If you let stream argument with default value as EventStream() , Python declares the EvenStream instance shared across all this function calls, no matter how much times you call it. To fix it, you must set default value to None and declare the instance into the function.

def print_from_stream(n, stream=None):
    if stream is None:
        stream = EvenStream()
        
    for _ in range(n):
        value = stream.get_next()
        print(value)