Re: How do you control _all_ items added to a list?
Xif wrote: Overiding all those methods is too much of an effort. I don't really need them. Hmm, it might be nice if there was a UserList.ListMixin that was the counterpart to UserDict.DictMixin that let's you provide the full dictionary API with just __getitem__, __setitem__, __delitem__ and keys() With an appropriate ListMixin, providing the first three methods would suffice to support the full list API. Cheers, Nick. -- Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia --- http://boredomandlaziness.skystorm.net -- http://mail.python.org/mailman/listinfo/python-list
ListMixin (WAS: How do you control _all_ items added to a list?)
Nick Coghlan wrote: Hmm, it might be nice if there was a UserList.ListMixin that was the counterpart to UserDict.DictMixin I've thought this occasionally too. One of the tricky issues though is that often you'd like to define __getitem__ for single items and have ListMixin add the code for slices. I haven't figured out how to do this cleanly yet... STeVe -- http://mail.python.org/mailman/listinfo/python-list
Re: ListMixin (WAS: How do you control _all_ items added to a list?)
Steven Bethard wrote: Nick Coghlan wrote: Hmm, it might be nice if there was a UserList.ListMixin that was the counterpart to UserDict.DictMixin I've thought this occasionally too. One of the tricky issues though is that often you'd like to define __getitem__ for single items and have ListMixin add the code for slices. I haven't figured out how to do this cleanly yet... STeVe I agree that would be useful. One solution would be to ask users to implement __getsingleitem__ (and not __getitem__) if they want the mixin to handle slice logic. The following illustrates that, and also falls back to slicing the iterator if it is provided: class ProtoListMixin(object): Prototype ListMixin, exploring slice interface and semantics def __getitem__(self, index): if isinstance(index, slice): start, stop, step = index.start or 0, index.stop, index.step or 1 if start 0 or stop 0 or not stop: try: start, stop, step = index.indices(len(self)) except TypeError: raise TypeError, unsized object try: getter = self.__getsingleitem__ return [getter(i) for i in range(start, stop, step)] except AttributeError: pass else: if index 0: try: index = len(self) + index except TypeError: raise TypeError, unsized object try: return self.__getsingleitem__(index) except AttributeError: pass start, stop, step = index, index + 1, None # Alternatively, try to use the iterator, if available import itertools try: args = [iter(self)] except AttributeError: raise TypeError, Must implement __getsingleitem__ or __iter__ if start: args.append(start) args.append(stop) if step: if step 1: raise ValueError, slicing an iterable requires step =1 args.append(step) iterator = itertools.islice(*args) if isinstance(index, slice): return list(iterator) else: try: return iterator.next() except StopIteration: raise IndexError, index out of range # Users should implement __getsingleitem__ for positive indices class Index(ProtoListMixin): def __init__(self, data): For testing, provide a list self._data = data def __getsingleitem__(self, index): return self._data[index] # If __len__ is implemented, negative indices are supported class IndexLen(Index): def __len__(self): return len(self._data) # If __getsingleitem__ is not implemented, positive slices are returned # from an iterator class Iter(ProtoListMixin): def __init__(self, data): For testing, provide an iterable self._data = data def __iter__(self): return iter(self._data) a = Index(range(10)) a[4] 4 a[4:8] [4, 5, 6, 7] a[-4] Traceback (most recent call last): File input, line 1, in ? File ListMixin, line 22, in __getitem__ TypeError: unsized object b = IndexLen(range(10)) b[-4] 6 c = Iter(xrange(10)) c[3] 3 c[3:6] [3, 4, 5] c[-3] Traceback (most recent call last): File input, line 1, in ? File ListMixin, line 22, in __getitem__ TypeError: unsized object -- http://mail.python.org/mailman/listinfo/python-list
Re: ListMixin (WAS: How do you control _all_ items added to a list?)
[Nick Coghlan] Hmm, it might be nice if there was a UserList.ListMixin that was the counterpart to UserDict.DictMixin [Steven Bethard] I've thought this occasionally too. One of the tricky issues though is that often you'd like to define __getitem__ for single items and have ListMixin add the code for slices. I haven't figured out how to do this cleanly yet... All that is needed is a helper function and a two line idiom for calling it from inside __getitem__: def sliceit(self, sliceobj): return [self[i] for i in xrange(*sliceobj.indices(len(self)))] class AlphaList: def __getitem__(self, i): if isinstance(i, slice): return sliceit(self, i) return chr(i+64) def __len__(self): return 26 a = AlphaList() print a[1], a[2], a[5] print a[2:5] -- http://mail.python.org/mailman/listinfo/python-list
How do you control _all_ items added to a list?
Hi I want to have control over all items added to a list. The first attempt was to subclass list and override its .append() method. Problem is, there are plenty of other ways the list can have items added to it - e.g. extend, insert - and theyr'e not at all affected by the changes I made to append. Is there some base item-adding method that all other item-adding methods use, so I can override it and have the changes affect all item-adding functions? Thanks, Xif -- http://mail.python.org/mailman/listinfo/python-list
Re: How do you control _all_ items added to a list?
Xif wrote: Hi I want to have control over all items added to a list. The first attempt was to subclass list and override its .append() method. Problem is, there are plenty of other ways the list can have items added to it - e.g. extend, insert - and theyr'e not at all affected by the changes I made to append. Is there some base item-adding method that all other item-adding methods use, so I can override it and have the changes affect all item-adding functions? If you subclass list then you will need to override all methods that can set items. This is because the built in methods work directly with the internal structure and are implemented in C. It's a bit of a pain... but not that much. Regards, Fuzzy http://www.voidspace.org.uk/python/index.shtml Thanks, Xif -- http://mail.python.org/mailman/listinfo/python-list
Re: How do you control _all_ items added to a list?
Quoting Fuzzyman: If you subclass list then you will need to override all methods that can set items. Overiding all those methods is too much of an effort. I don't really need them. I'll just have an object that uses a list internally, and define only the two methods I really need: extend() and getItems(). That way I retain full control over what goes into the private obj._list, without the superfluous work of overriding all those methods I don't really need. Thanks for your help, Xif -- http://mail.python.org/mailman/listinfo/python-list
