Is it possible to overload Python assignment?

Question

Is there a magic method that can overload the assignment operator, like __assign__(self, new_value)?

I'd like to forbid a re-bind for an instance:

class Protect():   def __assign__(self, value):     raise Exception("This is an ex-parrot")  var = Protect()  # once assigned... var = 1          # this should raise Exception() 

Is it possible? Is it insane? Should I be on medicine?

Answer 1

The way you describe it is absolutely not possible. Assignment to a name is a fundamental feature of Python and no hooks have been provided to change its behavior.

However, assignment to a member in a class instance can be controlled as you want, by overriding .__setattr__().

class MyClass(object):     def __init__(self, x):         self.x = x         self._locked = True     def __setattr__(self, name, value):         if self.__dict__.get("_locked", False) and name == "x":             raise AttributeError("MyClass does not allow assignment to .x member")         self.__dict__[name] = value  >>> m = MyClass(3) >>> m.x 3 >>> m.x = 4 Traceback (most recent call last):   File "<stdin>", line 1, in <module>   File "<stdin>", line 7, in __setattr__ AttributeError: MyClass does not allow assignment to .x member 

Note that there is a member variable, _locked, that controls whether the assignment is permitted. You can unlock it to update the value.

Answer 2

No, as assignment is a language intrinsic which doesn't have a modification hook.