Making A Class Constant Who's Type Is The Class In Which It Resides

Question

I have a Python class with special values, "EMPTY" and "UNIVERSE":

class RealSet:
    """Continuous open, half-open, and closed regions and discreet values of the Reals"""

    # implementation placeholder
    def __init__(self, intervals, *, canonicalize):
        pass

# Outside the class

RealSet.EMPTY = RealSet(tuple(), canonicalize=False)  # type: ignore
RealSet.UNIVERSE = RealSet(((None, None),), canonicalize=False)  # type: ignore

However, linting, code-completion, etc. don't like this because they're not seen as static attributes of the class. Even setting them is reported as a mypy error, hence the # type: ignore.

The below doesn't work because I can't construct a RealSet in the class scope because it doesn't exist yet:

class RealSet:
    """Continuous open, half-open, and closed regions and discreet values of the Reals"""
    ...
    ...

    EMPTY = RealSet(tuple(), canonicalize=False)  # error
    UNIVERSE = RealSet(((None, None),), canonicalize=False)  # error

And this doesn't work because it defines instance attributes, not class attributes:

class RealSet:
    """Continuous open, half-open, and closed regions and discreet values of the Reals"""
    ...
    ...

    EMPTY: "RealSet"
    UNIVERSE: "RealSet"

# Outside the class

RealSet.EMPTY = RealSet(tuple(), canonicalize=False)
RealSet.UNIVERSE = RealSet(((None, None),), canonicalize=False)

This seems to be a corner case in the design of Python classes. How can I make class attributes where the attribute's type is the class in which it resides? Bonus: make them constant.

how are you planning to use this class? What is your use case? — iurii_n, Feb 11 '21 at 22:21
@PeterWood Interesting. I'm not, but I think it would suffer from the same problem as the second code block. RealSet doesn't exist yet inside the class. — Brent, Feb 11 '21 at 22:22
@iurii_n The typical things you might expect, but also some operations that I haven't found in existing implementations. For instance, taking an array as input and returning the indices of the elements that the set contains in O(n log n). — Brent, Feb 11 '21 at 22:23
Python 3.9 only, but here we go https://stackoverflow.com/a/64738850/395029 — Brent, Feb 11 '21 at 22:45
@Brent Do you want to add the `@classmethod @property` solution as an answer? It should be included as an answer. — a_guest, Feb 11 '21 at 22:57
I haven't tested it myself so I'm not going to make an answer. — Brent, Feb 11 '21 at 23:36

score 2 · Accepted Answer · answered Feb 11 '21 at 22:51

You can use typing.ClassVar to annotate class variables:

class RealSet:
    def __init__(self, intervals, *, canonicalize):
        pass

    EMPTY: ClassVar['RealSet']
    UNIVERSE: ClassVar['RealSet']


RealSet.EMPTY = RealSet(tuple(), canonicalize=False)
RealSet.UNIVERSE = RealSet(((None, None),), canonicalize=False)

score 2 · Answer 2 · answered Feb 12 '21 at 00:13

Starting with Python 3.9 classmethod can decorate other descriptors such as property. This way one can create a "classproperty":

class RealSet:
    def __init__(self, intervals, *, canonicalize):
        pass

    @classmethod
    @property
    def EMPTY(cls):
        return cls(tuple(), canonicalize=False)

    @classmethod
    @property
    def UNIVERSE(cls):
        return cls(((None, None),), canonicalize=False)

Making A Class Constant Who's Type Is The Class In Which It Resides

2 Answers2