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+\[ [Index](index.md) | [Exercise 6.5](ex6_5.md) | [Exercise 7.2](ex7_2.md) \]
+
+# Exercise 7.1
+
+*Objectives:*
+
+- Learn how to define a simple decorator functions.
+
+*Files Created:* `logcall.py`
+
+*Files Modifie:* `validate.py`
+
+## (a) Your First Decorator
+
+To start with decorators, write a _very_ simple decorator
+function that simply prints out a message each time a function is
+called.  Create a file `logcall.py` and define the following
+function:
+
+```python
+# logcall.py
+
+def logged(func):
+    print('Adding logging to', func.__name__)
+    def wrapper(*args, **kwargs):
+        print('Calling', func.__name__)
+        return func(*args, **kwargs)
+    return wrapper
+```
+
+Now, make a separate file `sample.py` and apply it to a
+few function definitions:
+
+```python
+# sample.py
+
+from logcall import logged
+
+@logged
+def add(x,y):
+    return x+y
+
+@logged
+def sub(x,y):
+    return x-y
+```
+
+Test your code as follows:
+
+```python
+>>> import sample
+Adding logging to add
+Adding logging to sub
+>>> sample.add(3,4)
+Calling add
+7
+>>> sample.sub(2,3)
+Calling sub
+-1
+>>> 
+```
+
+## (b) A Real Decorator
+
+In [Exercise 6.6](ex6_6.md), you created a callable class `ValidatedFunction` that
+enforced type annotations.  Rewrite this class as a decorator function called `validated`.
+It should allow you to write code like this:
+
+```python
+from validate import Integer, validated
+
+@validated
+def add(x: Integer, y:Integer) -> Integer:
+    return x + y
+
+@validated
+def pow(x: Integer, y:Integer) -> Integer:
+    return x ** y
+```
+
+Here's how the decorated functions should work:
+
+```python
+>>> add(2, 3)
+5
+>>> add('2', '3')
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+  File "validate.py", line 75, in wrapper
+    raise TypeError('Bad Arguments\n' + '\n'.join(errors))
+TypeError: Bad Arguments
+    x: Expected <class 'int'>
+    y: Expected <class 'int'>
+
+>>> pow(2, 3)
+8
+>>> pow(2, -1)
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+  File "validate.py", line 83, in wrapper
+    raise TypeError(f'Bad return: {e}') from None
+TypeError: Bad return: Expected <class 'int'>
+>>>
+```
+
+Your decorator should try to patch up the exceptions so that they
+show more useful information as shown.  Also, the `@validated`
+decorator should work in classes (you don't need to do anything special).
+
+```python
+class Stock:
+    def __init__(self, name, shares, price):
+        self.name = name
+        self.shares = shares
+        self.price = price
+    
+    @property
+    def cost(self):
+        return self.shares * self.price
+
+    @validated
+    def sell(self, nshares:PositiveInteger):
+        self.shares -= nshares
+```
+
+Note:  This part doesn't involve a lot of code, but there are a lot of low-level
+fiddly bits.  The solution will look almost the same as for Exercise 6.6.  Don't
+be shy about looking at solution code though.
+
+\[ [Solution](soln7_1.md) | [Index](index.md) | [Exercise 6.5](ex6_5.md) | [Exercise 7.2](ex7_2.md) \]
+
+----
+`>>>` Advanced Python Mastery  
+`...` A course by [dabeaz](https://www.dabeaz.com)  
+`...` Copyright 2007-2023  
+
+![](https://i.creativecommons.org/l/by-sa/4.0/88x31.png). This work is licensed under a [Creative Commons Attribution-ShareAlike 4.0 International License](http://creativecommons.org/licenses/by-sa/4.0/)