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| 53 | <H2><A NAME="SECTION004250000000000000000"> |
| 54 | 2.2.5 Abstract Protocol Support</A> |
| 55 | </H2> |
| 56 | |
| 57 | <P> |
| 58 | Python supports a variety of <em>abstract</em> `protocols;' the specific |
| 59 | interfaces provided to use these interfaces are documented in the |
| 60 | <em class="citetitle"><a |
| 61 | href="../api/api.html" |
| 62 | title="Python/C API Reference Manual" |
| 63 | >Python/C API Reference Manual</a></em> in the |
| 64 | chapter ``<a class="ulink" href="../api/abstract.html" |
| 65 | >Abstract Objects Layer</a>.'' |
| 66 | |
| 67 | <P> |
| 68 | A number of these abstract interfaces were defined early in the |
| 69 | development of the Python implementation. In particular, the number, |
| 70 | mapping, and sequence protocols have been part of Python since the |
| 71 | beginning. Other protocols have been added over time. For protocols |
| 72 | which depend on several handler routines from the type implementation, |
| 73 | the older protocols have been defined as optional blocks of handlers |
| 74 | referenced by the type object. For newer protocols there are |
| 75 | additional slots in the main type object, with a flag bit being set to |
| 76 | indicate that the slots are present and should be checked by the |
| 77 | interpreter. (The flag bit does not indicate that the slot values are |
| 78 | non-<tt class="constant">NULL</tt>. The flag may be set to indicate the presence of a slot, |
| 79 | but a slot may still be unfilled.) |
| 80 | |
| 81 | <P> |
| 82 | <div class="verbatim"><pre> |
| 83 | PyNumberMethods tp_as_number; |
| 84 | PySequenceMethods tp_as_sequence; |
| 85 | PyMappingMethods tp_as_mapping; |
| 86 | </pre></div> |
| 87 | |
| 88 | <P> |
| 89 | If you wish your object to be able to act like a number, a sequence, |
| 90 | or a mapping object, then you place the address of a structure that |
| 91 | implements the C type <tt class="ctype">PyNumberMethods</tt>, |
| 92 | <tt class="ctype">PySequenceMethods</tt>, or <tt class="ctype">PyMappingMethods</tt>, respectively. |
| 93 | It is up to you to fill in this structure with appropriate values. You |
| 94 | can find examples of the use of each of these in the <span class="file">Objects</span> |
| 95 | directory of the Python source distribution. |
| 96 | |
| 97 | <P> |
| 98 | <div class="verbatim"><pre> |
| 99 | hashfunc tp_hash; |
| 100 | </pre></div> |
| 101 | |
| 102 | <P> |
| 103 | This function, if you choose to provide it, should return a hash |
| 104 | number for an instance of your data type. Here is a moderately |
| 105 | pointless example: |
| 106 | |
| 107 | <P> |
| 108 | <div class="verbatim"><pre> |
| 109 | static long |
| 110 | newdatatype_hash(newdatatypeobject *obj) |
| 111 | { |
| 112 | long result; |
| 113 | result = obj->obj_UnderlyingDatatypePtr->size; |
| 114 | result = result * 3; |
| 115 | return result; |
| 116 | } |
| 117 | </pre></div> |
| 118 | |
| 119 | <P> |
| 120 | <div class="verbatim"><pre> |
| 121 | ternaryfunc tp_call; |
| 122 | </pre></div> |
| 123 | |
| 124 | <P> |
| 125 | This function is called when an instance of your data type is "called", |
| 126 | for example, if <code>obj1</code> is an instance of your data type and the Python |
| 127 | script contains <code>obj1('hello')</code>, the <tt class="member">tp_call</tt> handler is |
| 128 | invoked. |
| 129 | |
| 130 | <P> |
| 131 | This function takes three arguments: |
| 132 | |
| 133 | <P> |
| 134 | |
| 135 | <OL> |
| 136 | <LI><var>arg1</var> is the instance of the data type which is the subject of |
| 137 | the call. If the call is <code>obj1('hello')</code>, then <var>arg1</var> is |
| 138 | <code>obj1</code>. |
| 139 | |
| 140 | <P> |
| 141 | </LI> |
| 142 | <LI><var>arg2</var> is a tuple containing the arguments to the call. You |
| 143 | can use <tt class="cfunction">PyArg_ParseTuple()</tt> to extract the arguments. |
| 144 | |
| 145 | <P> |
| 146 | </LI> |
| 147 | <LI><var>arg3</var> is a dictionary of keyword arguments that were passed. |
| 148 | If this is non-<tt class="constant">NULL</tt> and you support keyword arguments, use |
| 149 | <tt class="cfunction">PyArg_ParseTupleAndKeywords()</tt> to extract the |
| 150 | arguments. If you do not want to support keyword arguments and |
| 151 | this is non-<tt class="constant">NULL</tt>, raise a <tt class="exception">TypeError</tt> with a message |
| 152 | saying that keyword arguments are not supported. |
| 153 | </LI> |
| 154 | </OL> |
| 155 | |
| 156 | <P> |
| 157 | Here is a desultory example of the implementation of the call function. |
| 158 | |
| 159 | <P> |
| 160 | <div class="verbatim"><pre> |
| 161 | /* Implement the call function. |
| 162 | * obj1 is the instance receiving the call. |
| 163 | * obj2 is a tuple containing the arguments to the call, in this |
| 164 | * case 3 strings. |
| 165 | */ |
| 166 | static PyObject * |
| 167 | newdatatype_call(newdatatypeobject *obj, PyObject *args, PyObject *other) |
| 168 | { |
| 169 | PyObject *result; |
| 170 | char *arg1; |
| 171 | char *arg2; |
| 172 | char *arg3; |
| 173 | |
| 174 | if (!PyArg_ParseTuple(args, "sss:call", &arg1, &arg2, &arg3)) { |
| 175 | return NULL; |
| 176 | } |
| 177 | result = PyString_FromFormat( |
| 178 | "Returning -- value: [\%d] arg1: [\%s] arg2: [\%s] arg3: [\%s]\n", |
| 179 | obj->obj_UnderlyingDatatypePtr->size, |
| 180 | arg1, arg2, arg3); |
| 181 | printf("\%s", PyString_AS_STRING(result)); |
| 182 | return result; |
| 183 | } |
| 184 | </pre></div> |
| 185 | |
| 186 | <P> |
| 187 | XXX some fields need to be added here... |
| 188 | |
| 189 | <P> |
| 190 | <div class="verbatim"><pre> |
| 191 | /* Added in release 2.2 */ |
| 192 | /* Iterators */ |
| 193 | getiterfunc tp_iter; |
| 194 | iternextfunc tp_iternext; |
| 195 | </pre></div> |
| 196 | |
| 197 | <P> |
| 198 | These functions provide support for the iterator protocol. Any object |
| 199 | which wishes to support iteration over its contents (which may be |
| 200 | generated during iteration) must implement the <code>tp_iter</code> |
| 201 | handler. Objects which are returned by a <code>tp_iter</code> handler must |
| 202 | implement both the <code>tp_iter</code> and <code>tp_iternext</code> handlers. |
| 203 | Both handlers take exactly one parameter, the instance for which they |
| 204 | are being called, and return a new reference. In the case of an |
| 205 | error, they should set an exception and return <tt class="constant">NULL</tt>. |
| 206 | |
| 207 | <P> |
| 208 | For an object which represents an iterable collection, the |
| 209 | <code>tp_iter</code> handler must return an iterator object. The iterator |
| 210 | object is responsible for maintaining the state of the iteration. For |
| 211 | collections which can support multiple iterators which do not |
| 212 | interfere with each other (as lists and tuples do), a new iterator |
| 213 | should be created and returned. Objects which can only be iterated |
| 214 | over once (usually due to side effects of iteration) should implement |
| 215 | this handler by returning a new reference to themselves, and should |
| 216 | also implement the <code>tp_iternext</code> handler. File objects are an |
| 217 | example of such an iterator. |
| 218 | |
| 219 | <P> |
| 220 | Iterator objects should implement both handlers. The <code>tp_iter</code> |
| 221 | handler should return a new reference to the iterator (this is the |
| 222 | same as the <code>tp_iter</code> handler for objects which can only be |
| 223 | iterated over destructively). The <code>tp_iternext</code> handler should |
| 224 | return a new reference to the next object in the iteration if there is |
| 225 | one. If the iteration has reached the end, it may return <tt class="constant">NULL</tt> |
| 226 | without setting an exception or it may set <tt class="exception">StopIteration</tt>; |
| 227 | avoiding the exception can yield slightly better performance. If an |
| 228 | actual error occurs, it should set an exception and return <tt class="constant">NULL</tt>. |
| 229 | |
| 230 | <P> |
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