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Exercises/ex7_4.md

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+\[ [Index](index.md) | [Exercise 7.3](ex7_3.md) | [Exercise 7.5](ex7_5.md) \]
+
+# Exercise 7.4
+
+*Objectives:*
+
+- Learn about the low-level steps involved in creating a class
+
+*Files Modified:* `validate.py`, `structure.py`
+
+In this exercise, we look at the mechanics of how classes are actually
+created. 
+
+## (a) Class creation
+
+Recall, from earlier exercises, we defined a simple class
+`Stock` that looked like this:
+
+```python
+class Stock:
+    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
+```
+
+What we're going to do here is create the class manually.  Start out
+by just defining the methods as normal Python functions.
+
+```python
+>>> 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
+
+>>>
+```
+
+Next, make a methods dictionary:
+
+```python
+>>> methods = {
+         '__init__' : __init__,
+         'cost' : cost,
+         'sell' : sell } 
+
+>>>
+```
+
+Finally, create the `Stock` class object:
+
+```python
+>>> Stock = type('Stock',(object,),methods)
+>>> s = Stock('GOOG',100,490.10)
+>>> s.name
+'GOOG'
+>>> s.cost()
+49010.0
+>>> s.sell(25)
+>>> s.shares
+75
+>>>
+```
+
+Congratulations, you just created a class.   A class is really nothing
+more than a name, a tuple of base classes, and a dictionary holding
+all of the class contents. `type()` is a constructor that
+creates a class for you if you supply these three parts.
+
+## (b) Typed structures
+
+In the `structure.py` file, define the following function:
+
+```python
+# structure.py
+
+...
+def typed_structure(clsname, **validators):
+    cls = type(clsname, (Structure,), validators)
+    return cls
+```
+
+This function is somewhat similar to the `namedtuple()` function in that it creates a class. Try it out:
+
+```python
+>>> from validate import String, PositiveInteger, PositiveFloat
+>>> from structure import typed_structure
+>>> Stock = typed_structure('Stock', name=String(), shares=PositiveInteger(), price=PositiveFloat())
+>>> s = Stock('GOOG', 100, 490.1)
+>>> s.name
+'GOOG'
+>>> s
+Stock('GOOG', 100, 490.1)
+>>>
+```
+
+You might find the seams of your head starting to pull apart about now.
+
+## (c) Making a lot of classes
+
+
+There are other situations where direct usage of the `type()` constructor might be advantageous.
+Consider this bit of code:
+
+```python
+# validate.py
+...
+
+class Typed(Validator):
+    expected_type = object
+    @classmethod
+    def check(cls, value):
+        if not isinstance(value, cls.expected_type):
+            raise TypeError(f'expected {cls.expected_type}')
+        super().check(value)
+
+class Integer(Typed):
+    expected_type = int
+
+class Float(Typed):
+    expected_type = float
+
+class String(Typed):
+    expected_type = str
+...
+```
+
+Wow is the last part of that annoying and repetitive.  Change it
+to use a table of desired type classes like this:
+
+```python
+# validate.py
+...
+
+_typed_classes = [
+    ('Integer', int),
+    ('Float', float),
+    ('String', str) ]
+
+globals().update((name, type(name, (Typed,), {'expected_type':ty}))
+                 for name, ty in _typed_classes)
+```
+
+Now, if you want to have more type classes, you just add them to the
+table:
+
+```python
+_typed_classes = [
+    ('Integer', int),
+    ('Float', float),
+    ('Complex', complex),
+    ('Decimal', decimal.Decimal),
+    ('List', list),
+    ('Bool', bool),
+    ('String', str) ]
+```
+
+Admit it, that's kind of cool and saves a lot of typing (at the keyboard).
+
+\[ [Solution](soln7_4.md) | [Index](index.md) | [Exercise 7.3](ex7_3.md) | [Exercise 7.5](ex7_5.md) \]
+
+----
+`>>>` 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/)