Is it possible to overload Python assignment?

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.x3>>> m.x = 4Traceback (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.

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

I don't think it's possible. The way I see it, assignment to a variable doesn't do anything to the object it previously referred to: it's just that the variable "points" to a different object now.

In [3]: class My():   ...:     def __init__(self, id):   ...:         self.id=id   ...: In [4]: a = My(1)In [5]: b = aIn [6]: a = 1In [7]: bOut[7]: <__main__.My instance at 0xb689d14c>In [8]: b.idOut[8]: 1 # the object is unchanged!

However, you can mimic the desired behavior by creating a wrapper object with __setitem__() or __setattr__() methods that raise an exception, and keep the "unchangeable" stuff inside.