What does Python mean by a writeable attribute?

Question

I was reading the Python documentation and came across the following line in relation to the __dict__ attribute:

object.__dict__

A dictionary or other mapping object used to store an object’s (writable) attributes.

What is a writable attribute? What is the difference between a writable attribute and other types of attributes in Python?

Asked By: joepianoblaster

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Answer 1

Consider the following example:

class MyObject:
    def __init__(self, val):
        self.value = val

    @property
    def read_only_val(self):
        return self.value

my_obj = MyObject([10])

Writable attributes are those that can be set, e.g. using setattr or =

my_obj.value = [20]
setattr(my_obj, 'value', [20])

The values of these objects can be mutable or immutable, which will decide whether they can be modified in-place or not. "Writable" in the context of object attributes has nothing to do with mutability.

Non-writable attributes cannot be set. In our example, we defined read_only_val as a read-only property, which means we can’t set it. Both these statements throw an AttributeError: can't set attribute.

my_obj.read_only_val = [30]
setattr(my_obj, 'read_only_val', [30])

To further emphasize that "writable" in the context of object attributes has nothing to do with mutability, we can show that there is no problem in mutating the (non-writable) read_only_val attribute:

my_obj.read_only_val.append(40)
print(my_obj.read_only_val) # [20, 40]

Of course, if an attribute had an immutable value, we wouldn’t be able to mutate it, and we would have to set the attribute^{provided it’s writable} to give it another value.

When we inspect my_obj.__dict__, we see it contains only the writable attribute:

print(my_obj.__dict__)
# {'value': [20, 40]}

Answered By: Pranav Hosangadi

What does Python mean by a writeable attribute?

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