Question

我想了解python元类。对于练习，我正在实现一种编写类的声明方式（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被更改，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。我尝试给出一个最小，完整但可读的例子。

#! /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，但是使用第二个属性，尝试设置第一个属性（'locale'）的测试失败。导致单元测试失败的原因是什么？

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（正如您和我所知）。但如果我们只走在我们所知道的街道上，我们就不会学到任何新东西。请帮我扩展我的Python知识。

Answer 1

问题与元类或属性本身并没有任何关系。它与您如何定义get / set函数有关。您的fget和fset引用了封闭函数中的变量pattern。这会创建一个闭包。 pattern的值将在fget / fset 被称为时查找，而不是在定义它们时。因此，当您在下一个循环迭代中覆盖pattern时，您将导致所有fget / fset函数现在引用新模式。

这是一个更简单的例子，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，即使在thingy具有不同值时定义了三个函数，但是当我稍后调用它们时它们都返回相同的值。这是因为他们在调用它们时都会查找thingy，这是在循环完成后，因此thingy只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入。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计算的，而不是调用时间，所以这会强制每个函数“保存”它想要使用的模式的值。

为什么在metaclass-property-closure mix中添加第二个属性会改变第一个属性？

1 个答案: