| \chapter{Defining New Types |
| \label{defining-new-types}} |
| \sectionauthor{Michael Hudson}{mwh@python.net} |
| \sectionauthor{Dave Kuhlman}{dkuhlman@rexx.com} |
| |
| As mentioned in the last chapter, Python allows the writer of an |
| extension module to define new types that can be manipulated from |
| Python code, much like strings and lists in core Python. |
| |
| This is not hard; the code for all extension types follows a pattern, |
| but there are some details that you need to understand before you can |
| get started. |
| |
| \section{The Basics |
| \label{dnt-basics}} |
| |
| The Python runtime sees all Python objects as variables of type |
| \ctype{PyObject*}. A \ctype{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 \code{[].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: |
| |
| \verbatiminput{noddy.c} |
| |
| 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: |
| |
| \begin{verbatim} |
| staticforward PyTypeObject noddy_NoddyType; |
| \end{verbatim} |
| |
| 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 \code{staticforward} is required to placate various brain dead |
| compilers. |
| |
| \begin{verbatim} |
| typedef struct { |
| PyObject_HEAD |
| } noddy_NoddyObject; |
| \end{verbatim} |
| |
| 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 \code{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 |
| |
| \begin{verbatim} |
| typedef struct { |
| PyObject_HEAD |
| long ob_ival; |
| } PyIntObject; |
| \end{verbatim} |
| |
| is the corresponding definition for standard Python integers. |
| |
| Next up is: |
| |
| \begin{verbatim} |
| 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; |
| } |
| \end{verbatim} |
| |
| 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 \ctype{PyTypeObject} |
| structures is all very well, but if there's no way to actually |
| \emph{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: |
| |
| \begin{verbatim} |
| PyObject_New(<type>, &<type object>); |
| \end{verbatim} |
| |
| This allocates the memory and then initializes the object (sets |
| the reference count to one, makes the \member{ob_type} pointer point at |
| the right place and maybe some other stuff, depending on build options). |
| You \emph{can} do these steps separately if you have some reason to |
| --- but at this level we don't bother. |
| |
| Note that \cfunction{PyObject_New()} is a polymorphic macro rather |
| than a real function. The first parameter is the name of the C |
| structure that represents an object of our new type, and the return |
| value is a pointer to that type. This would be |
| \ctype{noddy_NoddyObject} in our example: |
| |
| \begin{verbatim} |
| noddy_NoddyObject *my_noddy; |
| |
| my_noddy = PyObject_New(noddy_NoddyObject, &noddy_NoddyType); |
| \end{verbatim} |
| |
| We cast the return value to a \ctype{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 \cfunction{noddy_new_noddy} to |
| return a \ctype{noddy_NoddyObject*} and then put a cast in the |
| definition of \cdata{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 |
| |
| \begin{verbatim} |
| static void |
| noddy_noddy_dealloc(PyObject* self) |
| { |
| PyObject_Del(self); |
| } |
| \end{verbatim} |
| |
| This is so short as to be self explanatory. This function will be |
| called when the reference count on a Noddy object reaches \code{0} (or |
| it is found as part of an unreachable cycle by the cyclic garbage |
| collector). \cfunction{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. |
| |
| \begin{verbatim} |
| static PyTypeObject noddy_NoddyType = { |
| PyObject_HEAD_INIT(NULL) |
| 0, /* ob_size */ |
| "Noddy", /* tp_name */ |
| sizeof(noddy_NoddyObject), /* tp_basicsize */ |
| 0, /* tp_itemsize */ |
| 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 */ |
| }; |
| \end{verbatim} |
| |
| Now if you go and look up the definition of \ctype{PyTypeObject} in |
| \file{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: |
| |
| \begin{verbatim} |
| PyObject_HEAD_INIT(NULL) |
| \end{verbatim} |
| |
| This line is a bit of a wart; what we'd like to write is: |
| |
| \begin{verbatim} |
| PyObject_HEAD_INIT(&PyType_Type) |
| \end{verbatim} |
| |
| 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 |
| \member{ob_type} field of \cdata{noddy_NoddyType} at the earliest |
| oppourtunity --- in \cfunction{initnoddy()}. |
| |
| \begin{verbatim} |
| 0, /* ob_size */ |
| \end{verbatim} |
| |
| The \member{ob_size} field of the header is not used; it's presence in |
| the type structure is a historical artifact that is maintained for |
| binary compatibility with extension modules compiled for older |
| versions of Python. Always set this field to zero. |
| |
| \begin{verbatim} |
| "Noddy", /* tp_name */ |
| \end{verbatim} |
| |
| The name of our type. This will appear in the default textual |
| representation of our objects and in some error messages, for example: |
| |
| \begin{verbatim} |
| >>> "" + noddy.new_noddy() |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in ? |
| TypeError: cannot add type "Noddy" to string |
| \end{verbatim} |
| |
| \begin{verbatim} |
| sizeof(noddy_NoddyObject), /* tp_basicsize */ |
| \end{verbatim} |
| |
| This is so that Python knows how much memory to allocate when you call |
| \cfunction{PyObject_New}. |
| |
| \begin{verbatim} |
| 0, /* tp_itemsize */ |
| \end{verbatim} |
| |
| 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. |
| |
| \begin{verbatim} |
| noddy_noddy_dealloc, /* tp_dealloc */ |
| \end{verbatim} |
| |
| From here, all the type methods are \NULL, 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 \cfunction{initnoddy}: |
| |
| \begin{verbatim} |
| noddy_NoddyType.ob_type = &PyType_Type; |
| \end{verbatim} |
| |
| This was alluded to above --- the \cdata{noddy_NoddyType} object should |
| have type ``type'', but \code{\&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 \file{noddymodule.c} and |
| |
| \begin{verbatim} |
| from distutils.core import setup, Extension |
| setup(name="noddy", version="1.0", |
| ext_modules=[Extension("noddy", ["noddymodule.c"])]) |
| \end{verbatim} |
| |
| in a file called \file{setup.py}; then typing |
| |
| \begin{verbatim} |
| $ python setup.py build |
| \end{verbatim} %$ <-- bow to font-lock ;-( |
| |
| at a shell should produce a file \file{noddy.so} in a subdirectory; |
| move to that directory and fire up Python --- you should be able to |
| \code{import noddy} and play around with Noddy objects. |
| |
| That wasn't so hard, was it? |
| |
| |
| \section{Type Methods |
| \label{dnt-type-methods}} |
| |
| This section aims to give a quick fly-by on the various type methods |
| you can implement and what they do. |
| |
| Here is the definition of \ctype{PyTypeObject}, with some fields only |
| used in debug builds omitted: |
| |
| \verbatiminput{typestruct.h} |
| |
| Now that's a \emph{lot} of methods. Don't worry too much though - if |
| you have a type you want to define, the chances are very good that you |
| will only implement a handful of these. |
| |
| As you probably expect by now, we're going to go over this and give |
| more information about the various handlers. We won't go in the order |
| they are defined in the structure, because there is a lot of |
| historical baggage that impacts the ordering of the fields; be sure |
| your type initializaion keeps the fields in the right order! It's |
| often easiest to find an example that includes all the fields you need |
| (even if they're initialized to \code{0}) and then change the values |
| to suit your new type. |
| |
| \begin{verbatim} |
| char *tp_name; /* For printing */ |
| \end{verbatim} |
| |
| The name of the type - as mentioned in the last section, this will |
| appear in various places, almost entirely for diagnostic purposes. |
| Try to choose something that will be helpful in such a situation! |
| |
| \begin{verbatim} |
| int tp_basicsize, tp_itemsize; /* For allocation */ |
| \end{verbatim} |
| |
| These fields tell the runtime how much memory to allocate when new |
| objects of this typed are created. Python has some builtin support |
| for variable length structures (think: strings, lists) which is where |
| the \member{tp_itemsize} field comes in. This will be dealt with |
| later. |
| |
| \begin{verbatim} |
| char *tp_doc; |
| \end{verbatim} |
| |
| Here you can put a string (or its address) that you want returned when |
| the Python script references \code{obj.__doc__} to retrieve the |
| docstring. |
| |
| Now we come to the basic type methods---the ones most extension types |
| will implement. |
| |
| |
| \subsection{Finalization and De-allocation} |
| |
| \index{object!deallocation} |
| \index{deallocation, object} |
| \index{object!finalization} |
| \index{finalization, of objects} |
| |
| \begin{verbatim} |
| destructor tp_dealloc; |
| \end{verbatim} |
| |
| This function is called when the reference count of the instance of |
| your type is reduced to zero and the Python interpreter wants to |
| reclaim it. If your type has memory to free or other clean-up to |
| perform, put it here. The object itself needs to be freed here as |
| well. Here is an example of this function: |
| |
| \begin{verbatim} |
| static void |
| newdatatype_dealloc(newdatatypeobject * obj) |
| { |
| free(obj->obj_UnderlyingDatatypePtr); |
| PyObject_DEL(obj); |
| } |
| \end{verbatim} |
| |
| One important requirement of the deallocator function is that it |
| leaves any pending exceptions alone. This is important since |
| deallocators are frequently called as the interpreter unwinds the |
| Python stack; when the stack is unwound due to an exception (rather |
| than normal returns), nothing is done to protect the deallocators from |
| seeing that an exception has already been set. Any actions which a |
| deallocator performs which may cause additional Python code to be |
| executed may detect that an exception has been set. This can lead to |
| misleading errors from the interpreter. The proper way to protect |
| against this is to save a pending exception before performing the |
| unsafe action, and restoring it when done. This can be done using the |
| \cfunction{PyErr_Fetch()}\ttindex{PyErr_Fetch()} and |
| \cfunction{PyErr_Restore()}\ttindex{PyErr_Restore()} functions: |
| |
| \begin{verbatim} |
| static void |
| my_dealloc(PyObject *obj) |
| { |
| MyObject *self = (MyObject *) obj; |
| PyObject *cbresult; |
| |
| if (self->my_callback != NULL) { |
| PyObject *err_type, *err_value, *err_traceback; |
| int have_error = PyErr_Occurred() ? 1 : 0; |
| |
| if (have_error) |
| PyErr_Fetch(&err_type, &err_value, &err_traceback); |
| |
| cbresult = PyObject_CallObject(self->my_callback, NULL); |
| if (cbresult == NULL) |
| PyErr_WriteUnraisable(); |
| else |
| Py_DECREF(cbresult); |
| |
| if (have_error) |
| PyErr_Restore(err_type, err_value, err_traceback); |
| |
| Py_DECREF(self->my_callback); |
| } |
| PyObject_DEL(obj); |
| } |
| \end{verbatim} |
| |
| |
| \subsection{Object Representation} |
| |
| In Python, there are three ways to generate a textual representation |
| of an object: the \function{repr()}\bifuncindex{repr} function (or |
| equivalent backtick syntax), the \function{str()}\bifuncindex{str} |
| function, and the \keyword{print} statement. For most objects, the |
| \keyword{print} statement is equivalent to the \function{str()} |
| function, but it is possible to special-case printing to a |
| \ctype{FILE*} if necessary; this should only be done if efficiency is |
| identified as a problem and profiling suggests that creating a |
| temporary string object to be written to a file is too expensive. |
| |
| These handlers are all optional, and most types at most need to |
| implement the \member{tp_str} and \member{tp_repr} handlers. |
| |
| \begin{verbatim} |
| reprfunc tp_repr; |
| reprfunc tp_str; |
| printfunc tp_print; |
| \end{verbatim} |
| |
| The \member{tp_repr} handler should return a string object containing |
| a representation of the instance for which it is called. Here is a |
| simple example: |
| |
| \begin{verbatim} |
| static PyObject * |
| newdatatype_repr(newdatatypeobject * obj) |
| { |
| return PyString_FromFormat("Repr-ified_newdatatype{{size:\%d}}", |
| obj->obj_UnderlyingDatatypePtr->size); |
| } |
| \end{verbatim} |
| |
| If no \member{tp_repr} handler is specified, the interpreter will |
| supply a representation that uses the type's \member{tp_name} and a |
| uniquely-identifying value for the object. |
| |
| The \member{tp_str} handler is to \function{str()} what the |
| \member{tp_repr} handler described above is to \function{repr()}; that |
| is, it is called when Python code calls \function{str()} on an |
| instance of your object. It's implementation is very similar to the |
| \member{tp_repr} function, but the resulting string is intended for |
| human consumption. It \member{tp_str} is not specified, the |
| \member{tp_repr} handler is used instead. |
| |
| Here is a simple example: |
| |
| \begin{verbatim} |
| static PyObject * |
| newdatatype_str(newdatatypeobject * obj) |
| { |
| return PyString_FromFormat("Stringified_newdatatype{{size:\%d}}", |
| obj->obj_UnderlyingDatatypePtr->size); |
| } |
| \end{verbatim} |
| |
| The print function will be called whenever Python needs to "print" an |
| instance of the type. For example, if 'node' is an instance of type |
| TreeNode, then the print function is called when Python code calls: |
| |
| \begin{verbatim} |
| print node |
| \end{verbatim} |
| |
| There is a flags argument and one flag, \constant{Py_PRINT_RAW}, and |
| it suggests that you print without string quotes and possibly without |
| interpreting escape sequences. |
| |
| The print function receives a file object as an argument. You will |
| likely want to write to that file object. |
| |
| Here is a sampe print function: |
| |
| \begin{verbatim} |
| static int |
| newdatatype_print(newdatatypeobject *obj, FILE *fp, int flags) |
| { |
| if (flags & Py_PRINT_RAW) { |
| fprintf(fp, "<{newdatatype object--size: %d}>", |
| obj->obj_UnderlyingDatatypePtr->size); |
| } |
| else { |
| fprintf(fp, "\"<{newdatatype object--size: %d}>\"", |
| obj->obj_UnderlyingDatatypePtr->size); |
| } |
| return 0; |
| } |
| \end{verbatim} |
| |
| |
| \subsection{Attribute Management Functions} |
| |
| For every object which can support attributes, the corresponding type |
| must provide the functions that control how the attributes are |
| resolved. There needs to be a function which can retrieve attributes |
| (if any are defined), and another to set attributes (if setting |
| attributes is allowed). Removing an attribute is a special case, for |
| which the new value passed to the handler is \NULL. |
| |
| Python supports two pairs of attribute handlers; a type that supports |
| attributes only needs to implement the functions for one pair. The |
| difference is that one pair takes the name of the attribute as a |
| \ctype{char*}, while the other accepts a \ctype{PyObject*}. Each type |
| can use whichever pair makes more sense for the implementation's |
| convenience. |
| |
| \begin{verbatim} |
| getattrfunc tp_getattr; /* char * version */ |
| setattrfunc tp_setattr; |
| /* ... */ |
| getattrofunc tp_getattrofunc; /* PyObject * version */ |
| setattrofunc tp_setattrofunc; |
| \end{verbatim} |
| |
| If accessing attributes of an object is always a simple operation |
| (this will be explained shortly), there are generic implementations |
| which can be used to provide the \ctype{PyObject*} version of the |
| attribute management functions. The actual need for type-specific |
| attribute handlers almost completely disappeared starting with Python |
| 2.2, though there are many examples which have not been updated to use |
| some of the new generic mechanism that is available. |
| |
| |
| \subsubsection{Generic Attribute Management} |
| |
| \versionadded{2.2} |
| |
| Most extension types only use \emph{simple} attributes. So, what |
| makes the attributes simple? There are only a couple of conditions |
| that must be met: |
| |
| \begin{enumerate} |
| \item The name of the attributes must be known when |
| \cfunction{PyType_Ready()} is called. |
| |
| \item No special processing is need to record that an attribute |
| was looked up or set, nor do actions need to be taken based |
| on the value. |
| \end{enumerate} |
| |
| Note that this list does not place any restrictions on the values of |
| the attributes, when the values are computed, or how relevant data is |
| stored. |
| |
| When \cfunction{PyType_Ready()} is called, it uses three tables |
| referenced by the type object to create \emph{descriptors} which are |
| placed in the dictionary of the type object. Each descriptor controls |
| access to one attribute of the instance object. Each of the tables is |
| optional; if all three are \NULL, instances of the type will only have |
| attributes that are inherited from their base type, and should leave |
| the \member{tp_getattro} and \member{tp_setattro} fields \NULL{} as |
| well, allowing the base type to handle attributes. |
| |
| The tables are declared as three fields of the type object: |
| |
| \begin{verbatim} |
| struct PyMethodDef *tp_methods; |
| struct PyMemberDef *tp_members; |
| struct PyGetSetDef *tp_getset; |
| \end{verbatim} |
| |
| If \member{tp_methods} is not \NULL, it must refer to an array of |
| \ctype{PyMethodDef} structures. Each entry in the table is an |
| instance of this structure: |
| |
| \begin{verbatim} |
| typedef struct PyMethodDef { |
| char *ml_name; /* method name */ |
| PyCFunction ml_meth; /* implementation function */ |
| int ml_flags; /* flags */ |
| char *ml_doc; /* docstring */ |
| } PyMethodDef; |
| \end{verbatim} |
| |
| One entry should be defined for each method provided by the type; no |
| entries are needed for methods inherited from a base type. One |
| additional entry is needed at the end; it is a sentinel that marks the |
| end of the array. The \member{ml_name} field of the sentinel must be |
| \NULL. |
| |
| XXX Need to refer to some unified discussion of the structure fields, |
| shared with the next section. |
| |
| The second table is used to define attributes which map directly to |
| data stored in the instance. A variety of primitive C types are |
| supported, and access may be read-only or read-write. The structures |
| in the table are defined as: |
| |
| \begin{verbatim} |
| typedef struct PyMemberDef { |
| char *name; |
| int type; |
| int offset; |
| int flags; |
| char *doc; |
| } PyMemberDef; |
| \end{verbatim} |
| |
| For each entry in the table, a descriptor will be constructed and |
| added to the type which will be able to extract a value from the |
| instance structure. The \member{type} field should contain one of the |
| type codes defined in the \file{structmember.h} header; the value will |
| be used to determine how to convert Python values to and from C |
| values. The \member{flags} field is used to store flags which control |
| how the attribute can be accessed. |
| |
| XXX Need to move some of this to a shared section! |
| |
| The following flag constants are defined in \file{structmember.h}; |
| they may be combined using bitwise-OR. |
| |
| \begin{tableii}{l|l}{constant}{Constant}{Meaning} |
| \lineii{READONLY \ttindex{READONLY}} |
| {Never writable.} |
| \lineii{RO \ttindex{RO}} |
| {Shorthand for \constant{READONLY}.} |
| \lineii{READ_RESTRICTED \ttindex{READ_RESTRICTED}} |
| {Not readable in restricted mode.} |
| \lineii{WRITE_RESTRICTED \ttindex{WRITE_RESTRICTED}} |
| {Not writable in restricted mode.} |
| \lineii{RESTRICTED \ttindex{RESTRICTED}} |
| {Not readable or writable in restricted mode.} |
| \end{tableii} |
| |
| An interesting advantage of using the \member{tp_members} table to |
| build descriptors that are used at runtime is that any attribute |
| defined this way can have an associated docstring simply by providing |
| the text in the table. An application can use the introspection API |
| to retrieve the descriptor from the class object, and get the |
| docstring using its \member{__doc__} attribute. |
| |
| As with the \member{tp_methods} table, a sentinel entry with a |
| \member{name} value of \NULL{} is required. |
| |
| |
| % XXX Descriptors need to be explained in more detail somewhere, but |
| % not here. |
| % |
| % Descriptor objects have two handler functions which correspond to |
| % the \member{tp_getattro} and \member{tp_setattro} handlers. The |
| % \method{__get__()} handler is a function which is passed the |
| % descriptor, instance, and type objects, and returns the value of the |
| % attribute, or it returns \NULL{} and sets an exception. The |
| % \method{__set__()} handler is passed the descriptor, instance, type, |
| % and new value; |
| |
| |
| \subsubsection{Type-specific Attribute Management} |
| |
| For simplicity, only the \ctype{char*} version will be demonstrated |
| here; the type of the name parameter is the only difference between |
| the \ctype{char*} and \ctype{PyObject*} flavors of the interface. |
| This example effectively does the same thing as the generic example |
| above, but does not use the generic support added in Python 2.2. The |
| value in showing this is two-fold: it demonstrates how basic attribute |
| management can be done in a way that is portable to older versions of |
| Python, and explains how the handler functions are called, so that if |
| you do need to extend their functionality, you'll understand what |
| needs to be done. |
| |
| The \member{tp_getattr} handler is called when the object requires an |
| attribute look-up. It is called in the same situations where the |
| \method{__getattr__()} method of a class would be called. |
| |
| A likely way to handle this is (1) to implement a set of functions |
| (such as \cfunction{newdatatype_getSize()} and |
| \cfunction{newdatatype_setSize()} in the example below), (2) provide a |
| method table listing these functions, and (3) provide a getattr |
| function that returns the result of a lookup in that table. The |
| method table uses the same structure as the \member{tp_methods} field |
| of the type object. |
| |
| Here is an example: |
| |
| \begin{verbatim} |
| static PyMethodDef newdatatype_methods[] = { |
| {"getSize", (PyCFunction)newdatatype_getSize, METH_VARARGS, |
| "Return the current size."}, |
| {"setSize", (PyCFunction)newdatatype_setSize, METH_VARARGS, |
| "Set the size."}, |
| {NULL, NULL, 0, NULL} /* sentinel */ |
| }; |
| |
| static PyObject * |
| newdatatype_getattr(newdatatypeobject *obj, char *name) |
| { |
| return Py_FindMethod(newdatatype_methods, (PyObject *)obj, name); |
| } |
| \end{verbatim} |
| |
| The \member{tp_setattr} handler is called when the |
| \method{__setattr__()} or \method{__delattr__()} method of a class |
| instance would be called. When an attribute should be deleted, the |
| third parameter will be \NULL. Here is an example that simply raises |
| an exception; if this were really all you wanted, the |
| \member{tp_setattr} handler should be set to \NULL. |
| |
| \begin{verbatim} |
| static int |
| newdatatype_setattr(newdatatypeobject *obj, char *name, PyObject *v) |
| { |
| (void)PyErr_Format(PyExc_RuntimeError, "Read-only attribute: \%s", name); |
| return -1; |
| } |
| \end{verbatim} |
| |
| |
| \subsection{Object Comparison} |
| |
| \begin{verbatim} |
| cmpfunc tp_compare; |
| \end{verbatim} |
| |
| The \member{tp_compare} handler is called when comparisons are needed |
| are the object does not implement the specific rich comparison method |
| which matches the requested comparison. (It is always used if defined |
| and the \cfunction{PyObject_Compare()} or \cfunction{PyObject_Cmp()} |
| functions are used, or if \function{cmp()} is used from Python.) |
| It is analogous to the \method{__cmp__()} method. This function |
| should return \code{-1} if \var{obj1} is less than |
| \var{obj2}, \code{0} if they are equal, and \code{1} if |
| \var{obj1} is greater than |
| \var{obj2}. |
| (It was previously allowed to return arbitrary negative or positive |
| integers for less than and greater than, respectively; as of Python |
| 2.2, this is no longer allowed. In the future, other return values |
| may be assigned a different meaning.) |
| |
| A \member{tp_compare} handler may raise an exception. In this case it |
| should return a negative value. The caller has to test for the |
| exception using \cfunction{PyErr_Occurred()}. |
| |
| |
| Here is a sample implementation: |
| |
| \begin{verbatim} |
| static int |
| newdatatype_compare(newdatatypeobject * obj1, newdatatypeobject * obj2) |
| { |
| long result; |
| |
| if (obj1->obj_UnderlyingDatatypePtr->size < |
| obj2->obj_UnderlyingDatatypePtr->size) { |
| result = -1; |
| } |
| else if (obj1->obj_UnderlyingDatatypePtr->size > |
| obj2->obj_UnderlyingDatatypePtr->size) { |
| result = 1; |
| } |
| else { |
| result = 0; |
| } |
| return result; |
| } |
| \end{verbatim} |
| |
| |
| \subsection{Abstract Protocol Support} |
| |
| Python supports a variety of \emph{abstract} `protocols;' the specific |
| interfaces provided to use these interfaces are documented in the |
| \citetitle[../api/api.html]{Python/C API Reference Manual} in the |
| chapter ``\ulink{Abstract Objects Layer}{../api/abstract.html}.'' |
| |
| A number of these abstract interfaces were defined early in the |
| development of the Python implementation. In particular, the number, |
| mapping, and sequence protocols have been part of Python since the |
| beginning. Other protocols have been added over time. For protocols |
| which depend on several handler routines from the type implementation, |
| the older protocols have been defined as optional blocks of handlers |
| referenced by the type object, while newer protocols have been added |
| using additional slots in the main type object, with a flag bit being |
| set to indicate that the slots are present. (The flag bit does not |
| indicate that the slot values are non-\NULL.) |
| |
| \begin{verbatim} |
| PyNumberMethods tp_as_number; |
| PySequenceMethods tp_as_sequence; |
| PyMappingMethods tp_as_mapping; |
| \end{verbatim} |
| |
| If you wish your object to be able to act like a number, a sequence, |
| or a mapping object, then you place the address of a structure that |
| implements the C type \ctype{PyNumberMethods}, |
| \ctype{PySequenceMethods}, or \ctype{PyMappingMethods}, respectively. |
| It is up to you to fill in this structure with appropriate values. You |
| can find examples of the use of each of these in the \file{Objects} |
| directory of the Python source distribution. |
| |
| |
| \begin{verbatim} |
| hashfunc tp_hash; |
| \end{verbatim} |
| |
| This function, if you choose to provide it, should return a hash |
| number for an instance of your datatype. Here is a moderately |
| pointless example: |
| |
| \begin{verbatim} |
| static long |
| newdatatype_hash(newdatatypeobject *obj) |
| { |
| long result; |
| result = obj->obj_UnderlyingDatatypePtr->size; |
| result = result * 3; |
| return result; |
| } |
| \end{verbatim} |
| |
| \begin{verbatim} |
| ternaryfunc tp_call; |
| \end{verbatim} |
| |
| This function is called when an instance of your datatype is "called", |
| for example, if \code{obj1} is an instance of your datatype and the Python |
| script contains \code{obj1('hello')}, the \member{tp_call} handler is |
| invoked. |
| |
| This function takes three arguments: |
| |
| \begin{enumerate} |
| \item |
| \var{arg1} is the instance of the datatype which is the subject of |
| the call. If the call is \code{obj1('hello')}, then \var{arg1} is |
| \code{obj1}. |
| |
| \item |
| \var{arg2} is a tuple containing the arguments to the call. You |
| can use \cfunction{PyArg_ParseTuple()} to extract the arguments. |
| |
| \item |
| \var{arg3} is a dictionary of keyword arguments that were passed. |
| If this is non-\NULL{} and you support keyword arguments, use |
| \cfunction{PyArg_ParseTupleAndKeywords()} to extract the |
| arguments. If you do not want to support keyword arguments and |
| this is non-\NULL, raise a \exception{TypeError} with a message |
| saying that keyword arguments are not supported. |
| \end{enumerate} |
| |
| Here is a desultory example of the implementation of the call function. |
| |
| \begin{verbatim} |
| /* Implement the call function. |
| * obj1 is the instance receiving the call. |
| * obj2 is a tuple containing the arguments to the call, in this |
| * case 3 strings. |
| */ |
| static PyObject * |
| newdatatype_call(newdatatypeobject *obj, PyObject *args, PyObject *other) |
| { |
| PyObject *result; |
| char *arg1; |
| char *arg2; |
| char *arg3; |
| |
| if (!PyArg_ParseTuple(args, "sss:call", &arg1, &arg2, &arg3)) { |
| return NULL; |
| } |
| result = PyString_FromFormat( |
| "Returning -- value: [\%d] arg1: [\%s] arg2: [\%s] arg3: [\%s]\n", |
| obj->obj_UnderlyingDatatypePtr->size, |
| arg1, arg2, arg3); |
| printf("\%s", PyString_AS_STRING(result)); |
| return result; |
| } |
| \end{verbatim} |
| |
| XXX some fields need to be added here... |
| |
| |
| \begin{verbatim} |
| /* Added in release 2.2 */ |
| /* Iterators */ |
| getiterfunc tp_iter; |
| iternextfunc tp_iternext; |
| \end{verbatim} |
| |
| These functions provide support for the iterator protocol. Any object |
| which wishes to support iteration over it's contents (which may be |
| generated during iteration) must implement the \code{tp_iter} |
| handler. Objects which are returned by a \code{tp_iter} handler must |
| implement both the \code{tp_iter} and \code{tp_iternext} handlers. |
| Both handlers take exactly one parameter, the instance for which they |
| are being called, and return a new reference. In the case of an |
| error, they should set an exception and return \NULL. |
| |
| For an object which represents an iterable collection, the |
| \code{tp_iter} handler must return an iterator object. The iterator |
| object is responsible for maintaining the state of the iteration. For |
| collections which can support multiple iterators which do not |
| interfere with each other (as lists and tuples do), a new iterator |
| should be created and returned. Objects which can only be iterated |
| over once (usually due to side effects of iteration) should implement |
| this handler by returning a new reference to themselves, and should |
| also implement the \code{tp_iternext} handler. File objects are an |
| example of such an iterator. |
| |
| Iterator objects should implement both handlers. The \code{tp_iter} |
| handler should return a new reference to the iterator (this is the |
| same as the \code{tp_iter} handler for objects which can only be |
| iterated over destructively). The \code{tp_iternext} handler should |
| return a new reference to the next object in the iteration if there is |
| one. If the iteration has reached the end, it may return \NULL{} |
| without setting an exception or it may set \exception{StopIteration}; |
| avoiding the exception can yield slightly better performance. If an |
| actual error occurs, it should set an exception and return \NULL. |
| |
| |
| \subsection{More Suggestions} |
| |
| Remember that you can omit most of these functions, in which case you |
| provide \code{0} as a value. |
| |
| In the \file{Objects} directory of the Python source distribution, |
| there is a file \file{xxobject.c}, which is intended to be used as a |
| template for the implementation of new types. One useful strategy |
| for implementing a new type is to copy and rename this file, then |
| read the instructions at the top of it. |
| |
| There are type definitions for each of the functions you must |
| provide. They are in \file{object.h} in the Python include |
| directory that comes with the source distribution of Python. |
| |
| In order to learn how to implement any specific method for your new |
| datatype, do the following: Download and unpack the Python source |
| distribution. Go the the \file{Objects} directory, then search the |
| C source files for \code{tp_} plus the function you want (for |
| example, \code{tp_print} or \code{tp_compare}). You will find |
| examples of the function you want to implement. |
| |
| When you need to verify that the type of an object is indeed the |
| object you are implementing and if you use xxobject.c as an starting |
| template for your implementation, then there is a macro defined for |
| this purpose. The macro definition will look something like this: |
| |
| \begin{verbatim} |
| #define is_newdatatypeobject(v) ((v)->ob_type == &Newdatatypetype) |
| \end{verbatim} |
| |
| And, a sample of its use might be something like the following: |
| |
| \begin{verbatim} |
| if (!is_newdatatypeobject(objp1) { |
| PyErr_SetString(PyExc_TypeError, "arg #1 not a newdatatype"); |
| return NULL; |
| } |
| \end{verbatim} |