How to create a setter without a getter in Python?

Question

class My_Class:
    def __init__(self):
        self._x = 0

    @property
    def x(self):
        return self._x

    @x.setter
    def x(self, x):
        self._x = x

If I delete the following getter from the code above:

@property
def x(self):
    return self._x

The code stops working. How can I create a setter without a getter?

Can you explain the usage? Perhaps there's an alternative approach — Ofer Sadan, Jun 19 '18 at 19:47

zvone · Accepted Answer · 2018-06-19T19:51:39.640

6

The property function does not have to be used as a decorator:decorator can be used as a function:

class My_Class:
    def _set_x(self, value):
        self._x = value

    x = property(fset=_set_x)  # now value has only a setter

    del _set_x  # optional: delete the unneeded setter function

instance = My_Class()
instance.x= 8  # the setter works

print(instance._x) # the "private" value

print(instance.x) # raises: AttributeError: unreadable attribute

edited Jun 19 '18 at 19:51

answered Jun 19 '18 at 19:50

zvone

18,045
3
49
77

why `del _set_value`?? – juanpa.arrivillaga Jun 19 '18 at 19:50
1

@juanpa.arrivillaga That is the standard cookbook how `property` was used before `@decorator` syntax was introduced. It deletes the method before it even becomes a method. It is optional, as the comment says, if you want not to have `X._set_value` (or `X._set_x` after I modified the variable names in the answer to fit the question). – zvone Jun 19 '18 at 19:53

score 2 · Answer 2 · answered Jun 19 '18 at 19:49

2

class My_Class:
    def __init__(self):
        self._x = 0

    @property
    def x(self):
        raise RuntimeError('This property has no getter!')

    @x.setter
    def x(self, x):
        self._x = x

answered Jun 19 '18 at 19:49

nosklo

217,122
57
293
297

score 0 · Answer 3 · answered Jun 19 '18 at 20:06

Here's an alternative answer to what I already offered: make your own write-only descriptor.

class WriteOnly:
    def __init__(self, private_name):
        self.private_name = private_name

    def __set__(self, obj, value):
        obj.__dict__[self.private_name] = value

    def __get__(self, obj, type=None):
        raise AttributeError('unreadable attribute')


class My_Class:
    x = WriteOnly('_x')

instance = My_Class()
instance.x = 8  # the setter works

print(instance._x) # the "private" value

print(instance.x) # raises: AttributeError: unreadable attribute

How to create a setter without a getter in Python?

3 Answers3