源码：
def ensure_one(self):
""" Verifies that the current recorset holds a single record. Raises
an exception otherwise.
"""
try:
# unpack to ensure there is only one value is faster than len when true and
# has a significant impact as this check is largely called
_id, = self._ids # 开始没明白什么意思，应该是python的多重赋值，只不过这种写法的情况下等号右边必须是可迭代的
return self
except ValueError:
raise ValueError("Expected singleton: %s" % self)
这段代码如果换种方式就很好理解了：
def ensure_one(self):"""
换种写法"""if len(self) == 1:
return self
else:raise ValueError("Expected singleton: %s" % self)
...)
