解决字典dict中key不存在的情况

如果字典中key不存在，程序会报错'KeyError'，如下：

def test_no_key(self):
    a_dict = {}
    print(a_dict['a_key'])

Error
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/unittest/case.py", line 59, in testPartExecutor
    yield
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/unittest/case.py", line 605, in run
    testMethod()
  File "/Users/wmj/PycharmProjects/fluent/test/test_dict.py", line 30, in test_no_key
    print(a_dict['a_key'])
KeyError: 'a_key'

采用dict.get()的方式，如果key不存在，怎会返回None

a_dict = {}
    result = a_dict.get('a_key')
    print(type(result))

并且可以自定义默认值，比如如果返回‘nothing'

a_dict = {}
    print(a_dict.get('a_key', 'nothing'))