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.