[EMAIL PROTECTED] wrote: > On Apr 25, 1:56 am, Peter Otten <[EMAIL PROTECTED]> wrote: >> Aaron Brady wrote: >> >>>> f.func_defaults[0] >> > [2, 3] >> >>>> f.func_defaults[0]+=[4] >> > Traceback (most recent call last): >> > File "<stdin>", line 1, in <module> >> > TypeError: 'tuple' object does not support item assignment >> >>>> f.func_defaults[0] >> > [2, 3, 4] >> >> > V. interesting. Operation succeeds but with a throw. Er, raise. >> >> This is not specific to func_defaults: >> >> >>> t = ([1],) >> >>> t[0] += [2] >> >> Traceback (most recent call last): >> File "<stdin>", line 1, in <module> >> TypeError: 'tuple' object does not support item assignment>>> t >> >> ([1, 2],) >> >> t[0] += [2] >> >> is resolved to >> >> t.__setitem__(t[0].__iadd__([2])) >> >> where list.__iadd__() succeeds but __setitem__() fails (because tuples >> don't have that method). >> >> Peter > > Curious why t.__setitem__ is called. Wouldn't it still refer to the > same list?
How is Python to know? Consider >>> items = [[], 0] >>> items[0] += [42] # could do without __setitem__() >>> items[1] += 42 # __setitem__() required >>> items [[42], 42] When the item is immutable the += operation can only work by replacing it with another value. Peter -- http://mail.python.org/mailman/listinfo/python-list
