Implement histogram LH5 specification

[gipert]

Serialization/deserialization of hist.Hist objects to LH5.

Specification: https://legend-exp.github.io/legend-data-format-specs/dev/hdf5/#Histograms

[MoritzNeuberger]

So I experimented a bit with a possible implementation. @gipert had the idea to implement a LGDO wrapper for hist.Hist (or more precisely hist.BaseHist [link]) using multiple inheritance. One problem I ran into while playing around is that I get a TypeError mentioning that the metaclasses of LGDO and Hist are not compatible. I'm not an expert in this area, so I don't really know how to fix this. Do you have any idea how to work around this?

[MoritzNeuberger]

The implementation so far looks like this btw:

class Histogram(LGDO,Hist):
    def __init__(self, 
                *in_args: hist.AxisTypes | hist.Storage | str,
                storage: hist.Storage | str | None = None,
                metadata: Any = None,
                data: np.typing.NDArray[Any] | None = None,
                label: str | None = None,
                name: str | None = None,
                attrs: dict[str, Any] = None,) -> None:

        LGDO.__init__(self,attrs)
        hist.Hist.__init__(self,in_args,storage,metadata,data,label,name)

[gipert]

Indeed:

>>> from hist import Hist
>>> from lgdo import LGDO
>>>
>>> class Histogram(LGDO, Hist):
>>>     def __init__(
>>>         self,
>>>         attrs=None,
>>>         **kwargs,
>>>     ):
>>>         super(LGDO).__init__(attrs)
>>>         super(Hist).__init__(**kwargs)

TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

But I'm not an expert, I guess some more research is needed. Also hist ppl seem to use quite advanced python subclassing stuff in basehist.py, we could maybe ask them.

[gipert]

https://hist.readthedocs.io/en/latest/user-guide/subclassing.html!

[ManuelHu]

So the problem is really that LGDO has ABC as a metaclass, and Hist has something different. Even

from hist import Hist
from lgdo import LGDO

class Histogram(LGDO, Hist):
    pass

is already throwing.

---

But

from abc import ABCMeta
from hist import Hist
from lgdo import LGDO

class HistogramMeta(ABCMeta, Hist.__class__):
    pass

class Histogram(LGDO, Hist, metaclass=HistogramMeta):
    pass

is working :-) But I have to look further into this if this actually works...

---

edit: Further down the line this does not work. I failed to even call the Hist constructor with this wrapping.