Tuples and immutability

Peter Otten __peter__ at web.de
Fri Mar 7 12:38:07 CET 2014


Chris Angelico wrote:

> On Fri, Mar 7, 2014 at 8:33 PM, Duncan Booth
> <duncan.booth at invalid.invalid> wrote:
>> Is there any reason why tuples need to throw an exception on assigning to
>> the element if the old value and new value are the same object?
> 
> It'd be easy enough to implement your own tuple subclass that behaves
> that way. Try it! See how many situations it actually helps.

>>> class T(tuple):
...     def __setitem__(self, index, value):
...             if value is not self[index]:
...                     raise TypeError("{} is not {}".format(value, 
self[index]))
... 
>>> for i, k in zip(range(250, 260), range(250, 260)):
...     T([i])[0] = k
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 4, in __setitem__
TypeError: 257 is not 257

I'm not sure "help" is the right word here ;)




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