python, __slots__, and "attribute is read-only"

Question

I want to create an object in python that has a few attributes and I want to protect myself from accidentally using the wrong attribute name. The code is as follows:

class MyClass( object ) :
    m = None # my attribute
    __slots__ = ( "m" ) # ensure that object has no _m etc

a = MyClass() # create one
a.m = "?"  # here is a PROBLEM

But after running this simple code, I get a very strange error:

Traceback (most recent call last):
  File "test.py", line 8, in <module>
    a.m = "?"
AttributeError: 'test' object attribute 'm' is read-only

Is there any wise programmer who can spare a bit of their time and enlighten me about "read-only" errors?

Answer 1

When you declare instance variables using __slots__, Python creates a descriptor object as a class variable with the same name. In your case, this descriptor is overwritten by the class variable m that you are defining at the following line:

  m = None # my attribute

Here is what you need to do: Do not define a class variable called m, and initialize the instance variable m in the __init__ method.

class MyClass(object):
  __slots__ = ("m",)
  def __init__(self):
    self.m = None

a = MyClass()
a.m = "?"

As a side note, tuples with single elements need a comma after the element. Both work in your code because __slots__ accepts a single string or an iterable/sequence of strings. In general, to define a tuple containing the element 1, use (1,) or 1, and not (1).