**kw 字典传参问题

"""
dict() -> new empty dictionary
dict(mapping) -> new dictionary initialized from a mapping object's
(key, value) pairs
dict(iterable) -> new dictionary initialized as if via:
d = {}
for k, v in iterable:
d[k] = v
dict(**kwargs) -> new dictionary initialized with the name=value pairs
in the keyword argument list. For example: dict(one=1, two=2)
# (copied from class doc)
"""

dict(a,**b) = dict(a, **{'b': 'cc'}) = dict(a, b='cc') = {'a': 'bb', 'b': 'cc'}