Alan Gauld wrote: >>map calls, list comprehension, etc. For loops, etc. >>work by indexing a sequences from zero to a higher >>index until out-of-bounds is reached. > > > What makes you think that? > So far as I know for loops work by calling next on > an iterator until nothing gets returned, no indexes > involved.(At least not in the for loop) But they could > just as well work by calling the len() function and > iterating that number of times. And len() could be > stored as part of the data structure ala Pascal arrays.
Hmm. From the language ref description of 'for': "The expression list is evaluated once; it should yield an iterable object." which begs the question of, what is an iterable object? The iterator protocol was introduced in Python 2.2; the "What's New" document give a good description of the old and new methods of iterating. Prior to Python 2.2, the _only_ way to make an object iterable was to give in a __getitem__() method. With Python 2.2 you can alternatively define __iter__(). http://www.python.org/doc/2.2.3/whatsnew/node4.html From the language ref description of __getitem__(): "Note: for loops expect that an IndexError will be raised for illegal indexes to allow proper detection of the end of the sequence." In fact a class that just defines __getitem__() can be iterated in a for loop: >>> class stepper: ... def __getitem__(self, i): ... if i < 5: return i ... raise IndexError ... >>> for i in stepper(): print i ... 0 1 2 3 4 > > The point being that it is dangerous to assume how > a language feature works internally, it can change from > version to version. Dangerous to assume, maybe, but special methods are there to be used, and the usage is generally well understood if not always well documented. > > In this case the iterator solution means that the for > loop can work on any iterable entity - like files for > instance. Yes, and for backward compatibility it also works on anything implementing __getitem__(). In fact strings have no __iter__() method, they use __getitem__(): >>> ''.__iter__ Traceback (most recent call last): File "<stdin>", line 1, in ? AttributeError: 'str' object has no attribute '__iter__' >>> ''.__getitem__ <method-wrapper object at 0x00A32E50> > > >>But why does this work? >> >> >>>>>class stepper: >> >>... def __getitem__(self, i): >>... return self.data[i] >>... >> >>>>>'p' in X >> >>True >> >>What does 'in' have to do with indexing? > > > Nothing unless its implementation uses a while loop > and index, but thats unlikely. But that is pretty close to what actually happens, according to the language ref docs for 'in' (see my previous post). Kent _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
