TITLE OF PAPER: New Style Class Tutorial
URL OF PRESENTATION: _URL_of_powerpoint_presentation_
PRESENTED BY: Thomas Wouters
REPRESENTING: _name_of_the_company_they_represent_

CONFERENCE: PyCON 2004
DATE: 20040325
LOCATION: _venue_and_room_in_venue_

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REAL-TIME NOTES / ANNOTATIONS OF THE PAPER:
{If you've contributed, add your name, e-mail & URL at the bottom}

[...missed first 10 minutes]

Subclassing C types in Python just works (mostly)
- Cannot subclass from multiple C types if they have incompatible memory layouts
- And it's not always a good idea...

Creating immutable types: uses __new__, Python's constructor
- it is a *static* method, called with the class as the first argument
- responsible for obtaining memory
- may return an existing value
- may defined __slots__ instead of (or along with) __dict__
  - avoids memory overhead of dict if not used
<example: tristate subclasses int, can only be 0, 1, or 2>

Subclassing C types in C
- complex, but now possible
- make sure base type supports subclassing
  - no type-specific hardcoding, among other reqts
  
Type checking: a necessary evil in C
- Define Py<type>_Check() in terms of PyObject_TypeCheck()
- Can also have Py<type>_CheckExact() - ptr comparison against actual type struct
_ <example: PyList>

Definitions for members:
- PyMemberDef (tp_members) for instance data
- PyMethodDef (tp_methods) for methods
  - wraps the C functions in Python [descriptor] objects
- PyGetSetDef (tp_getset) for accessors [properties]
- [these tp_ fields end up pointing to arrays of these def objects]
- <example: standard xxsubtype from Python distribution>
- notes:
  - tp_flags must have Py_TPFLAGS_BASETYPE set to be subclassed
  - tp_type, tp_base filled in during module initialization (tp_type via PyType_Ready())

Subclassing a struct:
- Include base class struct as first field in subclass struct
- ramifications for multiple inheritance

Method Resolution Order (MRO):
- Old MRO also had no convenient way to access base classes
- New MRO algorithm uses "C3" algorithm from Dylan
  - Essentially same order as before, but eliminates all but last occurance of class
  - example: in diamond inheritance: A -> B, C -> D
    - was: D B A C A
    - now: D B C A
  - Correctly eliminates multiple calls to root base class

super():
- Proxy object for base class of current object
- Continues MRO where it left off
  - finds the method that "would have been called" if your class wasn't present
  - "Do it for your children"

Descriptors:
- objects with __get__, __set__, __delete__ methods that get called by getattr/setattr/delattr
- example: caching property
- Python properties are an application of descriptors
  - also hold docstrings for attributes
  - set and delete don't work with old-style classes
- demonstration of example

Special method types:
- classmethod
  - takes class as implicit first argument (not instance)
  - can be called through the class or an instance (object)
  - allows e.g. for factory functions that may return subclass objects
- staticmethod
  - takes no implicit first argument
  - turns out to be less necessary than originally thought
    - was designed for calling __new__ - can use super() for this
  - but allows for having a (non-method) function as a member of a class
    - [like static members in Java, C++, etc.]
- example:
  - Note use of builtin funcs classmethod()/staticmethod()
    - [This is partly why the decorator syntax PEP is desired]

Metaclasses:
- the class of a class (usually derives from type())
- Can "post-process" a class
- Assign meta-class to __metaclass__ attribute in class definition
- [this can make your brain hurt pretty quickly...]

Example: "my favorite metaclass" 'conveniencyTypeType'
- automethod(func) turns the func into the right method type (class, static) by looking at the first argument name ("cls", "self") and doing the "right thing"
- metaclass itself automagically creates self.__super that acts like super(self)
- also turns appropriately named methods into a property[?]

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REFERENCES: {as documents / sites are referenced add them below}

[reference to tp_flags in python doc?]
[reference to MRO in python doc?]
Slides:     http://www.xs4all.nl/~thomas/python/
            [example code will be there too]

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QUOTES:

"Do it for your children"

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CONTRIBUTORS: {add your name, e-mail address and URL below}
Russell Finn, rsf@sprucehill.com, http://www.sprucehill.com/rsf/blog/ [future]


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