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__new__

Hello All,

I'm running 2.3.4

I was reading the documentation for classes & types
http://www.python.org/2.2.3/descrintro.html
And stumbled on this paragraph:

"""
__new__ must return an object. There's nothing that requires that it return a
new object that is an instance of its class argument, although that is the
convention. If you return an existing object, the constructor call will still
call its __init__ method. If you return an object of a different class, its
__init__ method will be called.
"""

The quote implies that when I call carol, b.__init__ should be called.
However, this does not seem to be the case (see code below). What am I not
understanding? Shouldn't the interpreter call b.__init__ when b is returned
from carol.__new__?

James

py> class bob(object):
.... def __init__(self):
.... print self.x
.... x = 2
....
py> class carol(object):
.... def __new__(cls):
.... return b
....
py> b=bob()
py> b.x
2
py> c = carol() # should print "2"
py> c
<__main__.bob object at 0x404333cc>

--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/
Nov 6 '05 #1
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3 Replies
James Stroud wrote:
I'm running 2.3.4

I was reading the documentation for classes & types
http://www.python.org/2.2.3/descrintro.html
And stumbled on this paragraph:

"""
__new__ must return an object. There's nothing that requires that it
return a new object that is an instance of its class argument, although
that is the convention. If you return an existing object, the constructor
call will still call its __init__ method. If you return an object of a
different class, its __init__ method will be called.
"""

The quote implies that when I call carol, b.__init__ should be called.
However, this does not seem to be the case (see code below). What am I not
understanding? Shouldn't the interpreter call b.__init__ when b is
returned from carol.__new__?


Here's what "Python in a Nutshell" (p84) says:

"""
Each new-style class has a static method named __new__. When you call
C(*args, **kwds) to create a new instance of a new-style class C, Python
invokes C.__new__(C, *args, **kwds). Python uses __new__'s return value x
as the newly created instance. Then, Python calls C.__init__(x, *args,
**kwds), but only when x is indeed an instance of C (otherwise, x's state
is as __new__ had left it). Thus, for a new-style class C, the statement
x=C(23) is equivlent to the following code:

x = C.__new__(C, 23)
if isinstance(x, C): C.__init__(x, 23)
"""

If the following code says what I think it does

class A(object):
def __init__(self):
print "init A"
def __new__(cls):
return b
class B(A):
def __init__(self):
print "init B"
b = object.__new__(B)

print "---"
A() # prints 'init B'
A.__init__(b) # prints 'init A'
print "---"
b = 42
A() # prints nothing

the "Nutshell" example should be changed to

x = C.__new__(C, 23)
if isinstance(x, C): x.__init__(23)

to comply with the current implementation (I used Python 2.4).

Peter

Nov 6 '05 #2
Peter Otten <__*******@web.de> wrote:
...
is as __new__ had left it). Thus, for a new-style class C, the statement
x=C(23) is equivlent to the following code:

x = C.__new__(C, 23)
if isinstance(x, C): C.__init__(x, 23) ... the "Nutshell" example should be changed to

x = C.__new__(C, 23)
if isinstance(x, C): x.__init__(23)

to comply with the current implementation (I used Python 2.4).


Hmmm -- not quite, because in the new-style object model special methods
are taken from the type, NOT from the instance as the latter is saying.
E.g, if you change class B into:

class B(A):
def __init__(self):
print "init B"
def f(): print 'from instance'
self.__init__ = f

you'll see that while b.__init__() would print 'from instance',
instantiating A will not. I think the right correction to the Nutshell
is therefore:

x = C.__new__(C, 23)
if isinstance(x, C): type(x).__init__(x, 23)

and this is how I plan to have it in the 2nd edition.
Alex
Nov 6 '05 #3
James Stroud wrote:
Hello All,

I'm running 2.3.4

I was reading the documentation for classes & types
http://www.python.org/2.2.3/descrintro.html
And stumbled on this paragraph:

"""
__new__ must return an object. There's nothing that requires that it return a
new object that is an instance of its class argument, although that is the
convention. If you return an existing object, the constructor call will still
call its __init__ method. If you return an object of a different class, its
__init__ method will be called.
"""


Any reason why you're looking at 2.2 documentation when you're running 2.3?

Anyway, the current docs corrected this mistake[1]

"""
If __new__() returns an instance of cls, then the new instance's
__init__() method will be invoked like "__init__(self[, ...])", where
self is the new instance and the remaining arguments are the same as
were passed to __new__().

If __new__() does not return an instance of cls, then the new instance's
__init__() method will not be invoked.
"""

[1]http://docs.python.org/ref/customization.html

STeVe
Nov 8 '05 #4

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