Added example solutions to several chapters. Feel free to create a pull request with your answers. Also for the chapters that have no solutions yet :)

CH_05_functional_programming/exercise_03/solution_00.py

@@ -0,0 +1,29 @@
+# Write a groupby function that returns lists of results instead of 
+# generators.
+
+import pprint
+
+
+def groupby(iterable, key=None):
+    '''
+    Return a dictionary of lists of items grouped by the key function.
+
+    Note that as opposed to the itertools.groupby function, this function
+    does not require the iterable to be sorted.
+    '''
+    if key is None:
+        key = lambda x: x
+    groups = {}
+    for item in iterable:
+        groups.setdefault(key(item), []).append(item)
+    return groups
+
+
+def main():
+    # Demo data from the itertools docs
+    pprint.pprint(groupby('AAAABBBCCDAABBB'))
+    pprint.pprint(groupby('AAAABBBCCD'))
+
+
+if __name__ == '__main__':
+    main()