Re: Creating alot of class instances?
> Steven D'Aprano (SD) wrote: >SD> On Mon, 06 Jul 2009 05:47:18 -0700, Scott David Daniels wrote: >>> Steven D'Aprano wrote: ... That's the Wrong Way to do it -- you're using a screwdriver to hammer a nail >>> >>> Don't knock tool abuse (though I agree with you here). Sometimes tool >>> abuse can produce good results. For example, using hammers to drive >>> screws for temporary strong holds led to making better nails. >SD> Historically, nails were invented a long time before screws. Screws as >SD> fasteners weren't invented until the 1400s, nails were used thousands of >SD> years ago. >SD> And hammering screws makes temporary *weak* holds, not strong, because >SD> the screw only touches the sides of the hole lightly. Unless the screw >SD> has been specifically designed to be hammered, hammering screws is pretty >SD> much the definition of incompetence and/or laziness! In our language (Dutch, i.e. Guido's mother tongue) `American screwdriver' is an expression meaning `hammer' :=) -- Piet van Oostrum URL: http://pietvanoostrum.com [PGP 8DAE142BE17999C4] Private email: p...@vanoostrum.org -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating alot of class instances?
On Mon, 06 Jul 2009 05:47:18 -0700, Scott David Daniels wrote: > Steven D'Aprano wrote: >> ... That's the Wrong Way to do it -- >> you're using a screwdriver to hammer a nail > > Don't knock tool abuse (though I agree with you here). Sometimes tool > abuse can produce good results. For example, using hammers to drive > screws for temporary strong holds led to making better nails. Historically, nails were invented a long time before screws. Screws as fasteners weren't invented until the 1400s, nails were used thousands of years ago. And hammering screws makes temporary *weak* holds, not strong, because the screw only touches the sides of the hole lightly. Unless the screw has been specifically designed to be hammered, hammering screws is pretty much the definition of incompetence and/or laziness! -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating alot of class instances?
Steven D'Aprano wrote: ... That's the Wrong Way to do it -- you're using a screwdriver to hammer a nail Don't knock tool abuse (though I agree with you here). Sometimes tool abuse can produce good results. For example, using hammers to drive screws for temporary strong holds led to making better nails. --Scott David Daniels scott.dani...@acm.org -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating alot of class instances?
Steven, Before your post I was contemplating about the merits of using the globals(). After reading your post I am totally convinced that your suggestion that was also suggested by previous posters is the way to go. At first I thought it would be limiting to not to have the instance names properly setup, but now I understand it better. thank you all again. -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating alot of class instances?
On Sun, 05 Jul 2009 19:27:25 -0700, kk wrote: > Hi > > Thank you so much for wonderful tips and suggestions. > > I also found a solution to dynamic naming of the instances(I think). It > does not sound like a very secure method but since my application will > be just processing data one way I think it might be alright. I will > compare to the list and dictionary methods. > > globals()["Some_Instance_Name"] You're fighting the computer instead of working with it. That's the Wrong Way to solve the problem -- you're doing more work than needed, for little or no benefit. My bet is, you have code that looks something like this: for i in range(N): # N comes from somewhere else # Create a new variable globals()["Some_Instance_Name%s" % i] = instance() # Do something with the variables for i in range(N): # Look up the variable x = globals()["Some_Instance_Name%s" % i] process(x) Am I close? That's the Wrong Way to do it -- you're using a screwdriver to hammer a nail. The right way to work with an unknown number of data elements is to put them in a list, and process each element in the list, not to try giving them all unique names. The only reason for using named variables is so you can use the name in source code: my_value = Some_Instance87 + Some_Instance126 But you can't do that, because you don't know how many instances there are, you don't know whether to write Some_Instance87 or Some_Instance125 or Some_Instance19. So instead, do something like this: instances = [] for i in range(N): # Create a new instance and store it for later instances.append( instance() ) # Later on: for x in instances(): process(x) -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating alot of class instances?
Hi Thank you so much for wonderful tips and suggestions. I also found a solution to dynamic naming of the instances(I think). It does not sound like a very secure method but since my application will be just processing data one way I think it might be alright. I will compare to the list and dictionary methods. globals()["Some_Instance_Name"] -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating alot of class instances?
kk wrote: > I will be querying some data and create class instances based on the > data I gather. But the problem as I mentioned is that I do not know > the names and the number of the end class instances. They will be > based on the content of the data. So how can I create class instances > within a loop and when the loop is done how can I figure out the list > of instances via class membership? I can track the names by > introducing another list but I want to understand the class side of > things. Do you need an exact number or just a rough statistic? In order to estimate the number of instances you can query the reference count of the class. Since every instance usually increases the reference count by one it provides a good overview. Note that lots of other things like imports increase the reference count, too. >>> import sys >>> class Example(object): ... pass ... >>> sys.getrefcount(Example) 5 >>> examples = list(Example() for i in range(10)) >>> examples [<__main__.Example object at 0x7f2e5cd61110>, <__main__.Example object at 0x7f2e5cd61150>, <__main__.Example object at 0x7f2e5cd61190>, <__main__.Example object at 0x7f2e5cd611d0>, <__main__.Example object at 0x7f2e5cd61210>, <__main__.Example object at 0x7f2e5cd61250>, <__main__.Example object at 0x7f2e5cd61390>, <__main__.Example object at 0x7f2e5cd613d0>, <__main__.Example object at 0x7f2e5cd61410>, <__main__.Example object at 0x7f2e5cd61450>] >>> sys.getrefcount(Example) 15 -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating alot of class instances?
For example lets say I have
class MyMaterials:
and my instances might need to look like
material_01
material_02
or
light_01
light_02
or
Mesh_01
Mesh_02 etc
If you do not know how many instances you are going to create from the
beginning, there is no way for you to know which of the instances you mentioned
above will get created. So having names for all of the instances will not help
you since you will never know what names are "safe" to use.
On the other hand, if you have all instances in a list, you can refer to them
by index and you know exactly how many of them you have.
If you would like to get instances by some name you gave to them, maybe
something like this will work:
def get_instance(name):
for instance in instance_list:
if instance.name == name:
return instance
return None
Another option might be to use a counter in the class that keeps
track of the number of instances:
class MyMaterial:
instances = 0
def __init__(self, name):
self.name = name
self.instance = MyMaterial.instances
MyMaterial.instances += 1
def __str__(self):
return "%s_%02i" % (self.name, self.instance)
m = MyMaterial("Brick")
print m
# "Brick_00"
print repr([MyMaterial("Stone") for _ in range(5)])
# "[Stone_01, Stone_02, Stone_03, Stone_04, Stone_05]"
It's not thread-safe, but it may do the trick.
-tkc
--
http://mail.python.org/mailman/listinfo/python-list
Re: Creating alot of class instances?
> Thank you soo much for speedy and in detailed help. Your replies > really cleared out most of the cloud for me. I have one last issue to > resolve which is something I did not articulate properly, I realize > now. The last issue is actually automatically naming the instances. > The reason I was using the "instance_count" is for enumerating the > actual name of an instance. > > For example lets say I have > > class MyMaterials: > > and my instances might need to look like > > material_01 > material_02 > or > light_01 > light_02 > or > Mesh_01 > Mesh_02 etc If you do not know how many instances you are going to create from the beginning, there is no way for you to know which of the instances you mentioned above will get created. So having names for all of the instances will not help you since you will never know what names are "safe" to use. On the other hand, if you have all instances in a list, you can refer to them by index and you know exactly how many of them you have. If you would like to get instances by some name you gave to them, maybe something like this will work: def get_instance(name): for instance in instance_list: if instance.name == name: return instance return None Note that this might very well return None if no instance with that particular name was found. -- Rickard Lindberg -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating alot of class instances?
Hi Thank you soo much for speedy and in detailed help. Your replies really cleared out most of the cloud for me. I have one last issue to resolve which is something I did not articulate properly, I realize now. The last issue is actually automatically naming the instances. The reason I was using the "instance_count" is for enumerating the actual name of an instance. For example lets say I have class MyMaterials: and my instances might need to look like material_01 material_02 or light_01 light_02 or Mesh_01 Mesh_02 etc I will need to get the base names from the "some list" and create the names accordingly from the list array. Basically I also need to generate the instance names based on the some list. For example some_list[2] might denote a name for the instance. I will study the sampled codes in depth now. Maybe you have answered the naming issue as well, if so please frgove the noise. thanks -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating alot of class instances?
The solution might be dead simple but I just cannot figure out at the moment. For example this is what I need in the simplest form class myclass(): def __init__(self,name): self.name=name for count,data in enumerate(some list): instance_count=myclass() instance_count.name=data print instances Sounds like a use for a list: instances = [] for count, data in enumerate(some_list): # 1) camel-case is preferred for classnames # 2) since your __init__() expects the name # pass it in, instead of setting it later instance = MyClass(data) instances.append(instance) This can be written in a slightly more condensed-yet-readable list-comprehension form as: instances = [MyClass(data) for data in some_list] You then have a list/array of instances you can print: print instances or pick off certain items from it: print instances[42] -tkc -- http://mail.python.org/mailman/listinfo/python-list
Re: Creating alot of class instances?
On 7/5/09, kk wrote: > I am new to Python classes and trying to figure out this particular > issue here. I will need to create instances of a class. But at the > moment I do not know how many instances I will end up having, in every > case it might be different. Most of the documents I read makes this > simpl class-student analogy to explain python classes which is fine. > But in those examples the number and the names of the instances were > known and limited That's no problem. The only limit to the number of instances of a class you can create is your memory - and not even that if you don't need to 'keep' the instances. > I will be querying some data and create class instances based on the > data I gather. But the problem as I mentioned is that I do not know > the names and the number of the end class instances. They will be > based on the content of the data. So how can I create class instances > within a loop and when the loop is done how can I figure out the list > of instances via class membership? I can track the names by > introducing another list but I want to understand the class side of > things. > > The solution might be dead simple but I just cannot figure out at the > moment. > > For example this is what I need in the simplest form > > class myclass(): > def __init__(self,name): > self.name=name > > for count,data in enumerate(some list): > instance_count=myclass() > instance_count.name=data > > print instances Okay, to solve your problem, we add a list containing all the instances: class myclass(): def __init__(self,name): self.name=name instances = [] for count,data in enumerate(some list): instance_count=myclass() instance_count.name=data instances.append(instance_count) print instances = However, that won't work because myclass has an __init__ with 2 attributes, so you will have to call it using an attribute: class myclass(): def __init__(self,name): self.name=name instances = [] for count,data in enumerate(some list): instance_count=myclass(data) instances.append(instance_count) print instances = This works, but it can be done better: First we notice that count is not used at all, so why create it? class myclass(): def __init__(self,name): self.name=name instances = [] for data in some list: instance_count=myclass(data) instances.append(instance_count) print instances = Then, the variable instance_count is created once, then used in the next line. We can do that at once: class myclass(): def __init__(self,name): self.name=name instances = [] for data in some list: instances.append(myclass(data)) print instances Finally, "print instances" does not give very nice looking information, so I would change this to: class myclass(): def __init__(self,name): self.name=name instances = [] for data in some list: instances.append(myclass(data)) print (instance.name for instance in instances) -- André Engels, andreeng...@gmail.com -- http://mail.python.org/mailman/listinfo/python-list
Creating alot of class instances?
Hi I am new to Python classes and trying to figure out this particular issue here. I will need to create instances of a class. But at the moment I do not know how many instances I will end up having, in every case it might be different. Most of the documents I read makes this simpl class-student analogy to explain python classes which is fine. But in those examples the number and the names of the instances were known and limited. I will be querying some data and create class instances based on the data I gather. But the problem as I mentioned is that I do not know the names and the number of the end class instances. They will be based on the content of the data. So how can I create class instances within a loop and when the loop is done how can I figure out the list of instances via class membership? I can track the names by introducing another list but I want to understand the class side of things. The solution might be dead simple but I just cannot figure out at the moment. For example this is what I need in the simplest form class myclass(): def __init__(self,name): self.name=name for count,data in enumerate(some list): instance_count=myclass() instance_count.name=data print instances thanks -- http://mail.python.org/mailman/listinfo/python-list
