Program To Restrict Object Creation In Python

class object_restrict(object): _count = 0 def __new__(cls): if cls._count > 5: raise TypeError('Too many keys created') cls._count += 1

Solution 1:

as pointed out by Wilfred there's a problem with the _count check or variable initialization. but i would like to point out another problem which is that you are not returning the instance of the object. if you have this:

classA(object):

    _count = 0def__new__(cls):
        if cls._count > 5:
            raise Exception('Too many instances were created')
        cls._count += 1def__init__(self):
        pass

asd = A() 
print asd

will output:

$ None

you should return the instance of the object, which is actually __new__ responsibility:

classA(object):

    _count = 0def__new__(cls):
        if cls._count >= 5:  # @Wilfred fixraise Exception('Too many instances were created')
        cls._count += 1returnsuper(A, cls).__new__(cls)  # returning the instancedef__init__(self):
        pass

asd = A() 
print asd

output:

$ <__main__.A object at 0x7f5ddad7db10>

Solution 2:

class object_restrict(object):
    _count = 0

    def __new__(cls):
        cls._count += 1
        if cls._count > 5:
            raise TypeError("Too many keys created")

        print cls._count, "object created"

    def __init__(self):
        pass

k = object_restrict()
k1 = object_restrict()
k2 = object_restrict()
k3 = object_restrict()
k4 = object_restrict()
k5 = object_restrict()

Solution 3:

You init your _count variable to 0.

First call : _count = 0

Sixth call : _count = 5, So he can create your object.

Init _count = 1 or update your if condition to cls._count >= 5:

Solution 4:

A class that allows maximum 5 objects to be instantiated (Python 3):

class RestrictClass(object):
    __count = 0
    def __new__(cls):
        if cls.__count>=5:
            raise TypeError("Already instantiated")
        cls.__count+=1
        return super(RestrictClass,cls).__new__(cls)

    def __init__(self):
        pass

    def showNumberOfObjects(self):
        print("Number of objects are now: "+str(self.__count))


k1 = RestrictClass()
k1.showNumberOfObjects()
k2 = RestrictClass()
k2.showNumberOfObjects()
k3 = RestrictClass()
k3.showNumberOfObjects()
k4 = RestrictClass()
k4.showNumberOfObjects()
k5 = RestrictClass()
k5.showNumberOfObjects()
k6 = RestrictClass()
k6.showNumberOfObjects()

Output:

Numberof objects are now: 1Numberof objects are now: 2Numberof objects are now: 3Numberof objects are now: 4Numberof objects are now: 5Traceback (most recent call last):
  File"/media/arsho/Documents/pyPrac/object.py", line 27, in <module>
    k6 = RestrictClass()
  File"/media/arsho/Documents/pyPrac/object.py", line 5, in __new__
    raise TypeError("Already instantiated")
TypeError: Already instantiated