Programming Forums > Script Programming > Python

MR.T · Oct 28th, 2006, 8:07 PM

I'm having a bit of trouble figuring out classes. Here's a rough example of what I'm trying to do:

class Item_Class:
def __int__(self):
self.Name
self.Description
self.Price

List[]
List.append(Item_Class)
List[0].Name = "Something"
List[0].Description = "Something"
List[0].Price = "Something"

That works fine. But if I append the class to the list a second time, and assign some values to it, the previous instance of the class is changed to those values also.
What am I doing wrong? Isn't the idea of classes to be able to make different instances of it?

thanks

Sane · Oct 28th, 2006, 8:36 PM

You need to change "List.append(Item_Class)" to "List.append(Item_Class())". The () initiates a new object for that class. You were appending to the list the direct reference to the class.

The following is a working example...

Note that the class's contents are completely blank. This is because you're declaring and initiating the variables externally. There is no need to put anything inside.

(Toggle Plain Text)

class Item_Class:
    pass

List = []

List.append(Item_Class())

List[0].Name = "IDLE"
List[0].Description = "Python Interpreter"
List[0].Price = 0.00


List.append(Item_Class())

List[1].Name = "My Help"
List[1].Description = "Programming Help From Sane"
List[1].Price = 0.00


print List[0].Name
print List[0].Description
print List[0].Price

print List[1].Name
print List[1].Description
print List[1].Price

However, if you want to write it in a way that provides a better understanding of how classes work, then consider the following...

(Toggle Plain Text)

class Item_Class:
    def __init__(self, Name, Description, Price):
        self.Name = Name
        self.Description = Description
        self.Price = Price

List = []

List.append(Item_Class("IDLE", "Python Interpreter", 0.00))
List.append(Item_Class("My Help", "Programming Help From Sane", 0.00))

print List[0].Name
print List[0].Description
print List[0].Price

print List[1].Name
print List[1].Description
print List[1].Price

Furthermore expanding on the program, we can get the same output like so...

(Toggle Plain Text)

class Item_Class:
    def __init__(self, Name, Description, Price):
        self.Name = Name
        self.Description = Description
        self.Price = Price

    def Output(self):
        print self.Name
        print self.Description
        print self.Price

List = []

List.append(Item_Class("IDLE", "Python Interpreter", 0.00))
List.append(Item_Class("My Help", "Programming Help From Sane", 0.00))

List[0].Output()
List[1].Output()

If anything I'm doing confuses you, don't hesitate to ask any questions. I also strongly suggest that you use the search to look through the Python forum for more examples using classes. I believe there's some excellent information provided by Arevos lying around.

I also urge you to use [code][/code] tags next time you post code.

MR.T · Oct 28th, 2006, 9:26 PM

Ahh, that makes sense. Thank you very much for simple and clear example.
On a related note were can I find out more about the different ways classes are used in programming?

BTW sorry about not puting the code in tags, I was going to but forgot because I was doing a couple things at the same time(maybe I should cut down on the Dr.pepper).

thanks again

Sane · Oct 28th, 2006, 9:34 PM

The way classes are used in programming are for you to discover and apply yourself as a programmer. The applications of Object-Oriented programming are very vast, and can be applied basically anywhere. But the more you use it as a programmer, the more you'll see where it should and should not be applied.

Otherwise, I hope wherever you're learning classes from explains the general applications of classes, and where and where not to use them.

Oct 28th, 2006, 8:07 PM	#1
MR.T Newbie Join Date: Feb 2006 Location: USA,Michigan Posts: 22 Rep Power: 0	Classes question I'm having a bit of trouble figuring out classes. Here's a rough example of what I'm trying to do: class Item_Class: def __int__(self): self.Name self.Description self.Price List[] List.append(Item_Class) List[0].Name = "Something" List[0].Description = "Something" List[0].Price = "Something" That works fine. But if I append the class to the list a second time, and assign some values to it, the previous instance of the class is changed to those values also. What am I doing wrong? Isn't the idea of classes to be able to make different instances of it? thanks __________________ "Gee, Brain, what do you want to do tonight?" "The same thing we do every night, Pinky: Try to take over the world!"

Oct 28th, 2006, 8:36 PM	#2
Sane Programming Guru Join Date: Apr 2005 Posts: 1,799 Rep Power: 5	You need to change "List.append(Item_Class)" to "List.append(Item_Class())". The () initiates a new object for that class. You were appending to the list the direct reference to the class. The following is a working example... Note that the class's contents are completely blank. This is because you're declaring and initiating the variables externally. There is no need to put anything inside. (Toggle Plain Text) class Item_Class: pass List = [] List.append(Item_Class()) List[0].Name = "IDLE" List[0].Description = "Python Interpreter" List[0].Price = 0.00 List.append(Item_Class()) List[1].Name = "My Help" List[1].Description = "Programming Help From Sane" List[1].Price = 0.00 print List[0].Name print List[0].Description print List[0].Price print List[1].Name print List[1].Description print List[1].Price class Item_Class: pass List = [] List.append(Item_Class()) List[0].Name = "IDLE" List[0].Description = "Python Interpreter" List[0].Price = 0.00 List.append(Item_Class()) List[1].Name = "My Help" List[1].Description = "Programming Help From Sane" List[1].Price = 0.00 print List[0].Name print List[0].Description print List[0].Price print List[1].Name print List[1].Description print List[1].Price However, if you want to write it in a way that provides a better understanding of how classes work, then consider the following... (Toggle Plain Text) class Item_Class: def __init__(self, Name, Description, Price): self.Name = Name self.Description = Description self.Price = Price List = [] List.append(Item_Class("IDLE", "Python Interpreter", 0.00)) List.append(Item_Class("My Help", "Programming Help From Sane", 0.00)) print List[0].Name print List[0].Description print List[0].Price print List[1].Name print List[1].Description print List[1].Price class Item_Class: def __init__(self, Name, Description, Price): self.Name = Name self.Description = Description self.Price = Price List = [] List.append(Item_Class("IDLE", "Python Interpreter", 0.00)) List.append(Item_Class("My Help", "Programming Help From Sane", 0.00)) print List[0].Name print List[0].Description print List[0].Price print List[1].Name print List[1].Description print List[1].Price Furthermore expanding on the program, we can get the same output like so... (Toggle Plain Text) class Item_Class: def __init__(self, Name, Description, Price): self.Name = Name self.Description = Description self.Price = Price def Output(self): print self.Name print self.Description print self.Price List = [] List.append(Item_Class("IDLE", "Python Interpreter", 0.00)) List.append(Item_Class("My Help", "Programming Help From Sane", 0.00)) List[0].Output() List[1].Output() class Item_Class: def __init__(self, Name, Description, Price): self.Name = Name self.Description = Description self.Price = Price def Output(self): print self.Name print self.Description print self.Price List = [] List.append(Item_Class("IDLE", "Python Interpreter", 0.00)) List.append(Item_Class("My Help", "Programming Help From Sane", 0.00)) List[0].Output() List[1].Output() If anything I'm doing confuses you, don't hesitate to ask any questions. I also strongly suggest that you use the search to look through the Python forum for more examples using classes. I believe there's some excellent information provided by Arevos lying around. I also urge you to use [code][/code] tags next time you post code.

Oct 28th, 2006, 9:26 PM	#3
MR.T Newbie Join Date: Feb 2006 Location: USA,Michigan Posts: 22 Rep Power: 0	Ahh, that makes sense. Thank you very much for simple and clear example. On a related note were can I find out more about the different ways classes are used in programming? BTW sorry about not puting the code in tags, I was going to but forgot because I was doing a couple things at the same time(maybe I should cut down on the Dr.pepper). thanks again __________________ "Gee, Brain, what do you want to do tonight?" "The same thing we do every night, Pinky: Try to take over the world!"

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Oct 28th, 2006, 9:34 PM	#4
Sane Programming Guru Join Date: Apr 2005 Posts: 1,799 Rep Power: 5	The way classes are used in programming are for you to discover and apply yourself as a programmer. The applications of Object-Oriented programming are very vast, and can be applied basically anywhere. But the more you use it as a programmer, the more you'll see where it should and should not be applied. Otherwise, I hope wherever you're learning classes from explains the general applications of classes, and where and where not to use them.

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Programming Forums > Script Programming > Python

Classes question