blocks|key|31853|text|这是另一种方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|31854|"""+Subclass+of+tuple+with+named+fields+"""
from+operator+import+itemgetter
from+inspect+import+signature

class+MetaTuple(type):
++++"""+metaclass+for+NamedTuple+"""

++++def+__new__(mcs,+name,+bases,+namespace):
++++++++cls+=+type.__new__(mcs,+name,+bases,+namespace)
++++++++names+=+signature(cls._signature).parameters.keys()
++++++++for+i,+key+in+enumerate(names):
++++++++++++setattr(cls,+key,+property(itemgetter(i)))
++++++++return+cls

class+NamedTuple(tuple,+metaclass=MetaTuple):
++++"""+Subclass+of+tuple+with+named+fields+"""

++++@staticmethod
++++def+_signature():
++++++++"+Override+in+subclass+"

++++def+__new__(cls,+*args):
++++++++new+=+super().__new__(cls,+*args)
++++++++if+len(new)+==+len(signature(cls._signature).parameters):
++++++++++++return+new
++++++++return+new._signature(*new)

if+__name__+==+'__main__':
++++class+Point(NamedTuple):
++++++++"+Simple+test+"
++++++++@staticmethod
++++++++def+_signature(x,+y,+z):+#+pylint:+disable=arguments-differ
++++++++++++"+Three+coordinates+"
++++print(Point((1,+2,+4)))|code-block|syntax|javascript|31855|如果这种方法有任何优点的话，那就是它的简单性。如果没有NamedTuple.__new__，它会更简单，因为它只用于强制执行元素计数。否则，它允许在命名元素之外添加匿名元素，省略元素的主要效果是在按名称访问省略的元素时对省略的元素进行IndexError+(只需做一些可以转换为AttributeError的工作)。错误的元素计数的错误消息有点奇怪，但它让人明白了这一点。我不希望这在Python2上起作用。|offset|length|style|CODE|31856|还有进一步复杂化的空间，比如__repr__方法。我不知道与其他实现相比性能如何(缓存签名长度可能会有所帮助)，但与原生namedtuple实现相比，我更喜欢调用约定。|31857|entityMap^0|0|0|R|I|39|A|3V|E|0|E|8|1O|A|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@$I|T|J|U|K|L]|$I|V|J|W|K|L]|$I|X|J|Y|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|Z|8|@$I|10|J|11|K|L]|$I|12|J|13|K|L]]|9|@]|A|$]]|$1|O|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|P|$]]

Here is another approach.

<pre><code>""" Subclass of tuple with named fields """
from operator import itemgetter
from inspect import signature

class MetaTuple(type):
 """ metaclass for NamedTuple """

 def __new__(mcs, name, bases, namespace):
 cls = type.__new__(mcs, name, bases, namespace)
 names = signature(cls._signature).parameters.keys()
 for i, key in enumerate(names):
 setattr(cls, key, property(itemgetter(i)))
 return cls

class NamedTuple(tuple, metaclass=MetaTuple):
 """ Subclass of tuple with named fields """

 @staticmethod
 def _signature():
 " Override in subclass "

 def __new__(cls, *args):
 new = super().__new__(cls, *args)
 if len(new) == len(signature(cls._signature).parameters):
 return new
 return new._signature(*new)

if __name__ == '__main__':
 class Point(NamedTuple):
 " Simple test "
 @staticmethod
 def _signature(x, y, z): # pylint: disable=arguments-differ
 " Three coordinates "
 print(Point((1, 2, 4)))
</code></pre>

If this approach has any virtue at all, it's the simplicity. It would be simpler yet without <code>NamedTuple.__new__</code>, which serves only the purpose of enforcing the element count. Without that, it happily allows additional anonymous elements past the named ones, and the primary effect of omitting elements is the <code>IndexError</code> on omitted elements when accessing them by name (with a little work that could be translated to an <code>AttributeError</code>). The error message for an incorrect element count is a bit strange, but it gets the point across. I wouldn't expect this to work with Python 2.

There is room for further complication, such as a <code>__repr__</code> method. I have no idea how the performance compares to other implementations (caching the signature length might help), but I much prefer the calling convention as compared to the native <code>namedtuple</code> implementation.

I spent some time investigating the <a href="https://hg.python.org/cpython/file/3b920a778484/Lib/collections/__init__.py#l265"><code>collections.namedtuple</code> module</a> a few weeks ago. The module uses a factory function which populates the dynamic data (the name of the new <code>namedtuple</code> class, and the class attribute names) into a very large string. Then <code>exec</code> is executed with the string (which represents the code) as the argument, and the new class is returned. 

Does anyone know why it was done this way, when there is a specific tool for this kind of thing readily available, i.e. the metaclass? I haven't tried to do it myself, but it seems like everything that is happening in the <code>namedtuple</code> module could have been easily accomplished using a <code>namedtuple</code> metaclass, like so: 

<pre><code>class namedtuple(type):
</code></pre>

etc etc.

Why doesn't the namedtuple module use a metaclass to create nt class objects?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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几周前，我花了一些时间调查。该模块使用工厂函数将动态数据(新namedtuple类的名称和类属性名称)填充到一个非常大的字符串中。然后，以字符串(表示代码)作为参数执行exec，并返回新类。有没有人知道为什么要这样做，因为有一种特定的工具可以用来处理这种事情，即元类？我自己并没有尝试过这样做，但是似乎使用namedtu...

为什么namedtuple模块不使用元类来创建nt类对象？-腾讯云开发者社区-腾讯云

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