adding a mapping protocol example

Author: PythonCHB

Sources/code/dict_making/dict_making.py

@@ -0,0 +1,51 @@
+"""
+an exploration of how the dict constructor knows whether it is
+working with a MApping of a general iterable.  It looks like
+the code is something like this:
+
+if isinstance(input, collections.abc.Mapping):
+    self.update = {key: input[key] key in input}
+else:
+    self.update = {key: val for key, val in input}
+
+So if your custom class present themapping interface -- that is, iterating
+over it returns the keys, than you want to be a MApping ABC subclass.ABC
+
+But if your custom class is not a Mapping, then you want __iter__ to return an
+iterator over teh key, value pairs.
+"""
+
+from collections.abc import Mapping
+
+
+def test_iter(instance):
+    yield ("this")
+    yield ("that")
+
+
+class DictTest(Mapping):
+
+    def __init__(self, this="this", that="that"):
+        self.this = this
+        self.that = that
+
+    def __iter__(self):
+        print("iter called")
+        return test_iter(self)
+
+    def __getitem__(self, key):
+        return getattr(self, key)
+
+    def __len__(self):
+        print("__len__ called")
+
+
+if __name__ == "__main__":
+
+    dt = DictTest()
+    print(dict(dt))
+
+    dt = DictTest(this=45, that=654)
+    print(dict(dt))
+
+