missing.py example

03-dict-set/missing.py

@@ -0,0 +1,122 @@
+"""
+Semantics of ``__missing__`` across mappings.
+
+✅ = indicates ``__missing__`` was called
+
+Subclass of ``dict``::
+
+    >>> d = DictSub(A = 'letter A')
+    >>> d['a']  # ✅
+    'letter A'
+    >>> d.get('a', '')
+    ''
+    >>> 'a' in d
+    False
+
+Subclass of ``UserDict``::
+
+    >>> ud = UserDictSub(A = 'letter A')
+    >>> ud['a']  # ✅
+    'letter A'
+    >>> ud.get('a', '')  # ✅
+    'letter A'
+    >>> 'a' in ud
+    False
+
+
+Simple subclass of ``abc.Mapping``::
+
+    >>> sms = SimpleMappingSub(A = 'letter A')
+    >>> sms['a']
+    Traceback (most recent call last):
+      ...
+    KeyError: 'a'
+    >>> sms.get('a', '')
+    ''
+    >>> 'a' in sms
+    False
+
+
+Subclass of ``abc.Mapping`` with support for ``__missing__``::
+
+    >>> mms = MappingMissingSub(A = 'letter A')
+    >>> mms['a']  # ✅
+    'letter A'
+    >>> mms.get('a', '')  # ✅
+    'letter A'
+    >>> 'a' in mms  # ✅
+    True
+
+Subclass of ``abc.Mapping`` with support for ``__missing__``::
+
+    >>> dms = DictLikeMappingSub(A = 'letter A')
+    >>> dms['a']  # ✅
+    'letter A'
+    >>> dms.get('a', '')
+    ''
+    >>> 'a' in dms
+    False
+
+
+"""
+
+from collections import UserDict
+from collections import abc
+
+
+def _upper(x):
+    try:
+        return x.upper()
+    except AttributeError:
+        return x
+
+
+class DictSub(dict):
+    def __missing__(self, key):
+        return self[_upper(key)]
+
+
+class UserDictSub(UserDict):
+    def __missing__(self, key):
+        return self[_upper(key)]
+
+
+class SimpleMappingSub(abc.Mapping):
+    def __init__(self, *args, **kwargs):
+        self._data = dict(*args, **kwargs)
+
+    # next three methods: abstract in ABC
+    def __getitem__(self, key):
+        return self._data[key]
+
+    def __len__(self):
+        return len(self._data)
+
+    def __iter__(self):
+        return iter(self._data)
+
+    # never called by instances of this class
+    def __missing__(self, key):
+        return self[_upper(key)]
+
+
+class MappingMissingSub(SimpleMappingSub):
+    def __getitem__(self, key):
+        try:
+            return self._data[key]
+        except KeyError:
+            return self[_upper(key)]
+
+
+class DictLikeMappingSub(SimpleMappingSub):
+    def __getitem__(self, key):
+        try:
+            return self._data[key]
+        except KeyError:
+            return self[_upper(key)]
+
+    def get(self, key, default=None):
+        return self._data.get(key, default)
+
+    def __contains__(self, key):
+        return key in self._data