Showing class attributes in the PyCharm debugger when subclassing str

I want to see class attributes for a subclass of str in the debugger.

Refer to the picture - I want to have the dropdown arrow next to a (which is a POSString object). In other words, in the Pycharm debugger I want to have the option to doubleclick a and see the attribute bar with value "HELLO", exactly like for the foo object.

Is this possible in PyCharm, and if yes how?

The two classes:

class Foo:
    bar = "HELLO"


class POSString(str):
    bar = "HELLO"

    def __init__(self, pos="", *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.pos = pos

    def __hash__(self):
        return hash((str(self), self.pos))

    def __eq__(self, other):
        return super().__eq__(other) and self.pos == other.pos

And how I create my two objects:

foo = Foo()
a = POSString("banana")

I am using Python 3.7.1.

PyCharm:

PyCharm 2018.2.5 (Professional Edition)
Build #PY-182.5107.22, built on November 13, 2018
JRE: 1.8.0_152-release-1248-b22 amd64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
Linux 4.15.0-43-generic

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