Re: operator module isSequenceType with builtin set produces False
On 19 Dec, 05:24, Gabriel Genellina [EMAIL PROTECTED] wrote: En Tue, 18 Dec 2007 09:15:12 -0300, English, Mark [EMAIL PROTECTED] escribió: try: set except NameError: from sets import Set as set class myset_fails(set): pass class myset_works(set): def __getitem__(self): pass s = set() fails = myset_fails() works = myset_works() import operator operator.isSequenceType(s) #Not what I expected False operator.isSequenceType(fails) #Not what I expected either False operator.isSequenceType(works) #A hint at what isSequenceType does ? True Are sets not sequences ? I didn't think the isSequenceDisclaimer gave false negatives. No, sets aren't sequences, as they have no order. Same as dicts, which aren't sequences either. Oops. I was under the misapprehension that they were sequences in that they were sequential but of unknown ordering e.g. for x in set([someobject, someotherobject, ...]) always iterates in the same order Anywho, now I know. Thanks. Is there a short Pythonic way to determine whether an object is iterable (iteratable ??) that I haven't thought of (getattr(obj, '__iter__') ?). Would operator.isIterable() be at all a useful addition ? (And what did I do wrong when I posted my original post that I can't see it in groups.google) -- http://mail.python.org/mailman/listinfo/python-list
Re: operator module isSequenceType with builtin set produces False
On 19 Dec, 10:03, MarkE [EMAIL PROTECTED] wrote: No, sets aren't sequences, as they have no order. Same as dicts, which aren't sequences either. Oops. I was under the misapprehension that they were sequences I realise now that this is even explicitly documented: http://docs.python.org/lib/typesseq.html There are six sequence types: strings, Unicode strings, lists, tuples, buffers, and xrange objects. Is there a short Pythonic way to determine whether an object is iterable (iteratable ??) that I haven't thought of (getattr(obj, '__iter__') ?). Would operator.isIterable() be at all a useful addition ? And here I probably meant container (although the url says sequence when the article meant container, bit like me): http://docs.python.org/ref/sequence-types.html Containers usually are sequences (such as lists or tuples) or mappings (like dictionaries), but can represent other containers as well So same question, but perhaps isContainer() rather than isIterable() -- http://mail.python.org/mailman/listinfo/python-list
Re: operator module isSequenceType with builtin set produces False
En Wed, 19 Dec 2007 06:28:03 -0300, MarkE [EMAIL PROTECTED] escribi�: Is there a short Pythonic way to determine whether an object is iterable (iteratable ??) that I haven't thought of (getattr(obj, '__iter__') ?). Would operator.isIterable() be at all a useful addition ? Yes, I think the only way is to try iter(obj) and see if it succeeds (that's basically the same as what you do with getattr, but actually creates the iterator and checks that it's of the right type). And here I probably meant container (although the url says sequence when the article meant container, bit like me): http://docs.python.org/ref/sequence-types.html Containers usually are sequences (such as lists or tuples) or mappings (like dictionaries), but can represent other containers as well So same question, but perhaps isContainer() rather than isIterable() container is too generic. Perhaps you can look if it has a __len__ attribute. But anyone could implement a linked list (the common interfase don't fit well with those sequence/mapping methods) and would be hard to deduce whether it is a container or not without further knowledge about it. -- Gabriel Genellina -- http://mail.python.org/mailman/listinfo/python-list
Re: operator module isSequenceType with builtin set produces False
MarkE [EMAIL PROTECTED] wrote in message news:[EMAIL PROTECTED] || Is there a short Pythonic way to determine whether an object is | iterable (iteratable ??) Welcome to Python and its neat concept of iterables and iterators. An iterable is an object that has an __iter__ method that returns an iterator. An iterator is an object that has an __iter__ method that returns itself and a next method that either returns another item or raises StopIteration. (I omit an older alternative that will disappear in 3.0. Ignore it.) (Also, in 3.0, next methods will become __next__ methods.) So iterators are also, intentionally, iterables. So functions can take arguments that are either by beginning with it = iter(iterable) without worrying about whether the iterable already is an iterator. As Gabriel said, assume that an object returned by iter is an iterable. Writing an __iter__ method that does not return such would be either a bug or a nasty trick. I usually would not bother wrapping iter in a try: block. Just iterate with 'it' and if it is not an iterator, you will find out. | And here I probably meant container (although the url says sequence | when the article meant container, bit like me): | http://docs.python.org/ref/sequence-types.html | Containers usually are sequences (such as lists or tuples) or | mappings (like dictionaries), but can represent other containers as | well I think 'collection' is a better word that 'container' since 'container' implies an exclusivity (which is false) that 'collection' does not, at least not necessarily. | So same question, but perhaps isContainer() rather than | isIterable() Python 3 will have Abstract Base Classes that will better defined some of these concepts. So if class authors bother to inherit from them when appropriate, isinstance(obj, Sequence) or isinstance(obj, Number), for instance, will tell you. Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: operator module isSequenceType with builtin set produces False
En Tue, 18 Dec 2007 09:15:12 -0300, English, Mark [EMAIL PROTECTED] escribió: try: set except NameError: from sets import Set as set class myset_fails(set): pass class myset_works(set): def __getitem__(self): pass s = set() fails = myset_fails() works = myset_works() import operator operator.isSequenceType(s) #Not what I expected False operator.isSequenceType(fails) #Not what I expected either False operator.isSequenceType(works) #A hint at what isSequenceType does ? True Are sets not sequences ? I didn't think the isSequenceDisclaimer gave false negatives. No, sets aren't sequences, as they have no order. Same as dicts, which aren't sequences either. If you add a __getitem__ method, isSequenceType can only *guess* that it *could* be used with integer indexes, so this object *could* be (or behave like) a sequence. The object could be a mapping too, and there is no way to tell the difference without inspecting or executing __getitem__. That's all isSequenceType can do for arbitrary objects. See Raymond Hettinger's post here too: http://groups.google.co.uk/group/comp.lang.python/tree/browse_frm/thread /bd04db20cc1f23bb/36f1f48bb7be1e4b?hl=enrnum=1q=set+isSequenceType_do ne=%2Fgroup%2Fcomp.lang.python%2Fbrowse_frm%2Fthread%2Fbd04db20cc1f23bb% 2F0454f5debc01c20d%3Fhl%3Den%26lnk%3Dgst%26q%3Dset%2BisSequenceType%26#d oc_0454f5debc01c20d Although there are suggestions that it means simply that you're headed in the wrong direction design wise: http://www.mail-archive.com/[EMAIL PROTECTED]/msg11576.html but then why have isSequenceType at all ? There are *true* sequences that it can detect, like lists, strings and all their subclases. For classic classes (that couldn't inherit from builtin types) all it can do is to guess based on __getitem__. For new style classes, it checks whether the slot used for __getitem__ is filled. Not particularly vital. Just implementing __str__ on some class and if one of the members was a sequence I was going to format that bit of data in brackets. As R. Hettinger said, you may add your own knowledge to what isSequenceType says, to determine whay *you* consider a sequence in this context. -- Gabriel Genellina -- http://mail.python.org/mailman/listinfo/python-list
