ch17: update from book draft

17-it-generator/aritprog_v1.py

@@ -1,7 +1,7 @@
 """
 Arithmetic progression class
 
-# BEGIN ARITPROG_CLASS_DEMO
+# tag::ARITPROG_CLASS_DEMO[]
 
     >>> ap = ArithmeticProgression(0, 1, 3)
     >>> list(ap)
@@ -21,24 +21,25 @@ Arithmetic progression class
     >>> list(ap)
     [Decimal('0.0'), Decimal('0.1'), Decimal('0.2')]
 
-# END ARITPROG_CLASS_DEMO
+# end::ARITPROG_CLASS_DEMO[]
 """
 
 
-# BEGIN ARITPROG_CLASS
+# tag::ARITPROG_CLASS[]
 class ArithmeticProgression:
 
-    def __init__(self, begin, step, end=None):  # <1>
+    def __init__(self, begin, step, end=None):       # <1>
         self.begin = begin
         self.step = step
         self.end = end  # None -> "infinite" series
 
     def __iter__(self):
-        result = type(self.begin + self.step)(self.begin)  # <2>
-        forever = self.end is None  # <3>
+        result_type = type(self.begin + self.step)   # <2>
+        result = result_type(self.begin)             # <3>
+        forever = self.end is None                   # <4>
         index = 0
-        while forever or result < self.end:  # <4>
-            yield result  # <5>
+        while forever or result < self.end:          # <5>
+            yield result                             # <6>
             index += 1
-            result = self.begin + self.step * index  # <6>
-# END ARITPROG_CLASS
+            result = self.begin + self.step * index  # <7>
+# end::ARITPROG_CLASS[]