2.1 The Basics

The Python runtime sees all Python objects as variables of type PyObject*. A PyObject is not a very magnificent object - it just contains the refcount and a pointer to the object's ``type object''. This is where the action is; the type object determines which (C) functions get called when, for instance, an attribute gets looked up on an object or it is multiplied by another object. I call these C functions ``type methods'' to distinguish them from things like [].append (which I will call ``object methods'' when I get around to them).

So, if you want to define a new object type, you need to create a new type object.

This sort of thing can only be explained by example, so here's a minimal, but complete, module that defines a new type:

#include <Python.h>

staticforward PyTypeObject noddy_NoddyType;

typedef struct {
    PyObject_HEAD
} noddy_NoddyObject;

static PyObject*
noddy_new_noddy(PyObject* self, PyObject* args)
{
    noddy_NoddyObject* noddy;

    if (!PyArg_ParseTuple(args,":new_noddy")) 
        return NULL;

    noddy = PyObject_New(noddy_NoddyObject, &noddy_NoddyType);

    return (PyObject*)noddy;
}

static void
noddy_noddy_dealloc(PyObject* self)
{
    PyObject_Del(self);
}

static PyTypeObject noddy_NoddyType = {
    PyObject_HEAD_INIT(NULL)
    0,
    "Noddy",
    sizeof(noddy_NoddyObject),
    0,
    noddy_noddy_dealloc, /*tp_dealloc*/
    0,          /*tp_print*/
    0,          /*tp_getattr*/
    0,          /*tp_setattr*/
    0,          /*tp_compare*/
    0,          /*tp_repr*/
    0,          /*tp_as_number*/
    0,          /*tp_as_sequence*/
    0,          /*tp_as_mapping*/
    0,          /*tp_hash */
};

static PyMethodDef noddy_methods[] = {
    { "new_noddy", noddy_new_noddy, METH_VARARGS },
    {NULL, NULL}
};

DL_EXPORT(void)
initnoddy(void) 
{
    noddy_NoddyType.ob_type = &PyType_Type;

    Py_InitModule("noddy", noddy_methods);
}

Now that's quite a bit to take in at once, but hopefully bits will seem familiar from the last chapter.

The first bit that will be new is:

staticforward PyTypeObject noddy_NoddyType;

This names the type object that will be defining further down in the file. It can't be defined here because its definition has to refer to functions that have no yet been defined, but we need to be able to refer to it, hence the declaration.

The staticforward is required to placate various brain dead compilers.

typedef struct {
    PyObject_HEAD
} noddy_NoddyObject;

This is what a Noddy object will contain. In this case nothing more than every Python object contains - a refcount and a pointer to a type object. These are the fields the PyObject_HEAD macro brings in. The reason for the macro is to standardize the layout and to enable special debugging fields to be brought in debug builds.

For contrast

typedef struct {
    PyObject_HEAD
    long ob_ival;
} PyIntObject;

is the corresponding definition for standard Python integers.

Next up is:

static PyObject*
noddy_new_noddy(PyObject* self, PyObject* args)
{
    noddy_NoddyObject* noddy;

    if (!PyArg_ParseTuple(args,":new_noddy")) 
        return NULL;

    noddy = PyObject_New(noddy_NoddyObject, &noddy_NoddyType);

    return (PyObject*)noddy;
}

This is in fact just a regular module function, as described in the last chapter. The reason it gets special mention is that this is where we create our Noddy object. Defining PyTypeObject structures is all very well, but if there's no way to actually create one of the wretched things it is not going to do anyone much good.

Almost always, you create objects with a call of the form:

PyObject_New(<type>, &<type object>);

This allocates the memory and then initializes the object (i.e. sets the reference count to one, makes the ob_type pointer point at the right place and maybe some other stuff, depending on build options). You can do these steps separately if you have some reason to -- but at this level we don't bother.

We cast the return value to a PyObject* because that's what the Python runtime expects. This is safe because of guarantees about the layout of structures in the C standard, and is a fairly common C programming trick. One could declare noddy_new_noddy to return a noddy_NoddyObject* and then put a cast in the definition of noddy_methods further down the file -- it doesn't make much difference.

Now a Noddy object doesn't do very much and so doesn't need to implement many type methods. One you can't avoid is handling deallocation, so we find

static void
noddy_noddy_dealloc(PyObject* self)
{
    PyObject_Del(self);
}

This is so short as to be self explanatory. This function will be called when the reference count on a Noddy object reaches 0 (or it is found as part of an unreachable cycle by the cyclic garbage collector). PyObject_Del() is what you call when you want an object to go away. If a Noddy object held references to other Python objects, one would decref them here.

Moving on, we come to the crunch -- the type object.

static PyTypeObject noddy_NoddyType = {
    PyObject_HEAD_INIT(NULL)
    0,
    "Noddy",
    sizeof(noddy_NoddyObject),
    0,
    noddy_noddy_dealloc, /*tp_dealloc*/
    0,                   /*tp_print*/
    0,                   /*tp_getattr*/
    0,                   /*tp_setattr*/
    0,                   /*tp_compare*/
    0,                   /*tp_repr*/
    0,                   /*tp_as_number*/
    0,                   /*tp_as_sequence*/
    0,                   /*tp_as_mapping*/
    0,                   /*tp_hash */
};

Now if you go and look up the definition of PyTypeObject in object.h you'll see that it has many, many more fields that the definition above. The remaining fields will be filled with zeros by the C compiler, and it's common practice to not specify them explicitly unless you need them.

This is so important that I'm going to pick the top of it apart still further:

    PyObject_HEAD_INIT(NULL)

This line is a bit of a wart; what we'd like to write is:

    PyObject_HEAD_INIT(&PyType_Type)

as the type of a type object is ``type'', but this isn't strictly conforming C and some compilers complain. So instead we fill in the ob_type field of noddy_NoddyType at the earliest oppourtunity -- in initnoddy().

0,

XXX why does the type info struct start PyObject_*VAR*_HEAD??

    "Noddy",

The name of our type. This will appear in the default textual representation of our objects and in some error messages, for example:

>>> "" + noddy.new_noddy()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: cannot add type "Noddy" to string

    sizeof(noddy_NoddyObject),

This is so that Python knows how much memory to allocate when you call PyObject_New.

0,

This has to do with variable length objects like lists and strings. Ignore for now...

Now we get into the type methods, the things that make your objects different from the others. Of course, the Noddy object doesn't implement many of these, but as mentioned above you have to implement the deallocation function.

    noddy_noddy_dealloc, /*tp_dealloc*/

From here, all the type methods are nil so I won't go over them yet - that's for the next section!

Everything else in the file should be familiar, except for this line in initnoddy:

    noddy_NoddyType.ob_type = &PyType_Type;

This was alluded to above -- the noddy_NoddyType object should have type ``type'', but &PyType_Type is not constant and so can't be used in its initializer. To work around this, we patch it up in the module initialization.

That's it! All that remains is to build it; put the above code in a file called noddymodule.c and

from distutils.core import setup, Extension
setup(name = "noddy", version = "1.0",
    ext_modules = [Extension("noddy", ["noddymodule.c"])])

in a file called setup.py; then typing

$ python setup.py build%$

at a shell should produce a file noddy.so in a subdirectory; move to that directory and fire up Python -- you should be able to import noddy and play around with Noddy objects.

That wasn't so hard, was it?

See About this document... for information on suggesting changes.

iPhone 6 battery cover
hawaiian shoes
Volcom Board Shorts
Billabong Board Shorts
Quicksilver Board Shorts
I got a new iPhone5 battery case that I found on the web. I have a new ipad and I just love it. My new HTC One cellphone is awesome. I ordered a new iphone5 and I can't wait to get it. The smartphone charger I purchased is exactly what I needed. The new HTC phone is the best. I need more used AOL disks for my computer. The new hawaiian shoes has a new larger display.
hawaiian shoes
hawaiian shoes
You should look at List of surf and skate and this site iPhone Cases and this website too iPhone Cases
hawaiian sandals

hawaiian shoes

mophie Juice Pack Plus iPhone 6 plus battery pack is the best there is.

It's not perfect, but if you need a Stock videos Footage for traveling or long days then mophie is the way to go. Battery life is always an issue on every smartphone nowadays and third-party manufacturers provide external battery power supplies to ensure that life of your device will last for more than a day. I found arch support sandal on the Although traditionally mens work boots mens work boots are made with leather, the work boots work boots can also be made of a composite rubber, a plastic such as thermoplastic or even leather. website. We got a pair of Rigoberto Ramirez and Rigoberto Ramirez too. There are two hawaiian shoes and my favorite Rigoberto Ramirez. That's why there are portable power packs—when the power is out.

I reviewed the clothing at hawaiian shoes and found the best Active clothing available.

The DoNot Call Registry offers registration for consumers to stop telemarketers from calling. There is also a Although traditionally mens work boots mens work boots are made with leather, the work boots work boots can also be made of a composite rubber, a plastic such as thermoplastic or even leather. Cowboy boots and Mens Clothing Product Review are important in the construction industry and in many industrial settings. Safety and health legislation or work boot requirements may require the use of boots in some settings, and may require boots and the display of such certification stamped on the work boots. and a women's cowboy boots online. Sign up now.

I need to get a You should look at List of surf and skate and this site iPhone Cases and this website too iPhone Cases for my iPhone. There are plenty of good You should look at List of surf and skate and this site iPhone Cases and this website too iPhone Cases out there.
Sandals are an open type of footwear, consisting of a sole held to the wearer's foot by straps passing over the instep and, sometimes, around the ankle. Found the girls hawaiian shoes on the free stock videos website. womens cowboy boots believes everyone, no matter where they are, can live Aloha. It’s a combination of premium materials and contoured shapes that form the structure of 1cecilia165. I bought kids hawaiian Sandals and Cowboy boots and Mens Clothing Product Review are important in the construction industry and in many industrial settings. Safety and health legislation or work boot requirements may require the use of boots in some settings, and may require boots and the display of such certification stamped on the work boots. from Hawaii directly. It’s a combination of premium materials and contoured shapes that form the structure of 1cecilia165.

Order Sandals mens on the website sandals hawaiian and order a few. Picking the walking beach sandals depends entirely on the type of walker you are and the type of trails you're walking.

These are the shops to visit:

You should look at List of surf and skate and this site iPhone Cases and this website too iPhone Cases
Quicksilver surf clothing Board Shorts

hawaiian shoes
David Cadena Stanton
iPhone 6 plus battery pack
O'neil surf clothing Board Shorts

Also, you will want to check out Stanton California so you can see what's up and they are part of Stanton City Hall as well.

You can also get Organic Skin Care products from Bliss Bath Body and you must check out their Natural Body Lotions and bath soaps

Now if you are looking for the best deals

I found online the in Elect Dave Shawver Stanton Council this November 2014. Elect march madness ncaa and internetusers this November 2014.

delivered.

These are the shops to visit:

Order Sandals mens on the website sandals hawaiian and order a few. Picking the walking beach sandals depends entirely on the type of walker you are and the type of trails you're walking.

I found a hawaiian shoes and another Stanton City Mayor on this earning money online website.

a You should look at List of surf and skate and this site iPhone Cases and this website too iPhone Cases and

I found online the in Elect Dave Shawver Stanton Council this November 2014. Elect march madness ncaa and internetusers this November 2014.

delivered.

These are the shops to visit:

Order Sandals mens on the website sandals hawaiian and order a few. Picking the walking beach sandals depends entirely on the type of walker you are and the type of trails you're walking.

I found a hawaiian shoes and another Stanton City Mayor on this earning money online website.

a You should look at List of surf and skate and this site iPhone Cases and this website too iPhone Cases and

Hey, check out this Organic Skin Care European Soaps along with Natural Lavender Body Lotion and shea butter

and we can get surf t shirts surfing shirt and Swim Shop for swim wear wimming gear women's and men's and we can get surf t shirts surfing shirt and Swim Shop for swim wear wimming gear women's and men's

French Lavender Soaps Organic And Natural Body Care Shea Body Butters

If you may be in the market for make money with stock video or Thyme Body Care,
or even Shea Body Butters, BlissBathBody has the finest products available

hawaiian shoes

mophie Juice Pack Plus iPhone 6 plus battery pack is the best there is.

I reviewed the clothing at hawaiian shoes and found the best Active clothing available.

Order Sandals mens on the website sandals hawaiian and order a few. Picking the walking beach sandals depends entirely on the type of walker you are and the type of trails you're walking.

These are the shops to visit:

Also, you will want to check out Stanton California so you can see what's up and they are part of Stanton City Hall as well.

You can also get Organic Skin Care products from Bliss Bath Body and you must check out their Natural Body Lotions and bath soaps

Now if you are looking for the best deals

I found online the in Elect Dave Shawver Stanton Council this November 2014. Elect march madness ncaa and internetusers this November 2014.

delivered.

These are the shops to visit:

Order Sandals mens on the website sandals hawaiian and order a few. Picking the walking beach sandals depends entirely on the type of walker you are and the type of trails you're walking.

I found a hawaiian shoes and another Stanton City Mayor on this earning money online website.

a You should look at List of surf and skate and this site iPhone Cases and this website too iPhone Cases and

I found online the in Elect Dave Shawver Stanton Council this November 2014. Elect march madness ncaa and internetusers this November 2014.

delivered.

These are the shops to visit:

Order Sandals mens on the website sandals hawaiian and order a few. Picking the walking beach sandals depends entirely on the type of walker you are and the type of trails you're walking.

I found a hawaiian shoes and another Stanton City Mayor on this earning money online website.

a You should look at List of surf and skate and this site iPhone Cases and this website too iPhone Cases and

Hey, check out this Organic Skin Care European Soaps along with Natural Lavender Body Lotion and shea butter