[Python-Dev] List mutation in list_repr?

[Hrvoje Niksic]

 > and I also don’t see any clue in the source as to when [list mutation]
 > would actually happen. Since inside the loop, the list object `v` is
 > never accessed other than passing `v->ob_item[i]` to the recursive
 > repr call, there shouldn’t be any mutation on the list object itself.

The individual object can have a reference to the list and (in extreme 
cases) do with it what it pleases:

class Evil:
     def __init__(self, l):
         self.l = l

     def __repr__(self):
         del l[:]
         return "evil"

l = []
l.append(Evil(l))
l.append(Evil(l))
print(l)

That is not something normal Python code does, but it shouldn't be 
allowed to crash the interpreter, either.