什么_really_缓存了一个Django QuerySet？

Question

根据(我对)官方dox的解读：

https://docs.djangoproject.com/en/dev/ref/models/querysets/#when-querysets-are-evaluated

在评估Django QuerySet时，它应该被缓存。但事实似乎并非如此。在下面的示例中，TrackingImport是一个模型，后面有一个非常大的表。(为简洁起见，对输出进行了稍微编辑。)

recs = TrackingImport.objects.filter(...stuff...)

In [102]: time(recs[0])
Wall time: 1.84 s

In [103]: time(recs[0])
Wall time: 1.84 s

调用len()看起来像广告中宣传的那样工作：

In [104]: len(recs)
Out[104]: 1823

In [105]: time(recs[0])
Wall time: 0.00 s

我不明白为什么取消对数组的引用没有缓存QuerySet结果。它必须对其进行评估，对吗？那么我错过了什么呢？

Answer 1

你可以浏览一下源代码(django.db.model.query)，然后你就会明白了，这是django 1.3.4的query.py，

def __getitem__(self, k):
    """
    Retrieves an item or slice from the set of results.
    """
    if not isinstance(k, (slice, int, long)):
        raise TypeError
    assert ((not isinstance(k, slice) and (k >= 0))
            or (isinstance(k, slice) and (k.start is None or k.start >= 0)
                and (k.stop is None or k.stop >= 0))), \
            "Negative indexing is not supported."

    if self._result_cache is not None:
        if self._iter is not None:
            # The result cache has only been partially populated, so we may
            # need to fill it out a bit more.
            if isinstance(k, slice):
                if k.stop is not None:
                    # Some people insist on passing in strings here.
                    bound = int(k.stop)
                else:
                    bound = None
            else:
                bound = k + 1
            if len(self._result_cache) < bound:
                self._fill_cache(bound - len(self._result_cache))
        return self._result_cache[k]

    if isinstance(k, slice):
        qs = self._clone()
        if k.start is not None:
            start = int(k.start)
        else:
            start = None
        if k.stop is not None:
            stop = int(k.stop)
        else:
            stop = None
        qs.query.set_limits(start, stop)
        return k.step and list(qs)[::k.step] or qs
    try:
        qs = self._clone()
        qs.query.set_limits(k, k + 1)
        return list(qs)[0]
    except self.model.DoesNotExist, e:
        raise IndexError(e.args)

如果您不遍历查询集，则_result_cache为None，然后当您调用resc时，它将跳到以下行，

try:
   qs = self._clone()
   qs.query.set_limits(k, k + 1)
   return list(qs)[0]
except self.model.DoesNotExist, e:
   raise IndexError(e.args)

您会发现，在本例中，没有设置_result_cache。这就是为什么多个resc的持续时间花费相同的时间。

在调用len(resc)之后，您可以找到源代码，

def __len__(self):
    # Since __len__ is called quite frequently (for example, as part of
    # list(qs), we make some effort here to be as efficient as possible
    # whilst not messing up any existing iterators against the QuerySet.
    if self._result_cache is None:
        if self._iter:
            self._result_cache = list(self._iter)
        else:
            self._result_cache = list(self.iterator())
    elif self._iter:
        self._result_cache.extend(self._iter)
    return len(self._result_cache)

您可以看到_result_cache具有值，然后调用recs，它将只使用缓存，

 if self._result_cache is not None:
         ....
     return self._result_cache[k]

源码从来不会说谎，所以当你在文档中找不到答案时，最好阅读源码。