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I know this is a noob question but I am learning OOPs and can't able to figure the outputs got

This is the code I found and may i know how this runs?

class InstanceCounter(object):
    count = 0

    def __init__(self, val):
        self.val = val
        InstanceCounter.count += 1

    def set_val(self, newval):
        self.val = newval

    def get_val(self):
        print(self.val)

    def get_count(self):
        print(InstanceCounter.count)

a = InstanceCounter(5)
b = InstanceCounter(10)
c = InstanceCounter(15)

for obj in (a, b, c):
    print("value of obj: %s" % obj.get_val())
    print("Count : %s" % obj.get_count())
Sophie
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1 Answers1

3

You have a class called InstanceCounter which inherits from object. The inherince from object can be removed if you use Python3. This class has an attribute count and value and some methods (functions - for example set_val).

Now you create three objects of your class and set the value of value to 5, 10 and 15 by passing these values into the constructor. You also increase the static attribute (see here) count by one with each constructor call. A static attribute is used with the notation Class.Attribute.

In the last step you loop over a list of your three objects ((a, b, c)) and store each of this object in the object obj, so obj will represent a then b and then c. So you can call the methods of this object, because your object obj has the type InstanceCounter and so obj contain the same methods and attributes.

By the way I have reworked your code so make it more understandable and use Python3 syntax.

class InstanceCounter:
    count = 0

    def __init__(self, val):
        self.val = val
        InstanceCounter.count += 1

    def set_val(self, newval):
        self.val = newval

    def get_val(self):
        return self.val

    def get_count(self):
        return InstanceCounter.count

a = InstanceCounter(5)
print("Count : {}".format(a.get_count()))
b = InstanceCounter(10)
print("Count : {}".format(b.get_count()))
c = InstanceCounter(15)
print("Count : {}".format(c.get_count()))

for obj in (a, b, c):
    print("value of obj: {}".format(obj.get_val()))
    print("Count : {}".format(obj.get_count()))

This result in the following output:

Count : 1
Count : 2
Count : 3
value of obj: 5
Count : 3
value of obj: 10
Count : 3
value of obj: 15
Count : 3

For a better understanding of static attributes:

So if you have three objects with type InstanceCounter you have three different attributes with name val because each object with type InstanceCounter contains one attribute val - an instance attribute and one identical attribute with name count - a class attribute.

  • count is a class attribute for the class InstanceCounter. This attribute has the same value for all objects with type InstanceCounter. Used with Classname.Attributename - for example InstanceCounter.count.
  • val is a instance attribute because each instance of a class InstanceCounter has his own value. Used with Instancename.Attributename - for example a.val.

See here for more information.

Kampi
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