Initial commit

Exercises/ex7_5.md

@@ -0,0 +1,81 @@
+\[ [Index](index.md) | [Exercise 7.4](ex7_4.md) | [Exercise 7.6](ex7_6.md) \]
+
+# Exercise 7.5
+
+*Objectives:*
+
+- Create your first metaclass
+
+*Files Created:* `mymeta.py`
+
+## (a) Create your first metaclass
+
+Create a file called `mymeta.py`
+and put the following code in it (from the slides):
+
+```python
+# mymeta.py
+
+class mytype(type):
+    @staticmethod
+    def __new__(meta, name, bases, __dict__):
+        print("Creating class :", name)
+        print("Base classes   :", bases)
+        print("Attributes     :", list(__dict__))
+        return super().__new__(meta, name, bases, __dict__)
+
+class myobject(metaclass=mytype):
+    pass
+```
+
+Once you've done this, define a class that inherits from
+`myobject` instead of object.  For example:
+
+```python
+class Stock(myobject):
+    def __init__(self, name, shares, price):
+        self.name = name
+        self.shares = shares
+        self.price = price
+    def cost(self):
+        return self.shares * self.price
+    def sell(self, nshares):
+        self.shares -= nshares
+```
+
+Try running your code and creating instances of `Stock`.  See
+what happens. You should see the print statements from your
+`mytype` running once when the `Stock` class is
+defined.
+
+What happens if you inherit from `Stock`?
+
+```python
+class MyStock(Stock):
+    pass
+```
+
+You should still see your metaclass at work.  Metaclasses are "sticky" in that they
+get applied across an entire inheritance hierarchy.
+
+**Discussion**
+
+Why would you want to do something like this?
+The main power of a metaclass is that it gives a programmer the ability
+to capture details about classes just prior to their creation. For
+example, in the `__new__()` method, you are given all of the
+basic details including the name of the class, base classes, and
+methods data.   If you inspect this data, you can perform various
+types of diagnostic checks.  If you're more daring, you can modify the
+data and change what gets placed in the class definition when it is
+created.   Needless to say, there are many opportunities for horrible
+diabolical evil.
+
+\[ [Solution](soln7_5.md) | [Index](index.md) | [Exercise 7.4](ex7_4.md) | [Exercise 7.6](ex7_6.md) \]
+
+----
+`>>>` 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/)