python namedtuple函数的使用

目录

• 先看演示

• 像类一样的访问属性

• 类似字典的访问

• 为什么可以这样？

• 源码解析

• 为什么有类的影子？

• 为什么有字典的影子？

先看演示

像类一样的访问属性

from collections import namedtuple

Friend = namedtuple('Friend', ['name', 'gender', 'address', 'star', 'signature'])

RidingRoad = Friend('RidingRoad', 'male', 'Mars', 'The five-star high praise',
                   'Change the world by Program!\n'
                   'Do what you like!\n'
                   'Live what you want!')

print(RidingRoad.name)
print(RidingRoad.gender)
print(RidingRoad.address)
print(RidingRoad.star)
print(RidingRoad.signature)

RidingRoad
male
Mars
The five-star high praise
Change the world by Program!
Do what you like!
Live what you want!

类似字典的访问

像字典一样访问items、keys、values

for key, value in RidingRoad.__dict__.items():
   print(key, value)

print("*" * 30)

for key in RidingRoad.__dict__.keys():
   print('{}: '.format(key), eval('RidingRoad.{}'.format(key)))

print("*" * 30)

for value in RidingRoad.__dict__.values():
   print(value)

('name', 'RidingRoad')
('gender', 'male')
('address', 'Mars')
('star', 'The five-star high praise')
('signature', 'Change the world by Program!\nDo what you like!\nLive what you want!')
******************************
('name: ', 'RidingRoad')
('gender: ', 'male')
('address: ', 'Mars')
('star: ', 'The five-star high praise')
('signature: ', 'Change the world by Program!\nDo what you like!\nLive what you want!')
******************************
RidingRoad
male
Mars
The five-star high praise
Change the world by Program!
Do what you like!
Live what you want!

为什么可以这样？

到这里，你应该会有两个疑问：

1. 为什么有类的影子？

2. 为什么有字典的影子？
   

源码解析

为什么有类的影子？

看源码的_class_template部分，其实函数内部为我们创了一个类了

# Fill-in the class template
   class_definition = _class_template.format(
       typename = typename,
       field_names = tuple(field_names),
       num_fields = len(field_names),
       arg_list = repr(tuple(field_names)).replace("'", "")[1:-1],
       repr_fmt = ', '.join(_repr_template.format(name=name)
                            for name in field_names),
       field_defs = '\n'.join(_field_template.format(index=index, name=name)
                              for index, name in enumerate(field_names))
   )
   if verbose:
       print class_definition

然后_class_template干了什么？对类进行定义

_class_template = '''\
class {typename}(tuple):
   '{typename}({arg_list})'

__slots__ = ()

_fields = {field_names!r}

def __new__(_cls, {arg_list}):
       'Create new instance of {typename}({arg_list})'
       return _tuple.__new__(_cls, ({arg_list}))

@classmethod
   def _make(cls, iterable, new=tuple.__new__, len=len):
       'Make a new {typename} object from a sequence or iterable'
       result = new(cls, iterable)
       if len(result) != {num_fields:d}:
           raise TypeError('Expected {num_fields:d} arguments, got ％d' ％ len(result))
       return result

def __repr__(self):
       'Return a nicely formatted representation string'
       return '{typename}({repr_fmt})' ％ self

def _asdict(self):
       'Return a new OrderedDict which maps field names to their values'
       return OrderedDict(zip(self._fields, self))

def _replace(_self, **kwds):
       'Return a new {typename} object replacing specified fields with new values'
       result = _self._make(map(kwds.pop, {field_names!r}, _self))
       if kwds:
           raise ValueError('Got unexpected field names: ％r' ％ kwds.keys())
       return result

def __getnewargs__(self):
       'Return self as a plain tuple.  Used by copy and pickle.'
       return tuple(self)

__dict__ = _property(_asdict)

def __getstate__(self):
       'Exclude the OrderedDict from pickling'
       pass

{field_defs}
'''

为什么有字典的影子？

看源码的 _asdict部分，这里封装成了有序字典，所以我们可以通过__dict__访问字典的特性了

__dict__ = _property(_asdict)
def _asdict(self):
       'Return a new OrderedDict which maps field names to their values'
       return OrderedDict(zip(self._fields, self))

来源：https://juejin.cn/post/6973302732740362277