对 Django Q 对象执行逻辑异或

我想执行逻辑异或（XOR http://en.wikipedia.org/wiki/Exclusive_disjunction) on django.db.models.Q对象，使用operator http://docs.python.org/2/library/operator.html模块将模型字段的选择限制为外键的子集。我在 Django 1.4.3 和 Python 2.7.2 中执行此操作。我有这样的事情：

import operator

from django.conf import settings
from django.db import models
from django.db.models import Q
from django.contrib.auth.models import User, Group

def query_group_lkup(group_name):
    return Q(user__user__groups__name__exact=group_name)

class Book(models.Model):
    author = models.ForeignKey(
                 User,
                 verbose_name=_("Author"),
                 null=False,
                 default='',
                 related_name="%(app_label)s_%(class)s_author",
                 # This would have provide an exclusive OR on the selected group name for User
                 limit_choices_to=reduce(
                     operator.xor,
                     map(query_group_lkup, getattr(settings, 'AUTHORIZED_AUTHORS', ''))
                 )

AUTHORIZED_AUTHORS是现有组名称的列表。

但这并没有奏效，因为Q对象不支持^运算符（仅| and &运营商从docs https://docs.djangoproject.com/en/1.4/topics/db/queries/#complex-lookups-with-q-objects）。来自堆栈跟踪的消息（部分）如下：

File "/home/moi/.virtualenvs/venv/lib/python2.7/site-packages/django/db/models/loading.py", line 64, in _populate
    self.load_app(app_name, True)
  File "/home/moi/.virtualenvs/venv/lib/python2.7/site-packages/django/db/models/loading.py", line 88, in load_app
    models = import_module('.models', app_name)
  File "/home/moi/.virtualenvs/venv/lib/python2.7/site-packages/django/utils/importlib.py", line 35, in import_module
    __import__(name)
  File "/opt/dvpt/toto/apps/book/models.py", line 42, in <module>
    class Book(models.Model):
  File "/opt/dvpt/toto/apps/book/models.py", line 100, in Book
    map(query_group_lkup, getattr(settings, 'AUTHORIZED_AUTHORS', ''))
TypeError: unsupported operand type(s) for ^: 'Q' and 'Q'

因此，受此启发answer https://stackoverflow.com/questions/8903128/python-list-to-bitwise-operations我尝试为我的特定查找实现异或。它并不是很灵活，因为查找是硬编码的（我需要使用kwargs在论据中query_xor例如...）。我最终做了这样的事情：

from django.conf import settings
from django.db import models
from django.db.models import Q
from django.db.models.query import EmptyQuerySet
from django.contrib.auth.models import User, Group

def query_xor_group(names_group):
    """Get a XOR of the queries that match the group names in names_group."""

    if not len(names_group):
        return EmptyQuerySet()
    elif len(names_group) == 1:
        return Q(user__user__groups__name__exact=names_group[0])

    q_chain_or = Q(user__user__groups__name__exact=names_group[0])
    q_chain_and = Q(user__user__groups__name__exact=names_group[0])

    for name in names_group[1:]:
        query = Q(user__user__groups__name__exact=name)
        q_chain_or |= query
        q_chain_and &= query

    return q_chain_or & ~q_chain_and

class Book(models.Model):
    author = models.ForeignKey(
                 User,
                 verbose_name=_("author"),
                 null=False,
                 default='',
                 related_name="%(app_label)s_%(class)s_author",
                 # This provides an exclusive OR on the SELECT group name for User
                 limit_choices_to=query_xor_group(getattr(settings, 'AUTHORIZED_AUTHORS', ''))
                 )

它按照我想要的方式工作，但在我看来，我并不是Pythonic（尤其是query_xor_group方法）。 是否有更好（更直接的方式）来做到这一点？

基本上，我的问题可以被剥夺limit_choices_to部分并总结为：

如何对一组进行按位异或运算django.db.models.Q以姜戈式的方式对象？

你可以添加一个__xor__()Q 的方法使用和/或/不执行异或逻辑。

from django.db.models import Q

class QQ:
    def __xor__(self, other):    
        not_self = self.clone()
        not_other = other.clone()
        not_self.negate()
        not_other.negate()

        x = self & not_other
        y = not_self & other

        return x | y

Q.__bases__ += (QQ, )

这样做之后我能够Q(...) ^ Q(...) in a filter() call.

Foobar.objects.filter(Q(blah=1) ^ Q(bar=2)) 

这意味着最初的尝试不再抛出不支持的操作数异常。

limit_choices_to=reduce(
                     operator.xor,
                     map(query_group_lkup, getattr(settings, 'AUTHORIZED_AUTHORS', ''))
                 )

测试于Django 1.6.1 on Python 2.7.5