Checking for list membership with Pytables where method

Checking for list membership with Pytables where method

I'm trying to select rows based on multiple criterion that cannot be
easily expressed with the conditional statements that [pytables allow]
(http://pytables.github.io/usersguide/condition_syntax.html).
I also don't want to format a really long string as a conditional argument
because that seems hacky.
So I was trying to collect the rows in an array, hoping that I can do
additional selection on rows
my_list1 = ['a', 'b', 'c']
my_list2 = ['5', '6', '7']
rows_selected = []
for an_item in my_list1:
for row in table_all.where("letters == an_item"):
for another_item in my_list2:
if row['numbers'] == another_item:
print row
rows_selected.append(row)
This type of setting would work (although not really ellegant). But the
problem is, all of the rows collected in rows_selected lose their identity
and become the last assigned row. So basically I end up with a list of
duplicate rows.
Is there an elegant way of using list membership as selection criteria for
pytables? Or how can I circumvent the duplicate row thing and make sure
the rows collected inside the loop remain unique?