Try collecting your queries into a list, and then use the * operator when you call filter:
queries = [X.y == 'a']
if b:
queries.append(X.z == 'b')
q.filter(*queries)
And BTW I don’t understand why you think chaining two filters would change your query, it would correspond to X.y = a AND X.z = b like when you use filter(X.y == 'a', X.z == 'b').