x2 = [{'a': 2, 'b': 'dog'}, {'a': 1, 'b': 'fish'}, {'a': 3}, {'a': 2}]
[(_x2, _x2['a'] in [_x1['a'] for _x1 in x1]) for _x2 in x2]
[({'a': 2, 'b': 'dog'}, True), ({'a': 1, 'b': 'fish'}, True), ({'a': 3},
    False), ({'a': 2}, True)]
Как: сравнивать словари в списках