Initial commit

Exercises/ex7_2.md

@@ -0,0 +1,178 @@
+\[ [Index](index.md) | [Exercise 7.1](ex7_1.md) | [Exercise 7.3](ex7_3.md) \]
+
+# Exercise 7.2
+
+*Objectives:*
+
+- Decorator chaining
+- Defining decorators that accept arguments.
+
+*Files Modified:* `logcall.py`, `validate.py`
+
+## (a) Copying Metadata
+
+When a function gets wrapped by a decorator, you often lose
+information about the name of the function, documentation strings, and
+other details.  Verify this:
+
+```python
+>>> @logged
+    def add(x,y):
+        'Adds two things'
+        return x+y
+
+>>> add
+<function wrapper at 0x4439b0>
+>>> help(add)
+... look at the output ...
+>>>
+```
+
+Fix the definition of the `logged` decorator so that it copies
+function metadata properly.  To do this, use the `@wraps(func)`
+decorator as shown in the notes.
+
+After you're done, make sure the decorator preserves the function name
+and doc string.
+
+```python
+>>> @logged
+    def add(x,y):
+        'Adds two things'
+        return x+y
+
+>>> add
+<function add at 0x4439b0>
+>>> add.__doc__
+'Adds two things'
+>>>
+```
+
+Fix the `@validated` decorator you wrote earlier so that it also preserves
+metadata using `@wraps(func)`.
+
+## (b) Your first decorator with arguments
+
+The `@logged` decorator you defined earlier always just
+prints a simple message with the function name.
+Suppose that you wanted the user to be able to specify a 
+custom message of some sort.
+
+Define a new decorator `@logformat(fmt)` that accepts
+a format string as an argument and uses `fmt.format(func=func)` to
+format a supplied function into a log message:
+
+```python
+# sample.py
+...
+from logcall import logformat
+
+@logformat('{func.__code__.co_filename}:{func.__name__}')
+def mul(x,y):
+    return x*y
+```
+
+To do this, you need to define a decorator that takes an argument.
+This is what it should look like when you test it:
+
+```python
+>>> import sample
+Adding logging to add
+Adding logging to sub
+Adding logging to mul
+>>> sample.add(2,3)
+Calling add
+5
+>>> sample.mul(2,3)
+sample.py:mul
+6
+>>>
+```
+
+To further simplify the code, show how you can define the original `@logged` decorator
+using the the `@logformat` decorator. 
+
+## (c) Multiple decorators and methods
+
+Things can get a bit dicey when decorators are applied to methods in a
+class.  Try applying your `@logged` decorator to the methods in the
+following class.
+
+```python
+class Spam:
+    @logged
+    def instance_method(self):
+        pass
+
+    @logged
+    @classmethod
+    def class_method(cls):
+        pass
+
+    @logged
+    @staticmethod
+    def static_method():
+        pass
+
+    @logged
+    @property
+    def property_method(self):
+        pass
+```
+
+Does it even work at all? (hint: no). Is there any way to fix the code so
+that it works?  For example, can you make it so the following example
+works?
+
+```python
+>>> s = Spam()
+>>> s.instance_method()
+instance_method
+>>> Spam.class_method()
+class_method
+>>> Spam.static_method()
+static_method
+>>> s.property_method
+property_method
+>>>
+```
+
+## (d) Validation (Redux)
+
+In the last exercise, you wrote a `@validated` decorator that enforced
+type annotations. For example:
+
+```python
+@validated
+def add(x: Integer, y:Integer) -> Integer:
+    return x + y
+```
+
+Make a new decorator `@enforce()` that enforces types specified
+via keyword arguments to the decorator instead.  For example:
+
+```python
+@enforce(x=Integer, y=Integer, return_=Integer)
+def add(x, y):
+    return x + y
+```
+
+The resulting behavior of the decorated function should be identical.
+Note: Make the `return_` keyword specify the return type.  `return` is
+a Python reserved word so you have to pick a slightly different name.
+
+
+**Discussion**
+
+Writing robust decorators is often a lot harder than it looks.
+Recommended reading:
+
+
+\[ [Solution](soln7_2.md) | [Index](index.md) | [Exercise 7.1](ex7_1.md) | [Exercise 7.3](ex7_3.md) \]
+
+----
+`>>>` 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/)