add setting a default value to a dictionary

docs/notes/python-dict.rst

@@ -38,6 +38,51 @@ Find Same Keys
     ['3', '2']
     [('1', 1), ('3', 3), ('2', 2)]
 
+Set a Default Value
+-------------------
+
+.. code-block:: python
+
+    # intuitive but not recommend
+    >>> d = {}
+    >>> key = "foo"
+    >>> if key not in d:
+    ...     d[key] = []
+    ...
+
+    # using d.setdefault(key[, default])
+    >>> d = {}
+    >>> key = "foo"
+    >>> d.setdefault(key, [])
+    []
+    >>> d[key] = 'bar'
+    >>> d
+    {'foo': 'bar'}
+
+    # using collections.defaultdict
+    >>> from collections import defaultdict
+    >>> d = defaultdict(list)
+    >>> d["key"]
+    []
+    >>> d["foo"]
+    []
+    >>> d["foo"].append("bar")
+    >>> d
+    defaultdict(<class 'list'>, {'key': [], 'foo': ['bar']})
+
+``dict.setdefault(key[, default])`` returns its default value if *key* is not in
+the dictionary. However, if the key exists in the dictionary, the function will
+return its value.
+
+.. code-block:: python
+
+    >>> d = {}
+    >>> d.setdefault("key", [])
+    []
+    >>> d["key"] = "bar"
+    >>> d.setdefault("key", [])
+    'bar'
+
 Update Dictionary
 -----------------