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17 | <title>9. The Python Debugger </title> | |
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54 | ||
55 | <H1><A NAME="SECTION0011000000000000000000"></A><A NAME="debugger"></A> | |
56 | <BR> | |
57 | 9. The Python Debugger | |
58 | </H1> | |
59 | ||
60 | <P> | |
61 | <A NAME="module-pdb"></A> | |
62 | ||
63 | <P> | |
64 | The module <tt class="module">pdb</tt> defines an interactive source code | |
65 | debugger<a id='l2h-3099' xml:id='l2h-3099'></a> for Python programs. It supports setting | |
66 | (conditional) breakpoints and single stepping at the source line | |
67 | level, inspection of stack frames, source code listing, and evaluation | |
68 | of arbitrary Python code in the context of any stack frame. It also | |
69 | supports post-mortem debugging and can be called under program | |
70 | control. | |
71 | ||
72 | <P> | |
73 | The debugger is extensible -- it is actually defined as the class | |
74 | <tt class="class">Pdb</tt><a id='l2h-3092' xml:id='l2h-3092'></a>. | |
75 | This is currently undocumented but easily understood by reading the | |
76 | source. The extension interface uses the modules | |
77 | <tt class="module">bdb</tt><a id='l2h-3100' xml:id='l2h-3100'></a> (undocumented) and | |
78 | <tt class="module"><a href="module-cmd.html">cmd</a></tt><a id='l2h-3101' xml:id='l2h-3101'></a>. | |
79 | ||
80 | <P> | |
81 | The debugger's prompt is "<tt class="samp">(Pdb) </tt>". | |
82 | Typical usage to run a program under control of the debugger is: | |
83 | ||
84 | <P> | |
85 | <div class="verbatim"><pre> | |
86 | >>> import pdb | |
87 | >>> import mymodule | |
88 | >>> pdb.run('mymodule.test()') | |
89 | > <string>(0)?() | |
90 | (Pdb) continue | |
91 | > <string>(1)?() | |
92 | (Pdb) continue | |
93 | NameError: 'spam' | |
94 | > <string>(1)?() | |
95 | (Pdb) | |
96 | </pre></div> | |
97 | ||
98 | <P> | |
99 | <span class="file">pdb.py</span> can also be invoked as | |
100 | a script to debug other scripts. For example: | |
101 | ||
102 | <P> | |
103 | <div class="verbatim"><pre> | |
104 | python -m pdb myscript.py | |
105 | </pre></div> | |
106 | ||
107 | <P> | |
108 | When invoked as a script, pdb will automatically enter post-mortem debugging | |
109 | if the program being debugged exits abnormally. After post-mortem debugging | |
110 | (or after normal exit of the program), pdb will restart the program. | |
111 | Automatic restarting preserves pdb's state (such as breakpoints) and in most | |
112 | cases is more useful than quitting the debugger upon program's exit. | |
113 | ||
114 | <span class="versionnote">New in version 2.4: | |
115 | Restarting post-mortem behavior added.</span> | |
116 | ||
117 | <P> | |
118 | Typical usage to inspect a crashed program is: | |
119 | ||
120 | <P> | |
121 | <div class="verbatim"><pre> | |
122 | >>> import pdb | |
123 | >>> import mymodule | |
124 | >>> mymodule.test() | |
125 | Traceback (most recent call last): | |
126 | File "<stdin>", line 1, in ? | |
127 | File "./mymodule.py", line 4, in test | |
128 | test2() | |
129 | File "./mymodule.py", line 3, in test2 | |
130 | print spam | |
131 | NameError: spam | |
132 | >>> pdb.pm() | |
133 | > ./mymodule.py(3)test2() | |
134 | -> print spam | |
135 | (Pdb) | |
136 | </pre></div> | |
137 | ||
138 | <P> | |
139 | The module defines the following functions; each enters the debugger | |
140 | in a slightly different way: | |
141 | ||
142 | <P> | |
143 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
144 | <td><nobr><b><tt id='l2h-3093' xml:id='l2h-3093' class="function">run</tt></b>(</nobr></td> | |
145 | <td><var>statement</var><big>[</big><var>, globals</var><big>[</big><var>, locals</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt> | |
146 | <dd> | |
147 | Execute the <var>statement</var> (given as a string) under debugger | |
148 | control. The debugger prompt appears before any code is executed; you | |
149 | can set breakpoints and type "<tt class="samp">continue</tt>", or you can step through | |
150 | the statement using "<tt class="samp">step</tt>" or "<tt class="samp">next</tt>" (all these commands are | |
151 | explained below). The optional <var>globals</var> and <var>locals</var> | |
152 | arguments specify the environment in which the code is executed; by | |
153 | default the dictionary of the module <tt class="module"><a href="module-main.html">__main__</a></tt> is | |
154 | used. (See the explanation of the <tt class="keyword">exec</tt> statement or the | |
155 | <tt class="function">eval()</tt> built-in function.) | |
156 | </dl> | |
157 | ||
158 | <P> | |
159 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
160 | <td><nobr><b><tt id='l2h-3094' xml:id='l2h-3094' class="function">runeval</tt></b>(</nobr></td> | |
161 | <td><var>expression</var><big>[</big><var>, globals</var><big>[</big><var>, locals</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt> | |
162 | <dd> | |
163 | Evaluate the <var>expression</var> (given as a string) under debugger | |
164 | control. When <tt class="function">runeval()</tt> returns, it returns the value of the | |
165 | expression. Otherwise this function is similar to | |
166 | <tt class="function">run()</tt>. | |
167 | </dl> | |
168 | ||
169 | <P> | |
170 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
171 | <td><nobr><b><tt id='l2h-3095' xml:id='l2h-3095' class="function">runcall</tt></b>(</nobr></td> | |
172 | <td><var>function</var><big>[</big><var>, argument, ...</var><big>]</big><var></var>)</td></tr></table></dt> | |
173 | <dd> | |
174 | Call the <var>function</var> (a function or method object, not a string) | |
175 | with the given arguments. When <tt class="function">runcall()</tt> returns, it returns | |
176 | whatever the function call returned. The debugger prompt appears as | |
177 | soon as the function is entered. | |
178 | </dl> | |
179 | ||
180 | <P> | |
181 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
182 | <td><nobr><b><tt id='l2h-3096' xml:id='l2h-3096' class="function">set_trace</tt></b>(</nobr></td> | |
183 | <td><var></var>)</td></tr></table></dt> | |
184 | <dd> | |
185 | Enter the debugger at the calling stack frame. This is useful to | |
186 | hard-code a breakpoint at a given point in a program, even if the code | |
187 | is not otherwise being debugged (e.g. when an assertion fails). | |
188 | </dl> | |
189 | ||
190 | <P> | |
191 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
192 | <td><nobr><b><tt id='l2h-3097' xml:id='l2h-3097' class="function">post_mortem</tt></b>(</nobr></td> | |
193 | <td><var>traceback</var>)</td></tr></table></dt> | |
194 | <dd> | |
195 | Enter post-mortem debugging of the given <var>traceback</var> object. | |
196 | </dl> | |
197 | ||
198 | <P> | |
199 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
200 | <td><nobr><b><tt id='l2h-3098' xml:id='l2h-3098' class="function">pm</tt></b>(</nobr></td> | |
201 | <td><var></var>)</td></tr></table></dt> | |
202 | <dd> | |
203 | Enter post-mortem debugging of the traceback found in | |
204 | <code>sys.last_traceback</code>. | |
205 | </dl> | |
206 | ||
207 | <P> | |
208 | ||
209 | <p><br /></p><hr class='online-navigation' /> | |
210 | <div class='online-navigation'> | |
211 | <!--Table of Child-Links--> | |
212 | <A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a> | |
213 | ||
214 | <UL CLASS="ChildLinks"> | |
215 | <LI><A href="debugger-commands.html">9.1 Debugger Commands</a> | |
216 | <LI><A href="debugger-hooks.html">9.2 How It Works</a> | |
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253 | </div> | |
254 | <hr /> | |
255 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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