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53 | ||
54 | <H1><A NAME="SECTION0011200000000000000000"></A><A NAME="debugger-hooks"></A> | |
55 | <BR> | |
56 | 9.2 How It Works | |
57 | </H1> | |
58 | ||
59 | <P> | |
60 | Some changes were made to the interpreter: | |
61 | ||
62 | <P> | |
63 | ||
64 | <UL> | |
65 | <LI><code>sys.settrace(<var>func</var>)</code> sets the global trace function | |
66 | </LI> | |
67 | <LI>there can also a local trace function (see later) | |
68 | </LI> | |
69 | </UL> | |
70 | ||
71 | <P> | |
72 | Trace functions have three arguments: <var>frame</var>, <var>event</var>, and | |
73 | <var>arg</var>. <var>frame</var> is the current stack frame. <var>event</var> is a | |
74 | string: <code>'call'</code>, <code>'line'</code>, <code>'return'</code>, <code>'exception'</code>, | |
75 | <code>'c_call'</code>, <code>'c_return'</code>, or <code>'c_exception'</code>. <var>arg</var> | |
76 | depends on the event type. | |
77 | ||
78 | <P> | |
79 | The global trace function is invoked (with <var>event</var> set to | |
80 | <code>'call'</code>) whenever a new local scope is entered; it should return | |
81 | a reference to the local trace function to be used that scope, or | |
82 | <code>None</code> if the scope shouldn't be traced. | |
83 | ||
84 | <P> | |
85 | The local trace function should return a reference to itself (or to | |
86 | another function for further tracing in that scope), or <code>None</code> to | |
87 | turn off tracing in that scope. | |
88 | ||
89 | <P> | |
90 | Instance methods are accepted (and very useful!) as trace functions. | |
91 | ||
92 | <P> | |
93 | The events have the following meaning: | |
94 | ||
95 | <P> | |
96 | <DL> | |
97 | <DT><STRONG><code>'call'</code></STRONG></DT> | |
98 | <DD>A function is called (or some other code block entered). The global | |
99 | trace function is called; <var>arg</var> is <code>None</code>; | |
100 | the return value specifies the local trace function. | |
101 | ||
102 | <P> | |
103 | </DD> | |
104 | <DT><STRONG><code>'line'</code></STRONG></DT> | |
105 | <DD>The interpreter is about to execute a new line of code (sometimes | |
106 | multiple line events on one line exist). The local trace function is | |
107 | called; <var>arg</var> is <code>None</code>; the return value specifies the new | |
108 | local trace function. | |
109 | ||
110 | <P> | |
111 | </DD> | |
112 | <DT><STRONG><code>'return'</code></STRONG></DT> | |
113 | <DD>A function (or other code block) is about to return. The local trace | |
114 | function is called; <var>arg</var> is the value that will be returned. The | |
115 | trace function's return value is ignored. | |
116 | ||
117 | <P> | |
118 | </DD> | |
119 | <DT><STRONG><code>'exception'</code></STRONG></DT> | |
120 | <DD>An exception has occurred. The local trace function is called; | |
121 | <var>arg</var> is a triple <code>(<var>exception</var>, <var>value</var>, | |
122 | <var>traceback</var>)</code>; the return value specifies the new local trace | |
123 | function. | |
124 | ||
125 | <P> | |
126 | </DD> | |
127 | <DT><STRONG><code>'c_call'</code></STRONG></DT> | |
128 | <DD>A C function is about to be called. This may be an extension function | |
129 | or a builtin. <var>arg</var> is the C function object. | |
130 | ||
131 | <P> | |
132 | </DD> | |
133 | <DT><STRONG><code>'c_return'</code></STRONG></DT> | |
134 | <DD>A C function has returned. <var>arg</var> is <code>None</code>. | |
135 | ||
136 | <P> | |
137 | </DD> | |
138 | <DT><STRONG><code>'c_exception'</code></STRONG></DT> | |
139 | <DD>A C function has thrown an exception. <var>arg</var> is <code>None</code>. | |
140 | ||
141 | <P> | |
142 | </DD> | |
143 | </DL> | |
144 | ||
145 | <P> | |
146 | Note that as an exception is propagated down the chain of callers, an | |
147 | <code>'exception'</code> event is generated at each level. | |
148 | ||
149 | <P> | |
150 | For more information on code and frame objects, refer to the | |
151 | <em class="citetitle"><a | |
152 | href="../ref/ref.html" | |
153 | title="Python Reference Manual" | |
154 | >Python Reference Manual</a></em>. | |
155 | ||
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182 | <div class='online-navigation'> | |
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189 | </div> | |
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192 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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