问题
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?
回答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.
回答2:
No, as assignment is a language intrinsic which doesn't have a modification hook.
回答3:
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.
回答4:
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.
回答5:
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.
回答6:
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.
回答7:
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
回答8:
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
回答9:
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
回答10:
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
Note that even from within the module, you cannot write to the protected variable once the class change happens. The above example assumes the code resides in a module named tst. You can do this in the repl by changing tst to __main__.
来源：https://stackoverflow.com/questions/11024646/is-it-possible-to-overload-python-assignment