[英]How does Python's reversed() function work?
According to Python's docs , reversed()
uses __getitem__
and __len__
if __reversed__
is not implemented.根据Python 的 docs ,如果
__reversed__
未实现,则reversed()
使用__getitem__
和__len__
。
I've encountered a weird behavior and failed to explain it:我遇到了一个奇怪的行为并且无法解释它:
>>> class A(dict):
... pass
...
>>> reversed(A())
Traceback (most recent call last):
...
TypeError: 'A' object is not reversible
>>> class B(dict):
... def __getitem__(self, key):
... return super().__getitem__(key)
... def __len__(self):
... return super().__len__()
...
>>> reversed(B())
Traceback (most recent call last):
...
TypeError: 'B' object is not reversible
>>> class C:
... def __getitem__(self, key):
... return "item"
... def __len__(self):
... return 1
...
>>> reversed(C())
<reversed object at 0x00000000022BB9B0>
Although calling reversed()
on mapping types makes no sense, how does it know it's a mapping?尽管在映射类型上调用
reversed()
没有意义,但它如何知道这是一个映射? Does it internally check isinstance(inst, dict)
?它是否在内部检查
isinstance(inst, dict)
? Does it check for any general mapping like collections.abc.Mapping
?它是否检查任何一般映射,如
collections.abc.Mapping
? Is there any way to override this behavior without implementing __reversed__
?有没有办法在不实现
__reversed__
情况下覆盖这种行为?
I thought it might be due to dict
implementing a __reversed__
that throws a TypeError
, or one that equals None
much like how you disable __hash__
, but dict.__reversed__
turned out empty with AttributeError
thrown.我认为这可能是由于
dict
实现了一个TypeError
的__reversed__
,或者一个等于None
就像你如何禁用__hash__
,但是dict.__reversed__
结果是空的,抛出了AttributeError
。
New Python versions implement __reversed__
for dictionaries.新的 Python 版本为字典实现了
__reversed__
。 Mapping protocols (such as collections.abc.Mapping) set __reversed__
to None
.映射协议(例如 collections.abc.Mapping)将
__reversed__
设置为None
。
Yes, there's a check for dict
type in PySequence_Check
used by reversed
.是的,有一检查
dict
型PySequence_Check
使用reversed
。
// cpython/Objects/enumobject.c
if (!PySequence_Check(seq)) {
PyErr_Format(PyExc_TypeError,
"'%.200s' object is not reversible",
Py_TYPE(seq)->tp_name);
return NULL;
}
// cpython/Objects/abstract.c
int
PySequence_Check(PyObject *s)
{
if (PyDict_Check(s))
return 0;
return s != NULL && s->ob_type->tp_as_sequence &&
s->ob_type->tp_as_sequence->sq_item != NULL;
}
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