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    python namedtuple函数的使用

    作者:一撸程猿 时间:2021-08-08 18:36

    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))
    
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