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Exercises/ex6_5.md

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+\[ [Index](index.md) | [Exercise 6.4](ex6_4.md) | [Exercise 7.1](ex7_1.md) \]
+
+# Exercise 6.5
+
+*Objectives:*
+
+- Learn how to define a proper callable object
+
+Files Modified :  `validate.py`
+
+Back in [Exercise 4.3](ex4_3.md), you created a series of `Validator` classes
+for performing different kinds of type and value checks.  For example:
+
+```python
+>>> from validate import Integer
+>>> Integer.check(1)
+>>> Integer.check('hello')
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+  File "validate.py", line 21, in check
+    raise TypeError(f'Expected {cls.expected_type}')
+TypeError: Expected <class 'int'>
+>>>
+```
+
+You could use the validators in functions like this:
+
+```python
+>>> def add(x, y):
+        Integer.check(x)
+        Integer.check(y)
+        return x + y
+
+>>>
+```
+
+In this exercise, we're going to take it just one step further. 
+
+## (a) Creating a Callable Object
+
+In the file `validate.py`, start by creating a class like this:
+
+```python
+# validate.py
+...
+
+class ValidatedFunction:
+    def __init__(self, func):
+        self.func = func
+
+    def __call__(self, *args, **kwargs):
+        print('Calling', self.func)
+        result = self.func(*args, **kwargs)
+        return result
+```
+
+Test the class by applying it to a function:
+
+```python
+>>> def add(x, y):
+        return x + y
+
+>>> add = ValidatedFunction(add)
+>>> add(2, 3)
+Calling <function add at 0x1014df598>
+5
+>>>
+```
+
+## (b) Enforcement
+
+Modify the `ValidatedFunction` class so that it enforces value checks
+attached via function annotations.  For example:
+
+```python
+>>> def add(x: Integer, y:Integer):
+        return x + y
+>>> add = ValidatedFunction(add)
+>>> add(2,3)
+5
+>>> add('two','three')
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+  File "validate.py", line 67, in __call__
+    self.func.__annotations__[name].check(val)
+  File "validate.py", line 21, in check
+    raise TypeError(f'Expected {cls.expected_type}')
+TypeError: expected <class 'int'>
+>>>>
+```
+
+Hint: To do this, play around with signature binding. Use the `bind()`
+method of `Signature` objects to bind function arguments to argument
+names.  Then cross reference this information with the
+`__annotations__` attribute to get the different validator classes.
+
+Keep in mind, you're making an object that looks like a function, but
+it's really not.  There is magic going on behind the scenes.
+
+## (c) Use as a Method (Challenge)
+
+A custom callable often presents problems if used as a custom method.
+For example, try this:
+
+```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
+
+    def sell(self, nshares:Integer):
+        self.shares -= nshares
+    sell = ValidatedFunction(sell)     # Fails
+```
+
+You'll find that the wrapped `sell()` fails miserably:
+
+```python
+>>> s = Stock('GOOG', 100, 490.1)
+>>> s.sell(10)
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+  File "validate.py", line 64, in __call__
+    bound = self.signature.bind(*args, **kwargs)
+  File "/usr/local/lib/python3.6/inspect.py", line 2933, in bind
+    return args[0]._bind(args[1:], kwargs)
+  File "/usr/local/lib/python3.6/inspect.py", line 2848, in _bind
+    raise TypeError(msg) from None
+TypeError: missing a required argument: 'nshares'
+>>> 
+```
+
+Bonus: Figure out why it fails--but don't spend too much time fooling around with it.
+
+\[ [Solution](soln6_5.md) | [Index](index.md) | [Exercise 6.4](ex6_4.md) | [Exercise 7.1](ex7_1.md) \]
+
+----
+`>>>` 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/)