82 lines
2.4 KiB
Markdown
82 lines
2.4 KiB
Markdown
\[ [Index](index.md) | [Exercise 7.4](ex7_4.md) | [Exercise 7.6](ex7_6.md) \]
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# Exercise 7.5
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*Objectives:*
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- Create your first metaclass
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*Files Created:* `mymeta.py`
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## (a) Create your first metaclass
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Create a file called `mymeta.py`
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and put the following code in it (from the slides):
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```python
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# mymeta.py
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class mytype(type):
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@staticmethod
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def __new__(meta, name, bases, __dict__):
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print("Creating class :", name)
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print("Base classes :", bases)
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print("Attributes :", list(__dict__))
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return super().__new__(meta, name, bases, __dict__)
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class myobject(metaclass=mytype):
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pass
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```
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Once you've done this, define a class that inherits from
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`myobject` instead of object. For example:
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```python
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class Stock(myobject):
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def __init__(self, name, shares, price):
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self.name = name
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self.shares = shares
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self.price = price
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def cost(self):
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return self.shares * self.price
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def sell(self, nshares):
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self.shares -= nshares
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```
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Try running your code and creating instances of `Stock`. See
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what happens. You should see the print statements from your
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`mytype` running once when the `Stock` class is
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defined.
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What happens if you inherit from `Stock`?
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```python
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class MyStock(Stock):
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pass
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```
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You should still see your metaclass at work. Metaclasses are "sticky" in that they
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get applied across an entire inheritance hierarchy.
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**Discussion**
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Why would you want to do something like this?
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The main power of a metaclass is that it gives a programmer the ability
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to capture details about classes just prior to their creation. For
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example, in the `__new__()` method, you are given all of the
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basic details including the name of the class, base classes, and
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methods data. If you inspect this data, you can perform various
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types of diagnostic checks. If you're more daring, you can modify the
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data and change what gets placed in the class definition when it is
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created. Needless to say, there are many opportunities for horrible
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diabolical evil.
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\[ [Solution](soln7_5.md) | [Index](index.md) | [Exercise 7.4](ex7_4.md) | [Exercise 7.6](ex7_6.md) \]
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----
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`>>>` Advanced Python Mastery
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`...` A course by [dabeaz](https://www.dabeaz.com)
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`...` Copyright 2007-2023
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. This work is licensed under a [Creative Commons Attribution-ShareAlike 4.0 International License](http://creativecommons.org/licenses/by-sa/4.0/)
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