Program to restrict object creation in Python

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:

class A(object): _count = 0 def __new__(cls): if cls._count > 5: raise Exception('Too many instances were created') cls._count += 1 def __init__(self): pass asd = A() print asd 

will output:

$ None 

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

class A(object): _count = 0 def __new__(cls): if cls._count >= 5: # @Wilfred fix raise Exception('Too many instances were created') cls._count += 1 return super(A, cls).__new__(cls) # returning the instance def __init__(self): pass asd = A() print asd 

output:

$ <__main__.A object at 0x7f5ddad7db10> 

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() 

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:

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:

Number of objects are now: 1 Number of objects are now: 2 Number of objects are now: 3 Number of objects are now: 4 Number of objects are now: 5 Traceback (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