invert dict

snippets/py/invert_dictionary.py

@@ -0,0 +1,26 @@
+## Invert a python dictionary
+
+def invert_dict(d):
+    '''Invert a dictionary
+    values become keys, keys are inserted in a list of values
+    This allow to store in list when somes keys initially had the same value (since keys are unique)
+
+    New dict has this shape (here key1 and key2 had same value val2):
+    val1 : [key0]
+    val2 : [key1, key2]
+    '''
+
+    inverted_dict = {}
+    for k, v in d.items():
+        inverted_dict.setdefault(v, []).append(k)
+    return inverted_dict
+
+def invert_dict_strict(d):
+    '''Invert a dictionary
+    Return None if multiple keys have the same vvalue in initial dict 
+    '''
+
+    if len(d) != len(set(d.values())):
+        return None
+
+    return {v: k for k, v in d.items()}