Python，字典复制和更新不起作用？

Question

Why does the following code show None for b, and not {'a': 1, 'e': 2}? Python 2.7.3

>>>> d = {'a' :1 }
>>>> b = d.copy().update({'e':2})
>>>> print b
None
>>>> d.update({'c':3})
>>>> print d
{'a': 1, 'c': 3}

Answer 1

dict.update modifies the dict but returns None . This is why

b = d.copy().update({'e':2})

sets b to equal None , while

d.update({'c':3})

modifies d .

A lot of Python methods behave this way. For example, list.sort and random.shuffle also modify an object and return None . I think Python does this to discourage long "Law-of-Demeter-scoffing" chains of references because they do not improve readability and make finding where exceptions are raised harder.