python - What is a mapping object, according to dict type?

Question

The documentation lists 3 ways for creating a dict instance:

class dict(**kwarg)
class dict(mapping, **kwarg)
class dict(iterable, **kwarg)

What exactly is a mapping here? What's the minimal interface required for dict(mapping) to work?

Answer 1

From the source code for CPython, this comment:

/* 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.
 */

So, "the minimal interface required for dict(mapping) to work" appears to be .keys() and .__getitem__().

Example program:

class M:
    def keys(self):
        return [1,2,3]
    def __getitem__(self, x):
        return x*2

m = M()

d = dict(m)

assert d == {1:2, 2:4, 3:6}