用串口读取JSON数据，解析时报错，请问是什么原因？

[wujingyu]

用串口读取：{'a':'1111','b':'2222','c':'3333','d':'4444'} 时，JSON解析时报错，请问是什么原因？
程序如下：

import time, pyb
from pyb import UART
import ujson

led = pyb.LED(3)
uart = UART(1, 9600)

while(True):
    if(uart.any()):     # 是否有数据
        time.sleep(200)
        led.off()
        d = uart.read()
        da = str(d, "utf-8")  # bytes转str
        dd = eval((da))
        jsObj = ujson.loads(dd)
        print(type(jsObj))
        for key in jsObj.keys():
            print('key: %s   value: %s' % (key,jsObj.get(key)))
    else:
        led.on()
    time.sleep(1000)

Traceback (most recent call last):
File "", line 15, in
TypeError: can't convert 'dict' object to str implicitly
MicroPython v1.9.4-4510-g23e8457de on 2018-06-29; OPENMV3 with STM32F765
Type "help()" for more information.

哪位大师能给出解决方案，谢谢！

[wujingyu]

通过自己摸索，已自行解决了！

[kidswong999]

json标准里，字符串只能用双引号，而不是单引号

s = '{"a":"1111","b":"2222","c":"3333","d":"4444"}'

json.loads(s)