replacing one instance with another

[manstey]

Hi,
I'm not sure why this doesn't work:

>>>dic_myinstances={}

>>>class MyClass(object):

def __init__(self, id):
global dic_myinstances
if dic_myinstances.has_key(id):
self = dic_myinstances[id]
else:
dic_myinstances[id] = self

>>>ins1 = MyClass('xx')
ins2 = MyClass('yy')
ins3 = MyClass('xx')
ins3 is ins1

False

Why isn't ins3 the same as ins1? How can I get this to work?

thanks

[manstey]

Hi,

I solved it myself! I realised that __new__ creates self prior to
init, so this works:

>>>dic_myinstances={}

>>>class MyClass(object):

def __new__(self,id):
global dic_myinstances
if dic_myinstances.has_key(id):
return dic_myinstances[id]
else:
dic_myinstances[id] = self
return self

>>>ins1 = MyClass('xx')
ins2 = MyClass('yy')
ins3 = MyClass('xx')
print ins3 is ins1

True

Is this the best way to do this?

[manstey]

Hi, yet again to myself!

I've realised after further testing and reading that I actually need
to do this:

>>>dic_myinstances={}
class MyClass(object):

def __new__(cls,id):
global dic_myinstances
if dic_myinstances.has_key(id):
return dic_myinstances[id]
else:
dic_myinstances[id] = super(MyClass, cls).__new__(cls, id)
return dic_myinstances[id]
def __init__(self,id):
print id

>>>ins1 = MyClass('xx')

'xx'

>>>ins2 = MyClass('yy')

'yy'

>>>ins3 = MyClass('xx')

'xx'

>>>ins3 is ins1

True

[Gabriel Genellina]

En Sun, 25 Mar 2007 23:34:51 -0300, manstey <ma*****@csu.edu.auescribió:

I've realised after further testing and reading that I actually need
to do this:

>>>dic_myinstances={}
class MyClass(object):

def __new__(cls,id):
global dic_myinstances
if dic_myinstances.has_key(id):
return dic_myinstances[id]
else:
dic_myinstances[id] = super(MyClass, cls).__new__(cls, id)
return dic_myinstances[id]
def __init__(self,id):
print id

>>>ins1 = MyClass('xx')

'xx'

>>>ins2 = MyClass('yy')

'yy'

>>>ins3 = MyClass('xx')

'xx'

>>>ins3 is ins1

True

That's fine, but notice that __init__ is called even if the instance
already exists. That's usually undesirable, and you can use a factory
function instead:

def MyClassFactory(id):
inst = dic_myinstances.get(id)
if inst is None:
dic_myinstances[id] = inst = MyClass(id)
return inst

(Notice that no global statement is needed, even on your __new__)
You can make it a static method if you wish.

--
Gabriel Genellina