[EMAIL PROTECTED] wrote: > Can someone tell me why python doesn't crash when I do the following: > >>>> a=[] >>>> a.append(a) >>>> print a > [[...]] >>>> print a[0][0][0][0][0][0][0] > [[...]] > > How does python handle this internally? Will it crash or use up lot's > of memory in similar but more complicated cases? > >
No, it remembers internally which objects it has seen when converting them with str or repr. The C api includes functions Py_ReprEnter() and Py_ReprLeave() which the builtin objects call when they enter/leave repr. If Py_ReprEnter() returns 0 the object will return its representation, on subsequent calls Py_ReprEnter() returns 1 and the object knows it is being recursed. If you have your own recursive data structures then you may have to take your own precautions against recursion. The simplest way to do this is to make sure that the recursion always goes through a builtin object such as a list or dictionary. >>> class C: ... def __repr__(self): ... return "<C %r>" % self.inner ... inner = None ... >>> c = C() >>> c.inner = c >>> c File "<stdin>", line 3, in __repr__ File "<stdin>", line 3, in __repr__ File "<stdin>", line 3, in __repr__ ... and so on ... File "<stdin>", line 3, in __repr__ File "<stdin>", line 3, in __repr__ File "<stdin>", line 3, in __repr__ RuntimeError: maximum recursion depth exceeded >>> c.inner = [c] >>> c <C [<C [...]>]> N.B. Don't use a tuple here as tuples don't check for recursion. That's because tuples cannot be recursive except by containing another recursive data structure such as a list, dict, or user-defined object. -- http://mail.python.org/mailman/listinfo/python-list
