Initial commit

Exercises/soln6_4.md

@@ -0,0 +1,52 @@
+# Exercise 6.4 - Solution
+
+```python
+# structure.py
+
+class Structure:
+    _fields = ()
+
+    def __setattr__(self, name, value):
+        if name.startswith('_') or name in self._fields:
+            super().__setattr__(name, value)
+        else:
+            raise AttributeError('No attribute %s' % name)
+
+    def __repr__(self):
+        return '%s(%s)' % (type(self).__name__,
+                           ', '.join(repr(getattr(self, name)) for name in self._fields))
+
+    @classmethod
+    def create_init(cls):
+        args = ','.join(cls._fields)
+        code = 'def __init__(self, {0}):\n'.format(args)
+        statements = [ '    self.{0} = {0}'.format(name) for name in cls._fields]
+        code += '\n'.join(statements)
+        locs = { }
+        exec(code, locs)
+        cls.__init__ = locs['__init__']
+```
+
+Here is the `Stock` class in progress after this change:
+
+```python
+# stock.py
+
+from structure import Structure
+
+class Stock(Structure):
+    _fields = ('name', 'shares', 'price')
+
+    @property
+    def cost(self):
+        return self.shares * self.price
+
+    def sell(self, nshares):
+        self.shares -= nshares
+
+Stock.create_init()
+```
+
+
+
+[Back](ex6_4.md)