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我想了解 python 元类。为了练习，我正在实现一种用于编写类的声明性方式（类似于 sqlalchemy.ext.declarative）。只要我只有一个属性，这看起来很有希望。

但是当我添加另一个属性时，第一个属性的某些部分会发生变化，并且第一个属性的值会根据第二个属性的模式进行验证。这可能是由元类、闭包、属性或它们的组合引起的。我试着给出一个最小的、完整的但可读的例子。

#! /usr/bin/env python

"""
Something like:
    class Artist:
        locale = Pattern('[A-Z]{2}-[A-Z]{2}')

should be equivalent to:
    class Artist:
        def __init__(self):
            self._locale = None
        @property
        def locale(self):
            return self._locale
        @locale.setter
        def locale(self, value):
            validate(value, '[A-Z]{2}-[A-Z]{2}')
            self._locale = value

Problem:
    The code below works if Artist has only one attribute.
    When I add another one with a different pattern, only that last
    pattern is used in validation.
"""

import re
import unittest


# this class (and future siblings) are used to describe attributes
class Pattern(object):
    def __init__(self, pattern):
        self.pattern = pattern

    def validate(self, value):
        if value is None:
            return
        if not re.match("^%s$" % self.pattern, value):
            raise ValueError("invalid value: %r" % value)

    def __repr__(self):
        return "%s(pattern=%r)" % (self.__class__.__name__, self.pattern)


# __metaclass__ based class creation
def createClassFromDeclaration(name, bases, dct):
    """ Examine dct, create initialization in __init__ and property. """
    attributes = dict()
    properties = dict()
    for key, value in dct.iteritems():
        if not isinstance(value, Pattern):
            continue
        pattern = value
        pattern.attribute = "_%s" % key
        attributes[key] = pattern

        def fget(self):
            return getattr(self, pattern.attribute)
        def fset(self, value):
            pattern.validate(value)
            return setattr(self, pattern.attribute, value)
        properties[key] = property(fget, fset)

    def __init__(self, **kwargs):
        # set all attributes found in the keyword arguments
        for key, value in kwargs.iteritems():
            if key in self.__attributes__:
                setattr(self, key, value)
        # set all attributes _NOT_ found to None
        for key, declaration in attributes.iteritems():
            if not hasattr(self, declaration.attribute):
                setattr(self, key, None)

    dct = dict(dct)
    dct.update(properties)
    dct['__init__'] = __init__
    dct['__attributes__'] = attributes
    return type(name, bases, dct)


# declarative class
class Artist(object):
    __metaclass__ = createClassFromDeclaration

    # FIXME: adding a second attribute changes the first pattern
    locale = Pattern('[A-Z]{2}-[A-Z]{2}')
    date = Pattern('[0-9]{4}-[0-9]{2}-[0-9]{2}')


# some unit tests
class TestArtist(unittest.TestCase):
    def test_attributes_are_default_initialized(self):
        artist = Artist()
        self.assertIsNone(artist.date)
        self.assertIsNone(artist.locale)

    def test_attributes_are_initialized_from_keywords(self):
        artist = Artist(locale="EN-US", date="2013-02-04")
        self.assertEqual(artist.date, "2013-02-04")
        # FIXME: the following does not work.
        # it validates against the date pattern
        self.assertEqual(artist.locale, "EN-US")

    def test_locale_with_valid_value(self):
        artist = Artist()
        artist.date = "2013-02-04"
        self.assertEqual(artist.locale, "2013-02-04")
        # FIXME: the following does not work.
        # it validates against the date pattern
        artist.locale = "EN-US"
        self.assertEqual(artist.locale, "EN-US")

    def test_locale_with_invalid_value_throws(self):
        artist = Artist()
        with self.assertRaises(ValueError):
            artist.locale = ""
        with self.assertRaises(ValueError):
            artist.locale = "EN-USA"


if __name__ == '__main__':
    unittest.main()

# vim: set ft=python sw=4 et sta:

当我注释掉第二个属性（'date'）时，测试成功，但使用第二个属性，尝试设置第一个属性（'locale'）的测试失败。是什么导致单元测试失败？

免责声明：此代码仅用于培训。有一些方法可以创建不涉及元类、属性和闭包的相同功能（如你我所知）。但是，如果我们只走在我们熟悉的街道上，我们就不会学到任何新东西。请帮助我扩展我的 Python 知识。

4

1 回答 1

1

这个问题与元类或属性本身没有任何关系。它与您如何定义 get/set 函数有关。您fget和fset引用pattern封闭函数中的变量。这将创建一个闭包。的值将在/被调用pattern时查找，而不是在定义时查找。因此，当您在下一次循环迭代中覆盖时，您会导致所有/函数现在引用新模式。fgetfsetpatternfgetfset

这是一个更简单的示例，显示了正在发生的事情：

def doIt(x):
    funs = []
    for key, val in x.iteritems():
        thingy = val + 1
        def func():
            return thingy
        funs.append(func)
    return funs

>>> dct = {'a': 1, 'b': 2, 'c': 3}
>>> funs = doIt(dct)
>>> for f in funs:
...     print f()

3
3
3

请注意，即使这三个函数有时被定义为thingy具有不同的值，但当我稍后调用它们时，它们都返回相同的值。thingy这是因为当它们被调用时，它们都在查找，这是在循环完成之后，所以thingy只是等于它设置的最后一个值。

解决此问题的常用方法是将要关闭的变量作为附加函数参数的默认值传递。尝试像这样做你的getter和setter：

def fget(self, pattern=pattern):
    return getattr(self, pattern.attribute)
def fset(self, value, pattern=pattern):
    pattern.validate(value)
    return setattr(self, pattern.attribute, value)

默认参数是在函数定义时而不是调用时评估的，因此这会强制每个函数“保存”它想要使用的模式值。

于 2013-02-04T07:37:30.817 回答