How do I avoid a “TypeError: unhashable type: ‘dict'” when computing list differences?

The set approach is not working since your dictionaries elements of lists. It seems that turning lists of dictionaries into sets is not allowed.

Instead, you can use a list comprehension where you check if any element in the list existing_dict is in the cur_dicts,

 deleted_dicts = [x for x in existing_dicts if not (x in cur_dicts)]

If the dictionary is not in cur_dicts, it is added to deleted_dicts. This relies on the fact that dictionaries can be compared for equality with an == operator.

Full example, extended with duplicate entries and larger dictionaries:

existing_dicts = [{"id": 1}, {"id": 2}, {"id": 2}, {"id": 2, "id2" : 3}, {"id": 1, "id2": 2}]
cur_dicts = [{"id": 2}, {"id": 1, "id2": 2}]

deleted_dicts = [x for x in existing_dicts if not (x in cur_dicts)]
print(deleted_dicts)