你可以去看看源码(django.db.model.query),然后就清楚了,这里是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)
当您不遍历查询集时,_result_cache 为 None,然后当您调用 resc[0] 时,它只会跳到以下几行,
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,在这种情况下,_result_cache 没有被设置。这就是为什么多个 resc[0] 的持续时间成本相同的原因。
调用 len(resc) 后,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)
你可以看到 _result_cache 有值,然后你调用 recs[0],它只会使用缓存,
if self._result_cache is not None:
....
return self._result_cache[k]
源代码永远不会说谎,因此当您在文档中找不到答案时,最好阅读源代码。