added __new__

Author: PythonCHB

slides_sources/source/advanced_oo.rst

@@ -1,8 +1,8 @@
 .. _advanced_oo:
 
-===========================================
+############################################
 Advanced Object Oriented Features of Python
-===========================================
+############################################
 
 - Chris Barker
 
@@ -11,7 +11,7 @@ Advanced Object Oriented Features of Python
 
 
 Multiple Inheritance
-====================
+#####################
 
 
 Pulling methods from more than one class
@@ -323,6 +323,114 @@ and call super like
 
 
 
+__new__
+--------
+
+What *really* happens when a class instance is created?
+
+Class Creation
+----------------
+
+What happens when a class instance is created?
+
+::
+
+    class Class(object):
+        def __init__(self, arg1, arg2):
+            self.arg1 = arg1
+            self.arg2 = arg2
+            .....
+
+* A new instance is created
+* ``__init__`` is called
+* The code in ``__init__`` is run to initialize the instance
+
+
+Class Creation
+----------------=
+
+What if you need to do something before creation?
+
+Enter: ``__new__``
+
+::
+
+    class Class(object):
+        def __new__(cls, arg1, arg2):
+            some_code_here
+            return cls(....)
+            .....
+
+* ``__new__`` is called: it returns a new instance
+* The code in ``__new__`` is run to pre-initialize
+* ``__init__`` is called
+* The code in ``__init__`` is run to initialize the instance
+
+
+Class Creation
+----------------
+
+``__new__`` is a static method -- but it must be called with a class object as the first argument.
+
+::
+
+    class Class(superclass):
+        def __new__(cls, arg1, arg2):
+            some_code_here
+            return superclass.__new__(cls)
+            .....
+
+``cls`` is the class object.
+
+The arguments (arg1, arg2) are what's passed in when calling the class.
+
+It needs to return a class instance -- usually by directly calling the superclass ``__new__``
+
+If nothing else, you can call ``object.__new__``
+
+
+When to use ``__new__``
+------------------------
+
+When would  you need to use it:
+
+* subclassing an immutable type:
+
+  - It's too late to change it once you get to ``__init__``
+
+* When ``__init__`` is not called:
+
+  - unpickling
+
+  - copying
+
+You may need to put some code in ``__new__`` to make sure things go right
+
+More detail here:
+
+http://www.python.org/download/releases/2.2/descrintro/#__new__
+
+
+LAB
+----
+
+**Demo:**
+
+ ``code/new_example.py``
+
+**Exercise:**
+
+Write a subclass of int that will always be an even number: round the input to the closest even number:
+
+  ``code/__new__/even_int.py``
+
+
+  ``code/__new__/test_even_int.py``
+
+
+
+
+
 Wrap Up
 ---------