python中json解析报错，但是双引号就不报错是为什么

同样是解析字符串，为什么解析单引号与双引号就不一样呢 代码操作过程

a="{'a':'b'}"
a
"{'a':'b'}"
json.loads(a)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/__init__.py", line 348, in loads
    return _default_decoder.decode(s)
  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/local/Cellar/python/3.7.5/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/decoder.py", line 353, in raw_decode
    obj, end = self.scan_once(s, idx)
json.decoder.JSONDecodeError: Expecting property name enclosed in double quotes: line 1 column 2 (char 1)
a='{"a":"b"}'
json.loads(a)
{'a': 'b'}

操作截图