How to avoid object creation in python?

Question

I am new to python programming,I have one class,for this class i created one object( obj1).i don't want to create other than this object,if any body wants to create one more object for this class that should refer to first object only(instead of creating one more object).how to do this? please refer the below code?

class MyClass:
   def __init__(self):
      pass
obj1=MyClass()//create object
obj2=MyClass()//avoid creation and refer obj2 to obj1
obj3=MyClass()//avoid creation and refer obj3 to obj1

Answer 1

So you want something singleton-ish? Then do not use objects for this at all. Simply put the functions in a separate module (.py file) and put your variables in the module scope (e.g. global variables) - that's the pythonic way to do what you want if you do not need thread safety. Remember: It's not java and using classes for everything is not the way to go.

However, here's some code that allows only one instance:

class MyClass:
    def __init__(self):
        if getattr(self.__class__, '_has_instance', False):
            raise RuntimeError('Cannot create another instance')
        self.__class__._has_instance = True

If you want singletons, have a look at Python and the Singleton Pattern and Is there a simple, elegant way to define singletons?

Answer 2

Here's a simple way -- hide the class name:

class obj:
    pass

obj = obj()

Which will make class obj instances more difficult to create afterwards -- but not impossible, as pointed out in the comments.

Another alternative, delete the class after its first use:

class MyClass:
    def method(self): print 'spam'

obj1 = MyClass()
del MyClass
obj1.method()  # show instance still exists
obj2 = MyClass()

Output:

spam
Traceback (most recent call last):
  File "noclass.py", line 7, in <module>
    obj2 = MyClass()
NameError: name 'MyClass' is not defined

Answer 3

You could create the single object and make it a global i.e top-level object in the module using it if all you are coding would go in a single file or you could put it in a seperate module and import it.