Allow `@` queries for metadata

Metadata-based querying of Epochs is very powerful. For example, these work:

import mne
path_kword = mne.datasets.kiloword.data_path() + '/kword_metadata-epo.fif'
kword = mne.read_epochs(path_kword)
kword["NumberOfLetters == 4 & ConsonantVowelProportion < .5"], kword["WORD == 'area'"]

But what doesn’t work is pulling a variable from the current scope with @:

word = 'area'
kword.metadata.query("WORD == @word")

This works, in Pandas. But this errors:

kword["WORD == @word"]

Would be cool to have!

@choldgraf

[choldgraf]

I’m pretty sure that they accomplish mostly the same thing, but that eval returns a boolean mask according to whatever the query was, while query returns the DataFrame object itself. So probably we use eval because we want that mask somewhere else in the code, I’m guessing.

[drammock]

… 2 years later, I tried to get this to work by manipulating the level kwarg (how far up the call stack pandas looks for @variables), as well as passing local_dict=globals() and a few other hackish things; nothing worked. Given that it’s possible to generally replace use of @ with a fairly similar f-string nowadays (e.g., epochs['WORD in @words'] becomes epochs[f'WORD in {words}']) I think we can close this as YAGNI.