bit more, I decided I didn't like make_Field having to know the criteria for creating the different subclasses, but wanted to put the smarts into the subclasses themselves. Here is an excerpt that shows this working: class Field(object): def __init__(self, input): super(Field, self).__init__(input) self.data = input @staticmethod def make_Field(a): subs = (ListField, IntegerField, StringField) ret = None for cls in subs: try: ret = cls(a) except TypeError: continue else: break return ret class IntegerField(Field): def __new__(cls, a): if not is_integer(a): raise TypeError() return Field.__new__(cls, a) ... ListField has a similar __new__ method, and StringField just creates the object, with no validation. make_Field still has to know what order to list the subclasses in (StringField is the most permissive, and so must come last in the list of subclasses), but the specific type tests are moved into the subclasses, which is a more appropriate place I think. -- Paul -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating new instances of subclasses.
J. Cliff Dyer j...@unc.edu wrote: I want to be able to create an object of a certain subclass, depending on the argument given to the class constructor. I have three fields, and one might need to be a StringField, one an IntegerField, and the last a ListField. But I'd like my class to delegate to the proper subclass automatically, so I can just do: f1 = Field('abc') f2 = Field('123') f3 = Field('D,E,F') f1.data 'abc' f2.data 123 f3.data ['D','E','F'] type(f1) class '__main__.StringField' type(f2) class '__main__.StringField' type(f3) class '__main__.ListField' I've come up with a solution, but I suspect there's something cleaner I can do with the inheritance structure of __new__. I don't like explicitly leapfrogging over Field.__new__ to object.__new__. My attempt is below: def is_list(arg): if ',' in arg: return True else: return False def is_integer(arg): try: int(arg) except ValueError: return False else: return True class Field(object): def __new__(cls, a): if is_list(a): return ListField(a) elif is_integer(a): return IntegerField(a) else: return StringField(a) def __init__(self, input): super(Field, self).__init__(input) self.data = input class IntegerField(Field): def __new__(cls, a): return object.__new__(cls, a) def __init__(self, s): super(IntegerField, self).__init__(s) self.s = int(self.s) class ListField(Field): def __new__(cls, a): return object.__new__(cls, a) def __init__(self, s): super(ListField, self).__init__(s) self.s = s.split(',') class StringField(Field): def __new__(cls, a): return object.__new__(cls, a) Is there a cleaner way to do this? The main problem is that Field.__new__ gets in the way of properly constructing the subclasses once I've used it to select the proper subclass in the first place. How about this minor modification? # rest as above class Field(object): def __new__(cls, a): if cls != Field: return object.__new__(cls, a) if is_list(a): return ListField(a) elif is_integer(a): return IntegerField(a) else: return StringField(a) def __init__(self, input): super(Field, self).__init__(input) self.data = input class IntegerField(Field): def __init__(self, s): super(IntegerField, self).__init__(s) self.s = int(s) class ListField(Field): def __init__(self, s): super(ListField, self).__init__(s) self.s = s.split(',') class StringField(Field): pass Or you could go for the full metaclass self registration scheme like this, which is actually less code since I delegated the is this ok test to the subclass - failing it returns a ValueError. class Field(object): registry = [] class __metaclass__(type): def __init__(cls, name, bases, dict): cls.registry.append(cls) def __new__(cls, a): if cls != Field: return object.__new__(cls, a) for subcls in cls.registry: if subcls == Field: continue try: return subcls(a) except ValueError: pass raise ValueError(Couldn't find subclass) def __init__(self, input): super(Field, self).__init__(input) self.data = input # Raise a ValueError in init if not suitable args for this subtype class IntegerField(Field): def __init__(self, s): s = int(s) super(IntegerField, self).__init__(s) self.s = s class ListField(Field): def __init__(self, s): if ',' not in s: raise ValueError(Not a list) super(ListField, self).__init__(s) self.s = s.split(',') class StringField(Field): pass -- Nick Craig-Wood n...@craig-wood.com -- http://www.craig-wood.com/nick -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating new instances of subclasses.
On Jan 7, 12:00 pm, Paul McGuire pt...@austin.rr.com wrote: On Jan 7, 10:38 am, J. Cliff Dyer j...@unc.edu wrote: I want to be able to create an object of a certain subclass, depending on the argument given to the class constructor. I have three fields, and one might need to be a StringField, one an IntegerField, and the last a ListField. But I'd like my class to delegate to the proper subclass automatically, so I can just do: f1 = Field('abc') f2 = Field('123') f3 = Field('D,E,F') O-O is not always the solution to every problem. Since inheritance is getting in your way, try using a class-level factory method. Instead of using the Field constructor, use a staticmethod of Field, something like: @staticmethod def make_Field(a) if is_list(a): return ListField(a) elif is_integer(a): return IntegerField(a) else: return StringField(a) and then get rid of all those __new__ methods, too. -- Paul After looking this over a bit more, I decided I didn't like make_Field having to know the criteria for creating the different subclasses, but wanted to put the smarts into the subclasses themselves. Here is an excerpt that shows this working: class Field(object): def __init__(self, input): super(Field, self).__init__(input) self.data = input @staticmethod def make_Field(a): subs = (ListField, IntegerField, StringField) ret = None for cls in subs: try: ret = cls(a) except TypeError: continue else: break return ret class IntegerField(Field): def __new__(cls, a): if not is_integer(a): raise TypeError() return Field.__new__(cls, a) ... ListField has a similar __new__ method, and StringField just creates the object, with no validation. make_Field still has to know what order to list the subclasses in (StringField is the most permissive, and so must come last in the list of subclasses), but the specific type tests are moved into the subclasses, which is a more appropriate place I think. -- Paul -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating new instances of subclasses.
On Jan 7, 10:38 am, J. Cliff Dyer j...@unc.edu wrote: I want to be able to create an object of a certain subclass, depending on the argument given to the class constructor. I have three fields, and one might need to be a StringField, one an IntegerField, and the last a ListField. But I'd like my class to delegate to the proper subclass automatically, so I can just do: f1 = Field('abc') f2 = Field('123') f3 = Field('D,E,F') O-O is not always the solution to every problem. Since inheritance is getting in your way, try using a class-level factory method. Instead of using the Field constructor, use a staticmethod of Field, something like: @staticmethod def make_Field(a) if is_list(a): return ListField(a) elif is_integer(a): return IntegerField(a) else: return StringField(a) and then get rid of all those __new__ methods, too. -- Paul -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating new instances of subclasses.
J. Cliff Dyer wrote: I want to be able to create an object of a certain subclass, depending on the argument given to the class constructor. I have three fields, and one might need to be a StringField, one an IntegerField, and the last a ListField. But I'd like my class to delegate to the proper subclass automatically, so I can just do: f1 = Field('abc') f2 = Field('123') f3 = Field('D,E,F') f1.data 'abc' f2.data 123 f3.data ['D','E','F'] type(f1) class '__main__.StringField' type(f2) class '__main__.StringField' type(f3) class '__main__.ListField' I've come up with a solution, but I suspect there's something cleaner Make your master class _Field and make Field a factory function that returns the proper subclass instance. The body of Field could be the body of __new__ below. Then dump the __new__ methods. tjr I can do with the inheritance structure of __new__. I don't like explicitly leapfrogging over Field.__new__ to object.__new__. My attempt is below: def is_list(arg): if ',' in arg: return True else: return False def is_integer(arg): try: int(arg) except ValueError: return False else: return True class Field(object): def __new__(cls, a): if is_list(a): return ListField(a) elif is_integer(a): return IntegerField(a) else: return StringField(a) def __init__(self, input): super(Field, self).__init__(input) self.data = input class IntegerField(Field): def __new__(cls, a): return object.__new__(cls, a) def __init__(self, s): super(IntegerField, self).__init__(s) self.s = int(self.s) class ListField(Field): def __new__(cls, a): return object.__new__(cls, a) def __init__(self, s): super(ListField, self).__init__(s) self.s = s.split(',') class StringField(Field): def __new__(cls, a): return object.__new__(cls, a) Is there a cleaner way to do this? The main problem is that Field.__new__ gets in the way of properly constructing the subclasses once I've used it to select the proper subclass in the first place. Cheers, Cliff -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating new instances of subclasses.
J. Cliff Dyer a écrit : I want to be able to create an object of a certain subclass, depending on the argument given to the class constructor. I have three fields, and one might need to be a StringField, one an IntegerField, and the last a ListField. But I'd like my class to delegate to the proper subclass automatically, so I can just do: f1 = Field('abc') f2 = Field('123') f3 = Field('D,E,F') f1.data 'abc' f2.data 123 f3.data ['D','E','F'] type(f1) class '__main__.StringField' type(f2) class '__main__.StringField' type(f3) class '__main__.ListField' I've come up with a solution, but I suspect there's something cleaner I can do with the inheritance structure of __new__. I don't like explicitly leapfrogging over Field.__new__ to object.__new__. My attempt is below: def is_list(arg): if ',' in arg: return True else: return False def is_integer(arg): try: int(arg) except ValueError: return False else: return True class Field(object): def __new__(cls, a): if is_list(a): return ListField(a) elif is_integer(a): return IntegerField(a) else: return StringField(a) def __init__(self, input): super(Field, self).__init__(input) self.data = input class IntegerField(Field): def __new__(cls, a): return object.__new__(cls, a) def __init__(self, s): super(IntegerField, self).__init__(s) self.s = int(self.s) class ListField(Field): def __new__(cls, a): return object.__new__(cls, a) def __init__(self, s): super(ListField, self).__init__(s) self.s = s.split(',') class StringField(Field): def __new__(cls, a): return object.__new__(cls, a) Is there a cleaner way to do this? The main problem is that Field.__new__ gets in the way of properly constructing the subclasses once I've used it to select the proper subclass in the first place. Not only that, but the base class should know nothing of its subclasses. As Paul suggested, a better solution would be to make Field a factory function (and rename the Field class to BaseField). Also and FWIW, since your Field subclasses are responsible for doing the conversion, you could as well use this to find out the concrete class to use for a given value, ie (not tested): class Field(object): __classes = [] __default = None @classmethod def register(cls, subclass, is_default=False): if is_default: if cls.__default is not None: raise WhateverError(only one default subclass, thanks) cls.__default = subclass cls.__classes.append(subclass) def __new__(cls, value): for subclass in cls.__classes: try: return subclass(value) except (TypeError, ValueError): continue if cls.__default is not None: return cls.__default(value) raise ValueError(no appropriate subclass for '%s' % value) class BaseField(object): # common code here class FloatField(BaseField): def __init__(self, value): self.value = float(value) Field.register(FloatField) class ListField(BaseField): def __init__(self, value): if , in value: self.value = value.split(,) raise ValueError(could not convert '%s' to a list % value) Field.register(ListField) class IntegerField(BaseField): def __init__(self, value): self.value = int(value) Field.register(IntegerField) class StringField(self, value): def __init__(self, value): self.value = str(value) Field.register(StringField, is_default=True) Now this may just be overkill for your needs - I don't have enough context to know !-) -- http://mail.python.org/mailman/listinfo/python-list