Class that acts as mapping for **unpacking
The __getitem__()
and keys()
methods will suffice:
>>> class D: def keys(self): return ['a', 'b'] def __getitem__(self, key): return key.upper()>>> def f(**kwds): print kwds>>> f(**D()){'a': 'A', 'b': 'B'}
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__
.
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' object is not a mapping
If we search CPython's source for that error, we can find the code that causes that error to be raised:
case TARGET(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.