Question

I have two lists of tuples.

The first list has tuples with two elements:

list_a = [('a','apple'), ('b','banana'), ('c','cherry')]

A second list has tuples with only one element:

list_b = [('d',), ('e',), ('a',)]

I need to remove tuples in list_a for which the first element of the tuple is included in list_b. So the goal is that list_a is rendered:

list_a = [('b','banana'), ('c','cherry')]

I’ve tried:

for la in list_a:
   if la[0] in list_b:
      list_a.remove(la)

Answer 1

You can’t remove elements from a list you are looping over. Use a list comprehension instead:

list_a = [la for la in list_a if la[:1] not in list_b]

Note that we test for la[:1] to test with a (single element) tuple; la[0] is just the single character string.

If you have to do this very often, consider using a set for list_b instead:

set_b = set(list_b)

Lookups in a set take constant time, vs. linear time for a list membership test.