Python property returning property object

Question

I have a class like this:

class Foo(object):
    def __init__(self):
        self.bar = property(self.get_bar)

    def get_bar(self):
        return "bar"

print Foo().bar  #this prints <property object at 0x0051CB40>

I've seen How do Python properties work?, How to set a python property in __init__, but they all use the decorator method, which i don't because i want a different name. And I need access to self

How do i get the property to behave?

Why would you like to use different name? Is there a reasonable reason? — Stan Prokop, Feb 16 '14 at 14:01
Have you tried using property as a decorator or with a docstring? — Michael Aaron Safyan, Feb 16 '14 at 14:02
A quick solution will be to call `__get__` on `bar` explicitly: `f = Foo();f.bar.__get__(f, type(f))` — Ashwini Chaudhary, Feb 16 '14 at 14:16
(since this is the first web hit I found for my problem:) One other reason a property object can be returned instead of the property (even if the correct way of writing the property is used) is if the class is not instantiated into an object: print(Foo.bar) instead of print(Foo().bar) — Michael Scott Asato Cuthbert, Sep 09 '15 at 20:36
@MichaelScottCuthbert I think you should add this as an alternative answer, despite not being directly related with the question. I had this problem and your comment saved me! Thank you! — jmpcm, Mar 02 '18 at 15:21

jonrsharpe · Accepted Answer · 2018-09-15T13:49:02.393

19

You need to make a minor change:

class Foo(object):

    def get_bar(self):
        return "bar"

    bar = property(get_bar)

print Foo().bar # prints bar

The property needs to be an attribute of the class, not the instance; that's how the descriptor protocol works.

edited Sep 15 '18 at 13:49

answered Feb 16 '14 at 14:08

jonrsharpe

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I was defining the property before the function. All okay now. – gcq Feb 16 '14 at 14:51
@gcq Now, try and set the value of `bar`. – thefourtheye Feb 16 '14 at 15:00
1

Why does the output of property work differently if it's `bar = property()` at the top level vs. `self.bar = property()` within `__init__`? – EL_DON Sep 13 '18 at 18:58
3

@EL_DON because the property needs to be an attribute of the *class*, not the *instance*; that's how the [descriptor](https://docs.python.org/3/howto/descriptor.html) protocol works. – jonrsharpe Sep 13 '18 at 19:05
@jonrsharpe Thanks. Maybe put that in the answer? – EL_DON Sep 13 '18 at 20:44

score 8 · Answer 2 · answered Feb 16 '14 at 14:06

You can do it like this

class Foo(object):
    def __init__(self):
        self.__bar = None

    def get_bar(self):
        return self.__bar

    def set_bar(self, value):
        self.__bar = value

    bar = property(get_bar, set_bar)

foo = Foo()
print foo.bar    # None
foo.bar = 1
print foo.bar    # 1

score 6 · Answer 3 · answered Mar 01 '17 at 14:46

You can also do it like shown here:

class Foo(object):
    def __init__(self):
        self._bar = None

    @property
    def bar(self):
        return self._bar

    @bar.setter
    def bar(self, value):
        self._bar = value

    @bar.deleter
    def bar(self):
        self._bar = None # for instance

which is equivalent to:

class Also_Foo(object):
    def __init__(self):
        self._bar = None

    def get_bar(self):
        return self._bar

    def set_bar(self, value):
        self._bar = value

    def del_bar(self):
        self._bar = None # for instance

    bar = property(fget=get_bar, fset=set_bar, fdel=del_bar, doc=None)

BUT without polluting the class namespace with get and set methods for each attribute.

You retain external direct access to the variable by using ._bar instead of .bar.

score 4 · Answer 4 · edited Jun 19 '19 at 17:08

4

The object is not instantiated.

class Foo(object):
  def get_bar(self):
    return "bar"

bar = Foo()
print(bar.get_bar)

edited Jun 19 '19 at 17:08

David Parks

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answered Nov 21 '17 at 22:10

warchitect

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15

This was the correct answer for me. I had done `Foo.get_bar` rather than `Foo().get_bar`. E.g., the object was not instantiated. – David Parks Jun 19 '19 at 17:10
Lol, it was my case too. After ~30 minutes of debbugging I found some place deep in the code where the property was read from class, not instance... – Andrey Semakin Nov 05 '19 at 12:24

jcomeau_ictx · Answer 5 · 2021-02-21T23:26:31.267

My use case required defining bar as a property only under certain conditions, so I took jonrsharpe's advice and moved the definition into __new__ instead:

class Foo(object):
    def __new__(cls):
        cls.bar = property(cls.get_bar)
        return super(Foo, cls).__new__(cls)

    def get_bar(self):
        return "bar"

print(Foo().bar)  #this prints "bar"

However, nothing was gained by this "cleverness". Once the condition was met, the class contained the property. I might as well have defined it as jonrsharpe did.

Python property returning property object

5 Answers5

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