python3 nonlocal

Author: lyltj2010

Diff for: 07FunctionDecoratorsAndClosures/running_avg_with_nonlocal.py

@@ -0,0 +1,37 @@
+"""
+Keeping track all the series is not efficient
+Just keep the count and total.
+Use nonlocal in Python3
+    * Flag a variable as a free variable even it is assigned
+    * a new value within the function
+    * Otherwise, immutable types in closure can never be assigned,
+    * because it will be marked as local
+Workarounds of nonlocal in Python2
+    * Python2 lacks nonlocal keyword
+    * Store the variables the inner functions need to change as items
+    * or attributes of some mutable object, like dict or simple instance
+    * and bind that object to a free variable
+>>> avg = make_averager()
+>>> avg(10)
+10.0
+>>> avg(11)
+10.5
+>>> avg(12)
+11.0
+"""
+
+def make_averager():
+    count = 0
+    total = 0
+
+    def averager(new_value):
+        """Without nonlocal, UnboundLocalError will be raised."""
+        nonlocal count, total
+        count += 1
+        total += new_value
+        return total / count
+    return averager
+
+if __name__ == "__main__":
+    import doctest
+    doctest.testmod()