Overriding list.__new__

Michele Simionato

Let me show first how does it work for tuples:

class MyTuple(tuple): .... def __new__(cls,strng): # implicit conversion string of ints => tuple
.... return super(MyTuple,cls).__new__(cls,map(int,strng.split ())) MyTuple('1 2') (1, 2)

No wonder here, everything is fine. However, if I do the same for
lists I get the following:
class MyList(list): .... def __new__(cls,strng): #implicit conversion string of ints => tuple
.... return super(MyList,cls).__new__(cls,map(int,strng.split( ))) MyList('1 2') ['1', ' ', '2']

The same is true for
class MyList(list): .... def __new__(cls,strng):
.... return list.__new__(cls,map(int,strng.split())) MyList('1 2') ['1', ' ', '2']

therefore it is not a problem of super.
The 'map' expression does not seem to be executed or, if its executed,
it has no effect at all. If I replace 'map' with anything, still I have
the same result:
class MyList(list): .... def __new__(cls,strng):
.... return list.__new__(cls,map(int,[]) # !notice: empty list here! MyList('1 2') ['1', ' ', '2']

In other words I always get the result of
list('1 2')

['1', ' ', '2']

and it seems impossible to override list.__new__.

I am very puzzled about that; any suggestions?

Michele

Jul 18 '05 #1

Terry Reedy

"Michele Simionato" <mi**@pitt.edu> wrote in message
news:22**************************@posting.google.c om...

Let me show first how does it work for tuples:

class MyTuple(tuple): ... def __new__(cls,strng): # implicit conversion string of ints => tuple ... return

super(MyTuple,cls).__new__(cls,map(int,strng.split ()))

MyTuple('1 2')

(1, 2)

No wonder here, everything is fine. However, if I do the same for
lists I get the following:

Values of immutable objects must be set when created, because they
cannot be changed thereafter. Mutable objects can be initialized in
the __init__() method. I suspect this is true of lists, so that
overriding __new__ for lists has no effect.

TJR

Jul 18 '05 #2