[Python-ideas] Fast sum() for non-numbers

[Terry Reedy]

On 7/5/2013 2:43 AM, Sergey wrote:

> I don't like the idea that `a` implicitly changes when I change `b`.

When a and b are the same things or one is part of the other, that must 
happen. This is the nature of mutable structures (when all or part of a 
structure can be named, which should be always). Here is Andrew 
Barnett's example translated into Python.

a = [1, [2, [3, None]]]
b = a[1]
b[1] = [10, [20, None]]
print(a)
 >>>
[1, [2, [10, [20, None]]]]

The difference bettween that being a lisp list and a python list is that 
list sees len(a) == 4, which Python would say 2, but that is because 
lisp looks as simple nested structures as if there were flat, becuase 
the nesting is, in a sense, an internal detail. Here is a possible 
corresponding iadd.

class Lisp(list):
     # the second member of each Lisp is either a Lisp or None.
     def __iadd__(self, other):
         if not instance(other, Lisp):
             raise TypeError
         while self[1] is not None:
             self = self[1]
         self[1] = other

-- 
Terry Jan Reedy