insert into with a dictionary, or generally with a variable number of columns

To make it very simple, I want to save a dictionary to a database.

I have a function that is reading various entries and returns a dictionary.
But the content of this dictionary depends on what it was "given" before.
Sometimes I want to insert a new row with 4 attributes, sometimes with 6...

Therefore, I was hoping to be able to do something like
cursor.mogrify('''INSERT INTO processing.raster_dataset %(columns)s VALUES
%(values)s ''', {'columns' :tuple(metaValues.keys()), 'values'  :
tuple(metaValues.values())})

But it is not working, it returns all the column names in bracket and fails.

I tried specifying the columns as :
 'columns' : ', '.join(metaValues.keys())

And as you can expect, it neither works.

Does anybody has a suggestion ? I'm sorry I could not find anything on
forums or previous threads, but maybe I was missing some keywords.

For the real enthousiasts, my final request is in fact even a bit harder.
But I think to be able to make once I can do the previous step. One of the
entry of the dictionary can be a geometry, with the value being the WKT
text of a geometry. And I want to convert it to a PostGIS geometry using
the "GeometryFromText()" function when passing the value.

Best Regards,
Matthieu