Iterating over objects of a class
Hi,
How can I iterate over all the objects of a class?
I wrote the code like following:
class baseClass(object):
__registry = []
def __init__(self, name):
self.__registry.append(self)
self.name = name
def __iter__(self):
baseClass.item = 0
return self.__registry[0]
def next(self):
if baseClass.item >= len(self.__registry):
raise StopIteration
baseClass.item += 1
return self.__registry[baseClass.item - 1]
For testing, create the following objects-
a = baseClass("Test1")
b = baseClass("Test2")
class subClass (baseClass):
pass
c = subClass("Test3")
>Actual Iteration<
for i in a:
print i.name
Test1
Test2
Test3
---
I see the following problems in the code:
1. I have to iterate over any of the objects. For correctness, I
wanted to iterate over the class, like
for i in baseClass():
do x
but that will will create one more object - which I do not want.
2. If the subclass wants to do somethings in its constructor, I am not
sure how to update the registry.
class subClass (baseClass):
def __init__(self, name):
**do something**
super.init(self, name) > This errors out, saying it needs
super, not subClass
Another method I thought of implementing it was using generators -
where-in baseClass.objects() is a generator which will yield the
objects one by one - but even then the second issue remains.
If somebody can help me out, I would be very thankful.
Regards
K
--
http://mail.python.org/mailman/listinfo/python-list
Re: Iterating over objects of a class
Kottiyath schrieb:
Hi,
How can I iterate over all the objects of a class?
I wrote the code like following:
class baseClass(object):
Consider adopting PEP 8 coding conventions.
__registry = []
def __init__(self, name):
self.__registry.append(self)
self.name = name
def __iter__(self):
baseClass.item = 0
return self.__registry[0]
def next(self):
if baseClass.item >= len(self.__registry):
raise StopIteration
baseClass.item += 1
return self.__registry[baseClass.item - 1]
For testing, create the following objects-
a = baseClass("Test1")
b = baseClass("Test2")
class subClass (baseClass):
pass
c = subClass("Test3")
>Actual Iteration<
for i in a:
print i.name
Test1
Test2
Test3
---
I see the following problems in the code:
1. I have to iterate over any of the objects. For correctness, I
wanted to iterate over the class, like
for i in baseClass():
do x
but that will will create one more object - which I do not want.
2. If the subclass wants to do somethings in its constructor, I am not
sure how to update the registry.
class subClass (baseClass):
def __init__(self, name):
**do something**
super.init(self, name) > This errors out, saying it needs
super, not subClass
You don't show the actual traceback, however the idiom for invoking
super for new-style-classes is
super(subClass, self).__init__(name)
for your case.
Another method I thought of implementing it was using generators -
where-in baseClass.objects() is a generator which will yield the
objects one by one - but even then the second issue remains.
If somebody can help me out, I would be very thankful.
Using a generator or not isn't the issue here.
What you need is a *class*-based access, not instance-based. There are
various methods to accomplish this. The simplest is to ditch the
obnoxious __registry as name, and just do
class BaseClass(object):
REGISTRY = []
Then iterating is a simple matter of
for instance in BaseClass.REGISTRY:
...
Case solved. Alternatively, if you insist on the concept of privacy for
that registry, you can use a classmethod:
class BaseClass(object):
@classmethod
def registry(cls):
for i in cls.__registry:
yield i
Last but not least you *could* go for a __metaclass__ with an
__getitem__-method, that makes thinks look fancy because you then can do:
for instance in BaseClass:
...
I leave it as an exercise to you - gotta go christmas dining now :)
Diez
--
http://mail.python.org/mailman/listinfo/python-list
Re: Iterating over objects of a class
Diez B. Roggisch wrote:
Kottiyath schrieb:
Hi,
How can I iterate over all the objects of a class?
I wrote the code like following:
class baseClass(object):
Consider adopting PEP 8 coding conventions.
__registry = []
def __init__(self, name):
self.__registry.append(self)
self.name = name
def __iter__(self):
baseClass.item = 0
return self.__registry[0]
def next(self):
if baseClass.item >= len(self.__registry):
raise StopIteration
baseClass.item += 1
return self.__registry[baseClass.item - 1]
For testing, create the following objects-
a = baseClass("Test1")
b = baseClass("Test2")
class subClass (baseClass):
pass
c = subClass("Test3")
>Actual Iteration<
for i in a:
print i.name
Test1
Test2
Test3
---
I see the following problems in the code:
1. I have to iterate over any of the objects. For correctness, I
wanted to iterate over the class, like
for i in baseClass():
do x
but that will will create one more object - which I do not want.
2. If the subclass wants to do somethings in its constructor, I am not
sure how to update the registry.
class subClass (baseClass):
def __init__(self, name):
**do something**
super.init(self, name) > This errors out, saying it needs
super, not subClass
You don't show the actual traceback, however the idiom for invoking
super for new-style-classes is
super(subClass, self).__init__(name)
for your case.
Another method I thought of implementing it was using generators -
where-in baseClass.objects() is a generator which will yield the
objects one by one - but even then the second issue remains.
If somebody can help me out, I would be very thankful.
Using a generator or not isn't the issue here.
What you need is a *class*-based access, not instance-based. There are
various methods to accomplish this. The simplest is to ditch the
obnoxious __registry as name, and just do
class BaseClass(object):
REGISTRY = []
Then iterating is a simple matter of
for instance in BaseClass.REGISTRY:
...
Case solved. Alternatively, if you insist on the concept of privacy for
that registry, you can use a classmethod:
class BaseClass(object):
@classmethod
def registry(cls):
for i in cls.__registry:
yield i
Last but not least you *could* go for a __metaclass__ with an
__getitem__-method, that makes thinks look fancy because you then can do:
for instance in BaseClass:
...
I leave it as an exercise to you - gotta go christmas dining now :)
The other thing to remember is that because the 'registry' contains
references to the instances, they won't be garbage collected.
--
http://mail.python.org/mailman/listinfo/python-list
Re: Iterating over objects of a class
On Dec 24, 10:52 pm, "Diez B. Roggisch" wrote:
> Kottiyath schrieb:
>
> > Hi,
> > How can I iterate over all the objects of a class?
> > I wrote the code like following:
> > class baseClass(object):
>
> Consider adopting PEP 8 coding conventions.
>
>
>
> > __registry = []
>
> > def __init__(self, name):
> > self.__registry.append(self)
> > self.name = name
>
> > def __iter__(self):
> > baseClass.item = 0
> > return self.__registry[0]
>
> > def next(self):
> > if baseClass.item >= len(self.__registry):
> > raise StopIteration
> > baseClass.item += 1
> > return self.__registry[baseClass.item - 1]
>
> > For testing, create the following objects-
> > a = baseClass("Test1")
> > b = baseClass("Test2")
>
> > class subClass (baseClass):
> > pass
> > c = subClass("Test3")
>
> > >Actual Iteration<
> > for i in a:
> > print i.name
>
> > Test1
> > Test2
> > Test3
>
> > ---
> > I see the following problems in the code:
> > 1. I have to iterate over any of the objects. For correctness, I
> > wanted to iterate over the class, like
> > for i in baseClass():
> > do x
> > but that will will create one more object - which I do not want.
>
> > 2. If the subclass wants to do somethings in its constructor, I am not
> > sure how to update the registry.
> > class subClass (baseClass):
> > def __init__(self, name):
> > **do something**
> > super.init(self, name) > This errors out, saying it needs
> > super, not subClass
>
> You don't show the actual traceback, however the idiom for invoking
> super for new-style-classes is
>
> super(subClass, self).__init__(name)
>
> for your case.
>
> > Another method I thought of implementing it was using generators -
> > where-in baseClass.objects() is a generator which will yield the
> > objects one by one - but even then the second issue remains.
> > If somebody can help me out, I would be very thankful.
>
> Using a generator or not isn't the issue here.
>
> What you need is a *class*-based access, not instance-based. There are
> various methods to accomplish this. The simplest is to ditch the
> obnoxious __registry as name, and just do
>
> class BaseClass(object):
>
> REGISTRY = []
>
> Then iterating is a simple matter of
>
> for instance in BaseClass.REGISTRY:
> ...
>
> Case solved. Alternatively, if you insist on the concept of privacy for
> that registry, you can use a classmethod:
>
> class BaseClass(object):
>
> @classmethod
> def registry(cls):
> for i in cls.__registry:
> yield i
>
> Last but not least you *could* go for a __metaclass__ with an
> __getitem__-method, that makes thinks look fancy because you then can do:
>
> for instance in BaseClass:
> ...
>
> I leave it as an exercise to you - gotta go christmas dining now :)
>
> Diez
Thank you Very much, Diez. I was able to do the Generator and the
super part of it, but I never even thought of the metaclass option.
I will try it out. Thank you very much.
Merry Christmas.
P.S - >Also, I will use the PEP 8 coding conventions
--
http://mail.python.org/mailman/listinfo/python-list
Re: Iterating over objects of a class
On Dec 24, 11:04 pm, MRAB wrote:
> Diez B. Roggisch wrote:
> > Kottiyath schrieb:
> >> Hi,
> >> How can I iterate over all the objects of a class?
> >> I wrote the code like following:
> >> class baseClass(object):
>
> > Consider adopting PEP 8 coding conventions.
>
> >> __registry = []
>
> >> def __init__(self, name):
> >> self.__registry.append(self)
> >> self.name = name
>
> >> def __iter__(self):
> >> baseClass.item = 0
> >> return self.__registry[0]
>
> >> def next(self):
> >> if baseClass.item >= len(self.__registry):
> >> raise StopIteration
> >> baseClass.item += 1
> >> return self.__registry[baseClass.item - 1]
>
> >> For testing, create the following objects-
> >> a = baseClass("Test1")
> >> b = baseClass("Test2")
>
> >> class subClass (baseClass):
> >> pass
> >> c = subClass("Test3")
>
> >> >Actual Iteration<
> >> for i in a:
> >> print i.name
>
> >> Test1
> >> Test2
> >> Test3
>
> >> ---
> >> I see the following problems in the code:
> >> 1. I have to iterate over any of the objects. For correctness, I
> >> wanted to iterate over the class, like
> >> for i in baseClass():
> >> do x
> >> but that will will create one more object - which I do not want.
>
> >> 2. If the subclass wants to do somethings in its constructor, I am not
> >> sure how to update the registry.
> >> class subClass (baseClass):
> >> def __init__(self, name):
> >> **do something**
> >> super.init(self, name) > This errors out, saying it needs
> >> super, not subClass
>
> > You don't show the actual traceback, however the idiom for invoking
> > super for new-style-classes is
>
> > super(subClass, self).__init__(name)
>
> > for your case.
>
> >> Another method I thought of implementing it was using generators -
> >> where-in baseClass.objects() is a generator which will yield the
> >> objects one by one - but even then the second issue remains.
> >> If somebody can help me out, I would be very thankful.
>
> > Using a generator or not isn't the issue here.
>
> > What you need is a *class*-based access, not instance-based. There are
> > various methods to accomplish this. The simplest is to ditch the
> > obnoxious __registry as name, and just do
>
> > class BaseClass(object):
>
> > REGISTRY = []
>
> > Then iterating is a simple matter of
>
> > for instance in BaseClass.REGISTRY:
> > ...
>
> > Case solved. Alternatively, if you insist on the concept of privacy for
> > that registry, you can use a classmethod:
>
> > class BaseClass(object):
>
> > �...@classmethod
> > def registry(cls):
> > for i in cls.__registry:
> > yield i
>
> > Last but not least you *could* go for a __metaclass__ with an
> > __getitem__-method, that makes thinks look fancy because you then can do:
>
> > for instance in BaseClass:
> > ...
>
> > I leave it as an exercise to you - gotta go christmas dining now :)
>
> The other thing to remember is that because the 'registry' contains
> references to the instances, they won't be garbage collected.
Is there any other way out in this case?
I have factory methods - and I have to loop over them - sort of Chain
of Responsibility pattern.
Having a registry inside the class instance and looping through them
was the only clean thing I could think of.
I understand that garbage collection would be an issue - but is there
any way out?
--
http://mail.python.org/mailman/listinfo/python-list
Re: Iterating over objects of a class
En Wed, 24 Dec 2008 16:18:55 -0200, Kottiyath escribió: The other thing to remember is that because the 'registry' contains references to the instances, they won't be garbage collected. Is there any other way out in this case? I have factory methods - and I have to loop over them - sort of Chain of Responsibility pattern. Having a registry inside the class instance and looping through them was the only clean thing I could think of. I understand that garbage collection would be an issue - but is there any way out? You may keep all that structures - just use weak references (see the weakref module). There isn't a WeakList nor WeakSet out-of-the-box but you may use a WeakKeyDictionary (set the value to anything, None by example). -- Gabriel Genellina -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterating over objects of a class
On Dec 24, 11:48 pm, "Gabriel Genellina" wrote: > En Wed, 24 Dec 2008 16:18:55 -0200, Kottiyath > escribió: > > >> The other thing to remember is that because the 'registry' contains > >> references to the instances, they won't be garbage collected. > > > Is there any other way out in this case? > > I have factory methods - and I have to loop over them - sort of Chain > > of Responsibility pattern. > > Having a registry inside the class instance and looping through them > > was the only clean thing I could think of. > > I understand that garbage collection would be an issue - but is there > > any way out? > > You may keep all that structures - just use weak references (see the > weakref module). > There isn't a WeakList nor WeakSet out-of-the-box but you may use a > WeakKeyDictionary (set the value to anything, None by example). > > -- > Gabriel Genellina Thank you very much, Gabriel. I am very thankful to everyone. -- http://mail.python.org/mailman/listinfo/python-list
Re: Iterating over objects of a class
Kottiyath wrote: ... Having a registry inside the class instance and looping through them was the only clean thing I could think of. I understand that garbage collection would be an issue - but is there any way out? Search for weakref in the documentatione. In this case, I'd use a WeakValueDictionary() from id(obj) to obj. Note id(obj) is guaranteed to be unique for all objects in existance, but the id of a collected object may match the id of a new object. >>> import weakref >>> d = weakref.WeakValueDictionary() >>> vs = [Int(n) for n in range(3, 500, 70)] >>> for n in vs: d[id(n)] = n v = Int(n+1) d[id(v)] = v >>> for obj in d.values(): # values is safer than itervalues print obj >>> # note the above was the following, which fails: >>> for v in d.values(): # values is safer than itervalues print v For extra credit, explain why values is better. --Scott David Daniels scott.dani...@acm.org -- http://mail.python.org/mailman/listinfo/python-list
