On Mar 19, 4:56 am, Duncan Booth <[EMAIL PROTECTED]> wrote: > [EMAIL PROTECTED] wrote: > > I am puzzled by the failure on 'a in a' for a=[a]. >>> a== [a] also > > fails. Can we assume/surmise/deduce/infer it's intentional? > > It may be less confusing if instead of an assignment following by a test > you just consider doing the test at the same time as the assignment > (then again it changes the behaviour so it may just be more confusing). > > There is a very limited set of values for which you would expect this > output: > > >>> a==[a] > True > >>> a in [a] > > True > > The most obvious one is a simple recursive list: > > >>> a = [] > >>> a.append(a) > >>> a==[a]; a in [a] > > True > True > > Pushing the recursion down a level breaks some of the comparisons: > > >>> a = [[]] > >>> a[0].append(a) > >>> a==[a] > > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > RuntimeError: maximum recursion depth exceeded in cmp>>> a in [a] > True > >>> a in a > > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > RuntimeError: maximum recursion depth exceeded in cmp > > which may be considered a bug or possibly it is just a limit of the > implementation. > > This also produces an overflow (in fact I think it is just the same > situation as above, recursing on the same list is handled, recursing on > different but equivalent lists is not): > > >>> a = []; a.append(a) > >>> b = []; b.append(b) > >>> a==b > > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > RuntimeError: maximum recursion depth exceeded in cmp
Pickle helps, >>> pickle.dumps( a ) b'\x80\x02]q\x00h\x00a.' >>> pickle.dumps( b ) b'\x80\x02]q\x00h\x00a.' >>> pickle.dumps( a )== pickle.dumps( b ) True However, >>> a= [] >>> b= [] >>> a.append( b ) >>> b.append( a ) >>> a [[[...]]] >>> b [[[...]]] >>> a== b Traceback (most recent call last): File "<stdin>", line 1, in <module> RuntimeError: maximum recursion depth exceeded in cmp >>> pickle.dumps( a )== pickle.dumps( b ) True Liar's paradox, q.e.d. -- http://mail.python.org/mailman/listinfo/python-list
