Re: Is it correct this way to inherit from a list?
On 02/03/2013 9:30 PM, gialloporpora wrote: Risposta al messaggio di Rick Johnson : What are you trying to achieve exactly? I would like to implement a class (vector) to works with vectors, for example using scalar multiplication: a*v = [a*v1, a*vn] and a dual class for dual vector (the only method that I'll change is the __str__ method to print it as colun. Sandro Numpy facilitates this sort of thing more efficiently than using a List. Colin W. -- http://mail.python.org/mailman/listinfo/python-list
Re: Is it correct this way to inherit from a list?
On Sun, Mar 3, 2013 at 9:21 AM, Colin J. Williams c...@ncf.ca wrote: On 02/03/2013 9:30 PM, gialloporpora wrote: Risposta al messaggio di Rick Johnson : What are you trying to achieve exactly? I would like to implement a class (vector) to works with vectors, for example using scalar multiplication: a*v = [a*v1, a*vn] and a dual class for dual vector (the only method that I'll change is the __str__ method to print it as colun. Sandro Numpy facilitates this sort of thing more efficiently than using a List. As a couple people have already pointed out, numpy is the way to go for most scientific applications. You have also been given good advice regarding 'properly' inheriting from 'list' by calling the list.__init__ function. The only thing I'll add here is that you can inherit from array.array instead of list if you want a 'truly' numeric-vector without introducing numpy as a dependency. The advantage array.array has over list in this instance is that it is type-restrictive for member data (it has to be pre-declared and throws TypeError if you try to pass it a bad variable type). You can then proceed to override the __add__, __sub__, __mul__, and __div__ methods (and the in-place versions of these operators) to mimic vector operations. (By default, __add__ appends the rhs to the lhs and returns a copy of that for array.array and list). You can avoid all this work, however, and just use numpy.ndarray instead ;). Good luck, Jason -- http://mail.python.org/mailman/listinfo/python-list
Is it correct this way to inherit from a list?
Hi all, I would like to inherit from the list native class. really I expected that was possible to use native list method without redefining them, for example the __repr__ method. I don't know if i have made something wrong, this is my code (I obmit customized methods that I have added): from os.path import exists class vector(list): def __init__(self, *args): self._list = list(args) self._index = 0 def __getitem__(self, key): return self._list[key] def __setitem__(self, key, value): self._list[key] = value def __len__(self): return len(self._list) def __iter__(self): return self def next(self): if self._index == len(self): self._index = 0 raise StopIteration next = self._index self._index += 1 return self[next Is it correct or it exists another way to inherit from list class? Sandro -- http://mail.python.org/mailman/listinfo/python-list
Re: Is it correct this way to inherit from a list?
gialloporpora wrote: I would like to inherit from the list native class. really I expected that was possible to use native list method without redefining them, for example the __repr__ method. I don't know if i have made something wrong, this is my code (I obmit customized methods that I have added): from os.path import exists class vector(list): def __getitem__(self, key): return self._list[key] [and many more] Is it correct or it exists another way to inherit from list class? Have you considered subclassing collections.MutableSequence instead? You cannot instantiate that class until you have overridden all its abstract methods. -- http://mail.python.org/mailman/listinfo/python-list
Re: Is it correct this way to inherit from a list?
On Sat, Mar 2, 2013 at 10:22 AM, Ian Kelly ian.g.ke...@gmail.com wrote: class Vector(list): def __new__(cls, *args): return super(Vector, cls).__new__(cls, args) def __init__(self, *args): super(Vector, self).__init__(args) The __new__ method here will receive the args in the style that you want, and then pass them up the inheritance chain to the superclass constructor, which will then just do the right thing. The __init__ method is also overridden to match the modified argspec. The super().__init__() call is included for completeness; AFAIK it doesn't actually do anything. I retract that. On further testing, it is actually the __init__ method that initializes the list contents, not the __new__ method. So this is all you need: class Vector(list): def __init__(self, *args): super(Vector, self).__init__(args) -- http://mail.python.org/mailman/listinfo/python-list
Re: Is it correct this way to inherit from a list?
On Sat, Mar 2, 2013 at 10:02 AM, gialloporpora gialloporp...@gmail.com wrote: Hi all, I would like to inherit from the list native class. really I expected that was possible to use native list method without redefining them, for example the __repr__ method. I don't know if i have made something wrong, this is my code (I obmit customized methods that I have added): from os.path import exists class vector(list): def __init__(self, *args): self._list = list(args) So here you have a list subclass, but instead of taking advantage of that is-a relationship, you're creating a secondary list from the arguments and attaching it to self._list in a has-a relationship. The net effect is that you actually have two separate list objects here, with some methods operating on the list itself and some operating on the attached list. Try this instead: class Vector(list): def __new__(cls, *args): return super(Vector, cls).__new__(cls, args) def __init__(self, *args): super(Vector, self).__init__(args) The __new__ method here will receive the args in the style that you want, and then pass them up the inheritance chain to the superclass constructor, which will then just do the right thing. The __init__ method is also overridden to match the modified argspec. The super().__init__() call is included for completeness; AFAIK it doesn't actually do anything. -- http://mail.python.org/mailman/listinfo/python-list
Re: Is it correct this way to inherit from a list?
On Saturday, March 2, 2013 11:02:14 AM UTC-6, gialloporpora wrote:
I would like to inherit from the list native class. really
I expected that was possible to use native list method
without redefining them, for example the __repr__ method.
[...]
class vector(list):
def __init__(self, *args):
self._list = list(args)
self._index = 0
Here is where you go wrong.
First of all why would you inherit from list and then create a new list as
attribute, that seems a bit silly huh? If you want your custom list to
inherit all the pre-defined methods of the python list type, then do so.
class MyList(list):
pass
ml = MyList()
dir(ml)
['__add__', '__class__', '__contains__', '__delattr__', '__delitem__',
'__delslice__', '__dict__', '__doc__', '__eq__', '__format__', '__ge__',
'__getattribute__', '__getitem__', '__getslice__', '__gt__', '__hash__',
'__iadd__', '__imul__', '__init__', '__iter__', '__le__', '__len__', '__lt__',
'__module__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__',
'__repr__', '__reversed__', '__rmul__', '__setattr__', '__setitem__',
'__setslice__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__',
'append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse',
'sort']
ml
[]
ml.append('cat')
ml
['cat']
ml.extend(range(5))
ml
['cat', 0, 1, 2, 3, 4]
ml.sort()
ml
[0, 1, 2, 3, 4, 'cat']
isinstance(ml, list)
True
Quacks like a list to me. In this case you did not need to call the superclass
constructor explicitly; for example.
class MyList(list):
def __init__(self):
list.__init__(self)
...is really a waste of time because you will not have any options to pass to
the super.
ml2 = MyList2()
dir(ml2)
['__add__', '__class__', '__contains__', '__delattr__', '__delitem__',
'__delslice__', '__dict__', '__doc__', '__eq__', '__format__', '__ge__',
'__getattribute__', '__getitem__', '__getslice__', '__gt__', '__hash__',
'__iadd__', '__imul__', '__init__', '__iter__', '__le__', '__len__', '__lt__',
'__module__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__',
'__repr__', '__reversed__', '__rmul__', '__setattr__', '__setitem__',
'__setslice__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__',
'append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse',
'sort']
ml2+[10,20,30]
[10, 20, 30]
ml2
[]
ml2.append('salt')
ml2
['salt']
isinstance(ml2, list)
True
If however you wanted to create a custom Tkinter widget, you would then need to
pass the options from the derived class __init__ method into the superclass
__init__ method, like this:
class MyButton(tk.Button):
def __init__(self, master, **kw):
tk.Button.__init__(self, master, **kw)
mb = MyButton(rootWindow, text='PushMe', command=helloButton)
What are you trying to achieve exactly?
--
http://mail.python.org/mailman/listinfo/python-list
Re: Is it correct this way to inherit from a list?
On Sun, Mar 3, 2013 at 1:30 PM, gialloporpora gialloporp...@gmail.com wrote: Risposta al messaggio di Rick Johnson : What are you trying to achieve exactly? I would like to implement a class (vector) to works with vectors, for example using scalar multiplication: a*v = [a*v1, a*vn] and a dual class for dual vector (the only method that I'll change is the __str__ method to print it as colun. Have you looked at NumPy? I haven't used it myself, but I understand it's good for this sort of thing. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
