Symptom

Trying to deduplicate a list of expenses using set() raises TypeError: unhashable type: 'Expense'.

expenses = [e1, e2, e1]  # e1 appears twice
unique = set(expenses)    # TypeError: unhashable type: 'Expense'

What you know

The Expense class has an __eq__ method that compares by id. Two expenses with the same id are considered equal. But putting them in a set fails.