Class That Acts As Mapping For **unpacking
Solution 1:
The __getitem__()
and keys()
methods will suffice:
>>> classD:
defkeys(self):
return ['a', 'b']
def__getitem__(self, key):
return key.upper()
>>> deff(**kwds):
print kwds
>>> f(**D())
{'a': 'A', 'b': 'B'}
Solution 2:
If you're trying to create a Mapping — not just satisfy the requirements for passing to a function — then you really should inherit from collections.abc.Mapping
. As described in the documentation, you need to implement just:
__getitem____len____iter__
The Mixin will implement everything else for you: __contains__
, keys
, items
, values
, get
, __eq__
, and __ne__
.
Solution 3:
The answer can be found by digging through the source.
When attempting to use a non-mapping object with **
, the following error is given:
TypeError: 'Foo'objectisnot a mapping
If we search CPython's source for that error, we can find the code that causes that error to be raised:
caseTARGET(DICT_UPDATE): {
PyObject *update = POP();
PyObject *dict = PEEK(oparg);
if (PyDict_Update(dict, update) < 0) {
if (_PyErr_ExceptionMatches(tstate, PyExc_AttributeError)) {
_PyErr_Format(tstate, PyExc_TypeError,
"'%.200s' object is not a mapping",
Py_TYPE(update)->tp_name);
PyDict_Update
is actually dict_merge
, and the error is thrown when dict_merge
returns a negative number. If we check the source for dict_merge
, we can see what leads to -1 being returned:
/* We accept for the argument either a concrete dictionary object,
* or an abstract "mapping" object. For the former, we can do
* things quite efficiently. For the latter, we only require that
* PyMapping_Keys() and PyObject_GetItem() be supported.
*/if (a == NULL || !PyDict_Check(a) || b == NULL) {
PyErr_BadInternalCall();
return-1;
The key part being:
For the latter, we only require that PyMapping_Keys() and PyObject_GetItem() be supported.
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