如何使用Python声明pySerial的全局变量

import serial

# 声明全局变量
ser = None

def initialize_serial(port, baudrate):
    global ser
    try:
        ser = serial.Serial(port, baudrate)
        print(f"Serial port {port} opened with baudrate {baudrate}")
    except serial.SerialException as e:
        print(f"Failed to open serial port {port}: {e}")

def read_from_serial():
    global ser
    if ser and ser.is_open:
        data = ser.readline().decode('utf-8').rstrip()
        return data
    else:
        print("Serial port is not open")
        return None

def close_serial():
    global ser
    if ser and ser.is_open:
        ser.close()
        print("Serial port closed")

# 使用示例
if __name__ == "__main__":
    initialize_serial('/dev/ttyUSB0', 9600)
    try:
        while True:
            data = read_from_serial()
            if data:
                print(f"Received data: {data}")
    except KeyboardInterrupt:
        close_serial()