variable scope explained

Author: lyltj2010

07FunctionDecoratorsAndClosures/variable_scope_rule.py

@@ -0,0 +1,38 @@
+"""
+# No Surprise
+>>> b = 6
+>>> def f(a):
+...     print(a)
+...     print(b)
+>>> f(3)
+3
+6
+
+# This may suprise you
+>>> b = 6
+>>> def f1(a):
+...     print(a)
+...     print(b)
+...     b = 9
+
+By calling f1(3) you will get
+3 
+Traceback(most recent call last):
+    File "<stdin>", line 1 ...
+    File "<stdin>", line 3 ...
+UnboundLocalError: local variable 'b' referenced before assignment
+
+It is Unbound(not assigned) Local(inside the function) Error!
+
+The fact is:
+1. When Python compiles the body of the function, it decides that b is a local
+   variable because it is assigned within the function
+2. It will fetch b from the local environment and find it is unbound
+3. It is a design choice
+   Python does not require you to declare variables
+   but assumes that a variable assigned in the body of a function is local
+"""
+
+if __name__ == "__main__":
+    import doctest
+    doctest.testmod()