BUG: EAs should not be hashable

[jbrockmendel]

This turns out to be a blocker for some cleanup in index.pyx

[gfyoung]

LGTM - do we need a whatsnew ?

[WillAyd]

Do we actually want to commit to this? Just wonder if this would conflict with some of the work I think @TomAugspurger did before to try and create a non-mutable dataframe

[jreback]

yes mutable things are not hashable
we do allow hash_pandas_object to work though

[TomAugspurger]

This change seems fine. Can you say more about the motivation though? As I can imagine an immutable array that is hasable, so we mange shouldn’t rely on unhashabilty.

[jbrockmendel]

Can you say more about the motivation though? As I can imagine an immutable array that is hasable, so we mange shouldn’t rely on unhashabilty.

I'm working on indexing code and want to simplify a check

cdef inline bint is_definitely_invalid_key(object val):
    if isinstance(val, tuple):
        try:
            hash(val)
        except TypeError:
            return True

    # we have a _data, means we are a NDFrame
    return (isinstance(val, slice) or util.is_array(val)
            or isinstance(val, list) or hasattr(val, '_data'))

    cpdef get_loc(self, object val):
        cdef:
            Py_ssize_t loc

        if is_definitely_invalid_key(val):
            raise TypeError(f"'{val}' is an invalid key")
    [...]

to just check for hashability. Doing that breaks a couple of tests because EAs are currently hashable

[TomAugspurger]

to just check for hashability. Doing that breaks a couple of tests because EAs are currently hashable

When an immutable, 3rd-party EA comes along that's hashable, will that break again?

[jbrockmendel]

When an immutable, 3rd-party EA comes along that's hashable, will that break again?

If that EA gets passed as a key to index._engine.(get_loc|get_value|contains) then it may give (KeyError|KeyError|False) instead of raising a TypeError

[jreback]

may want to add some text to the top of the EA that says EA by default are not hashable

[jreback]

thanks @jbrockmendel