Using isinstance() to check class variable's belonging

Question

I would like to use isinstance() method to identify class variable is it belongs to the given class.

I created an own Enum() base class to list class variables of subclasses. I did't detail body of source code, not important.

class Enum(object):

    @classmethod
    def keys(cls):
        pass  # Returns all names of class varables.

    @classmethod
    def values(cls):
        pass  # Returns all values of class varables

    @classmethod
    def items(cls):
        pass # Returns all class variable and its value pairs.

class MyEnum(Enum):
    MyConstantA = 0
    MyConstantB = 1

>>>MyEnum.keys()
['MyConstantA', 'MyConstantB']

I would like to use this one:

>>>isinstance(MyEnum.MyConstantB, MyEnum)
True

Python 3.4 has enums on board. For other versions you can use the port of [enum34](https://pypi.python.org/pypi/enum34). — HelloWorld, Sep 29 '15 at 09:35
This kind of looks like you want to reimplement a dictionary. — mkrieger1, Sep 29 '15 at 09:35
If you don't care about how you check if a class got a certain attribute, this question might be helpful for you: http://stackoverflow.com/questions/9748678/which-is-the-best-way-to-check-for-the-existence-of-an-attribute — Igl3, Sep 29 '15 at 09:36
@jonrsharpe Not true. It can be done with metaclasses, for example the python implementation of enums is designed in this way. — wim, Sep 29 '15 at 09:56
@wim so `isinstance(SomeEnum.member, SomeEnum)` is true? That seems counter-intuitive. — jonrsharpe, Sep 29 '15 at 09:57
@wim - alright; retracted, thanks. *Goes off to find out how and why...* — jonrsharpe, Sep 29 '15 at 10:01
I have to use Python 2.6 and I would like to use this behavior that written above. — David Lantos, Sep 29 '15 at 11:33
@DavidLantos ...then just use the backport, which is good from 2.4 up — jonrsharpe, Sep 29 '15 at 12:13
@jonrsharpe - I can just drop into my package the _enum34_ and use it directly, Cheers guys the quick answer! — David Lantos, Sep 29 '15 at 14:43

score 0 · Answer 1 · answered Sep 29 '15 at 15:30

Enum became an official data type in Python 3.4, and there is a backport here, with docs here.

isinstance() works as you would like, and to get the names of the members you would use:

myEnum.__members__.keys()

While both MyEnum['MyConstantA'] and MyEnum(0) would return the MyEnum.MyConstantA member.

mkrieger1 · Answer 2 · 2015-09-29T11:33:28.793

-2

It seems that an ordinary dictionary can do what you want:

>>> myEnum = {'MyConstantA': 0, 'MyConstantB': 1}

Or let enumerate do the counting:

>>> myEnum = {k: i for i, k in enumerate(['MyConstantA', 'MyConstantB'])}

Then:

>>> myEnum.keys()
['MyConstantA', 'MyConstantB']
>>> myEnum.values()
[0, 1]
>>> myEnum.items()
[('MyConstantA', 0), ('MyConstantB', 1)]
>>> 'MyConstantB' in myEnum
True

In case you really want to write your own class, use hasattr to test the existence of class variables:

>>> class Foo:
...     bar = 5
...
>>> hasattr(Foo, 'bar')
True

edited Sep 29 '15 at 11:33

answered Sep 29 '15 at 11:27

mkrieger1

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The goal is to achieve the isinstance() usage. This implementation is a part of bigger structure, that's why would be great if isinstance() can work some way. – David Lantos Sep 29 '15 at 11:38

Using isinstance() to check class variable's belonging

2 Answers2