Is it possible to overload Python assignment?

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?

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.

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 = a

In [6]: a = 1

In [7]: b
Out[7]: <__main__.My instance at 0xb689d14c>

In [8]: b.id
Out[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.

Inside a module, this is absolutely possible, via a bit of dark magic.

import sys
tst = sys.modules['tst']

class Protect():
  def __assign__(self, value):
    raise Exception("This is an ex-parrot")

var = Protect()  # once assigned...

Module = type(tst)
class ProtectedModule(Module):
  def __setattr__(self, attr, val):
    exists = getattr(self, attr, None)
    if exists is not None and hasattr(exists, '__assign__'):
      exists.__assign__(val)
    super().__setattr__(attr, val)

tst.__class__ = ProtectedModule

The above example assumes the code resides in a module named tst. You can do this in the repl by changing tst to __main__.

If you want to protect access through the local module, make all writes to it through tst.var = newval.

Using the top-level namespace, this is impossible. When you run

var = 1

It stores the key var and the value 1 in the global dictionary. It is roughly equivalent to calling globals().__setitem__('var', 1). The problem is that you cannot replace the global dictionary in a running script (you probably can by messing with the stack, but that is not a good idea). However you can execute code in a secondary namespace, and provide a custom dictionary for its globals.

class myglobals(dict):
    def __setitem__(self, key, value):
        if key=='val':
            raise TypeError()
        dict.__setitem__(self, key, value)

myg = myglobals()
dict.__setitem__(myg, 'val', 'protected')

import code
code.InteractiveConsole(locals=myg).interact()

That will fire up a REPL which almost operates normally, but refuses any attempts to set the variable val. You could also use execfile(filename, myg). Note this doesn't protect against malicious code.

No there isn't

Think about it, in your example you are rebinding the name var to a new value. You aren't actually touching the instance of Protect.

If the name you wish to rebind is in fact a property of some other entity i.e myobj.var then you can prevent assigning a value to the property/attribute of the entity. But I assume thats not what you want from your example.

Generally, the best approach I found is overriding __ilshift__ as a setter and __rlshift__ as a getter, being duplicated by the property decorator. It is almost the last operator being resolved just (| & ^) and logical are lower. It is rarely used (__lrshift__ is less, but it can be taken to account).

Within using of PyPi assign package only forward assignment can be controlled, so actual 'strength' of the operator is lower. PyPi assign package example:

class Test:

    def __init__(self, val, name):
        self._val = val
        self._name = name
        self.named = False

    def __assign__(self, other):
        if hasattr(other, 'val'):
            other = other.val
        self.set(other)
        return self

    def __rassign__(self, other):
        return self.get()

    def set(self, val):
        self._val = val

    def get(self):
        if self.named:
            return self._name
        return self._val

    @property
    def val(self):
        return self._val

x = Test(1, 'x')
y = Test(2, 'y')

print('x.val =', x.val)
print('y.val =', y.val)

x = y
print('x.val =', x.val)
z: int = None
z = x
print('z =', z)
x = 3
y = x
print('y.val =', y.val)
y.val = 4

output:

x.val = 1
y.val = 2
x.val = 2
z = <__main__.Test object at 0x0000029209DFD978>
Traceback (most recent call last):
  File "E:\packages\pyksp\pyksp\compiler2\simple_test2.py", line 44, in <module>
    print('y.val =', y.val)
AttributeError: 'int' object has no attribute 'val'

The same with shift:

class Test:

    def __init__(self, val, name):
        self._val = val
        self._name = name
        self.named = False

    def __ilshift__(self, other):
        if hasattr(other, 'val'):
            other = other.val
        self.set(other)
        return self

    def __rlshift__(self, other):
        return self.get()

    def set(self, val):
        self._val = val

    def get(self):
        if self.named:
            return self._name
        return self._val

    @property
    def val(self):
        return self._val


x = Test(1, 'x')
y = Test(2, 'y')

print('x.val =', x.val)
print('y.val =', y.val)

x <<= y
print('x.val =', x.val)
z: int = None
z <<= x
print('z =', z)
x <<= 3
y <<= x
print('y.val =', y.val)
y.val = 4

output:

x.val = 1
y.val = 2
x.val = 2
z = 2
y.val = 3
Traceback (most recent call last):
  File "E:\packages\pyksp\pyksp\compiler2\simple_test.py", line 45, in <module>
    y.val = 4
AttributeError: can't set attribute

So <<= operator within getting value at a property is the much more visually clean solution and it is not attempting user to make some reflective mistakes like:

var1.val = 1
var2.val = 2

# if we have to check type of input
var1.val = var2

# but it could be accendently typed worse,
# skipping the type-check:
var1.val = var2.val

# or much more worse:
somevar = var1 + var2
var1 += var2
# sic!
var1 = var2

In the global namespace this is not possible, but you could take advantage of more advanced Python metaprogramming to prevent multiple instances of a the Protect object from being created. The Singleton pattern is good example of this.

In the case of a Singleton you would ensure that once instantiated, even if the original variable referencing the instance is reassigned, that the object would persist. Any subsequent instances would just return a reference to the same object.

Despite this pattern, you would never be able to prevent a global variable name itself from being reassigned.

Yes, It's possible, you can handle __assign__ via modify ast.

pip install assign

Test with:

class T():
    def __assign__(self, v):
        print('called with %s' % v)
b = T()
c = b

You will get

>>> import magic
>>> import test
called with c

The project is at https://github.com/RyanKung/assign And the simpler gist: https://gist.github.com/RyanKung/4830d6c8474e6bcefa4edd13f122b4df

I will burn in Python hell, but what's life without a little fun.

---

Important disclaimers:

• I only provide this example for fun
• I'm 100% sure I don't understand this well
• It might not even be safe to do this, in any sense
• I don't think this is practical
• I don't think this is a good idea
• I don't even want to seriously try to implement this
• This doesn't work for jupyter (probably ipython too)*

---

Maybe you can't overload assignment, but you can (at least with Python ~3.9) achieve what you want even at the top-level namespace. It will be hard doing it "properly" for all cases, but here's a small example by hacking audithooks:

import sys
import ast
import inspect
import dis
import types


def hook(name, tup):
    if name == "exec" and tup:
        if tup and isinstance(tup[0], types.CodeType):
            # Probably only works for my example
            code = tup[0]
            
            # We want to parse that code and find if it "stores" a variable.
            # The ops for the example code would look something like this:
            #   ['LOAD_CONST', '<0>', 'STORE_NAME', '<0>', 
            #    'LOAD_CONST', 'POP_TOP', 'RETURN_VALUE', '<0>'] 
            store_instruction_arg = None
            instructions = [dis.opname[op] for op in code.co_code]
            
            # Track the index so we can find the '<NUM>' index into the names
            for i, instruction in enumerate(instructions):
                # You might need to implement more logic here
                # or catch more cases
                if instruction == "STORE_NAME":
                    
                    # store_instruction_arg in our case is 0.
                    # This might be the wrong way to parse get this value,
                    # but oh well.
                    store_instruction_arg = code.co_code[i + 1]
                    break
            
            if store_instruction_arg is not None:
                # code.co_names here is:  ('a',)
                var_name = code.co_names[store_instruction_arg]
                
                # Check if the variable name has been previously defined.
                # Will this work inside a function? a class? another
                # module? Well... :D 
                if var_name in globals():
                    raise Exception("Cannot re-assign variable")


# Magic
sys.addaudithook(hook)

And here's the example:

>>> a = "123"
>>> a = 123
Traceback (most recent call last):
  File "<stdin>", line 21, in hook
Exception: Cannot re-assign variable

>>> a
'123'

---

*For Jupyter I found another way that looked a tiny bit cleaner because I parsed the AST instead of the code object:

import sys
import ast


def hook(name, tup):
    if name == "compile" and tup:
        ast_mod = tup[0]
        if isinstance(ast_mod, ast.Module):
            assign_token = None
            for token in ast_mod.body:
                if isinstance(token, ast.Assign):
                    target, value = token.targets[0], token.value
                    var_name = target.id
                    
                    if var_name in globals():
                        raise Exception("Can't re-assign variable")
    
sys.addaudithook(hook)

A ugly solution is to reassign on destructor. But it's no real overload assignment.

import copy
global a

class MyClass():
    def __init__(self):
            a = 1000
            # ...

    def __del__(self):
            a = copy.copy(self)


a = MyClass()
a = 1

As mentioned by other people, there is no way to do it directly. It can be overridden for class members though, which is good for many cases.

As Ryan Kung mentioned, the AST of a package can be instrumented so that all assignments can have a side effect if the class assigned implements specific method(s). Building on his work to handle object creation and attribute assignment cases, the modified code and a full description is available here:

https://github.com/patgolez10/assignhooks

The package can be installed as: pip3 install assignhooks

Example :

class SampleClass():

   name = None

   def __assignpre__(self, lhs_name, rhs_name, rhs):
       print('PRE: assigning %s = %s' % (lhs_name, rhs_name))
       # modify rhs if needed before assignment
       if rhs.name is None:
           rhs.name = lhs_name
       return rhs

   def __assignpost__(self, lhs_name, rhs_name):
       print('POST: lhs', self)
       print('POST: assigning %s = %s' % (lhs_name, rhs_name))


def myfunc(): 
    b = SampleClass()
    c = b
    print('b.name', b.name)

to instrument it, e.g.

import assignhooks

assignhooks.instrument.start()  # instrument from now on

import testmod

assignhooks.instrument.stop()   # stop instrumenting

# ... other imports and code bellow ...

testmod.myfunc()

Will produce:

> $ python3 ./test.py

POST: lhs <testmod.SampleClass object at 0x1041dcc70>
POST: assigning b = SampleClass
PRE: assigning c = b
POST: lhs <testmod.SampleClass object at 0x1041dcc70>
POST: assigning c = b
b.name b