diff --git a/python_3_oop/chapter06/lists.py b/python_3_oop/chapter06/lists.py
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index 0000000..ff519ef
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+++ b/python_3_oop/chapter06/lists.py
@@ -0,0 +1,57 @@
+import string
+
+
+# list with all valid characters
+CHARACTERS = list(string.ascii_letters) + [" ", ",", "'", "?", "Ä", "ö"]
+
+
+def letter_frequency(sentence):
+    # init a list of tuples for all valid characters
+    frequencies = [(c, 0) for c in CHARACTERS]
+    for letter in sentence:
+        # find index of selected character
+        index = CHARACTERS.index(letter)
+        # increment counter of selected character
+        frequencies[index] = (letter, frequencies[index][1] + 1)
+    return frequencies
+
+print(letter_frequency("Hello world, what's going on? Äötsch"))
+
+
+# sorting self constructed classes by overriding their __lt__ method
+
+class WeirdSortee:
+    def __init__(self, string, number, sort_num):
+        self.string = string
+        self.number = number
+        self.sort_num = sort_num
+
+    def __lt__(self, object):
+        if self.sort_num:
+            return self.number < object.number
+        return self.string < object.string
+
+    def __repr__(self):
+        return "{}:{}".format(self.string, self.number)
+
+
+a = WeirdSortee('a', 4, True)
+b = WeirdSortee('b', 3, True)
+c = WeirdSortee('c', 2, True)
+d = WeirdSortee('d', 1, True)
+l = [a, d, c, b]
+print(str(l) + " unsorted")
+l.sort()
+print(str(l) + " sorted by number")
+for i in l:
+    i.sort_num = False
+l.sort()
+print(str(l) + " sorted by character")
+
+
+# using the 'key' argument of the sort() method
+x = [(1, 'c'), (2, 'a'), (3, 'b')]
+x.sort()
+print(str(x) + " sorted by 1st item")
+x.sort(key=lambda i: i[1])
+print(str(x) + " sorted by 2nd item")
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