lists v. tuples

[castironpi at gmail.com]

On Mar 19, 4:56 am, Duncan Booth <duncan.bo... at invalid.invalid> wrote:
> castiro... at gmail.com 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.