How to call a function stored in a Python class instance variable

Question

I'm passing a function as a parameter to a class constructor. The constructor stores it in an instance variable, and other class methods want to call it.

class ComboList(list) :
    def __init__(self,a,b,kg) :
        list.__init__(self)
        self.__kg = kg
        # More stuff follows

When I try to call the function in another method, for example:

x = self.__kg('a')

I get "{AttributeError}'' object has no attribute '__kg'."

When I evaluate the function call in my IDE's Evaluate Expression window, I get the same thing. But when I evaluate the instance variable itself ("self.__kg"), I get:

result = {function} <function <lambda> at 0x0000000002ED5C18>

...so it appears that the attribute __kg is defined.

What is going wrong?

I can call the function by referencing the parameter -- no problem.

kg(a')

I can also assign the parameter's value to a method variable and call the variable -- no problem.

_kag = kg
_kag('a')

It's only the reference to the instance variable that doesn't work.

This is name mangling - try _ComboList__kg. Or just ditch the double underscores. — jonrsharpe
– jonrsharpe, Commented Jun 18, 2015 at 16:20
Can you please update the code where you are creating object of ComboList and calling the kg function? — Anand S Kumar
– Anand S Kumar, Commented Jun 18, 2015 at 16:39
jonrsharpe, that makes sense but it's surprising. I know name mangling is supposed to make variables hard to get by accident outside the scope object where they're defined. I didn't expect it to make the hard to get on purpose WITHIN the scope object. @Anand, it's not clear what kind of update you mean. The code hasn't changed in any relevant way. If this is still want, please clarify and I'll try to accommodate you. — Jonathan Sachs
– Jonathan Sachs, Commented Jun 18, 2015 at 22:13
I mean i tried your simplified approach in python 3.3, and it does not give me any error, can you update atleast the complete method , where you are doing x = self.__kg('a') — Anand S Kumar
– Anand S Kumar, Commented Jun 19, 2015 at 3:49

CodingPenguins · Accepted Answer · 2015-06-18 16:40:00Z

1

Whenever a variable has a double underscore in a class, Python will invoke name mangling (see the Python documentation on classes). Using double underscores is pretty much only to avoid name clashes with subclasses or to make a variable 'really private'. (See PEP8 for more style suggestions).

If you want other inherited subclasses to use your private variable, you can either use one single underscore (_kg) or use the @property decorator.

Check out this really dumb example:

class ComboList(list) :
    def __init__(self,a,b,kg) :
        list.__init__(self)
        self.__kg = kg
        # More stuff follows

    @property
    def kg(self):
        return(self.__kg)

my_list = ComboList(1,2,'This will be printed')
print(my_list.kg)

answered Jun 18, 2015 at 16:40

CodingPenguins

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Comments

hspandher · Accepted Answer · 2015-06-18 19:42:03Z

1

Don't use double underscore, which makes python do some magic and append classname to variable names (called name mangling). Just use single underscore instead of double.

answered Jun 18, 2015 at 19:42

hspandher

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