4 :mod:`pdb` --- The Python Debugger
5 ==================================
8 :synopsis: The Python debugger for interactive interpreters.
11 .. index:: single: debugging
13 The module :mod:`pdb` defines an interactive source code debugger for Python
14 programs. It supports setting (conditional) breakpoints and single stepping at
15 the source line level, inspection of stack frames, source code listing, and
16 evaluation of arbitrary Python code in the context of any stack frame. It also
17 supports post-mortem debugging and can be called under program control.
20 single: Pdb (class in pdb)
24 The debugger is extensible --- it is actually defined as the class :class:`Pdb`.
25 This is currently undocumented but easily understood by reading the source. The
26 extension interface uses the modules :mod:`bdb` (undocumented) and :mod:`cmd`.
28 The debugger's prompt is ``(Pdb)``. Typical usage to run a program under control
33 >>> pdb.run('mymodule.test()')
42 :file:`pdb.py` can also be invoked as a script to debug other scripts. For
45 python -m pdb myscript.py
47 When invoked as a script, pdb will automatically enter post-mortem debugging if
48 the program being debugged exits abnormally. After post-mortem debugging (or
49 after normal exit of the program), pdb will restart the program. Automatic
50 restarting preserves pdb's state (such as breakpoints) and in most cases is more
51 useful than quitting the debugger upon program's exit.
54 Restarting post-mortem behavior added.
56 Typical usage to inspect a crashed program is::
61 Traceback (most recent call last):
62 File "<stdin>", line 1, in ?
63 File "./mymodule.py", line 4, in test
65 File "./mymodule.py", line 3, in test2
69 > ./mymodule.py(3)test2()
73 The module defines the following functions; each enters the debugger in a
74 slightly different way:
77 .. function:: run(statement[, globals[, locals]])
79 Execute the *statement* (given as a string) under debugger control. The
80 debugger prompt appears before any code is executed; you can set breakpoints and
81 type ``continue``, or you can step through the statement using ``step`` or
82 ``next`` (all these commands are explained below). The optional *globals* and
83 *locals* arguments specify the environment in which the code is executed; by
84 default the dictionary of the module :mod:`__main__` is used. (See the
85 explanation of the :keyword:`exec` statement or the :func:`eval` built-in
89 .. function:: runeval(expression[, globals[, locals]])
91 Evaluate the *expression* (given as a string) under debugger control. When
92 :func:`runeval` returns, it returns the value of the expression. Otherwise this
93 function is similar to :func:`run`.
96 .. function:: runcall(function[, argument, ...])
98 Call the *function* (a function or method object, not a string) with the given
99 arguments. When :func:`runcall` returns, it returns whatever the function call
100 returned. The debugger prompt appears as soon as the function is entered.
103 .. function:: set_trace()
105 Enter the debugger at the calling stack frame. This is useful to hard-code a
106 breakpoint at a given point in a program, even if the code is not otherwise
107 being debugged (e.g. when an assertion fails).
110 .. function:: post_mortem(traceback)
112 Enter post-mortem debugging of the given *traceback* object.
117 Enter post-mortem debugging of the traceback found in ``sys.last_traceback``.
120 .. _debugger-commands:
125 The debugger recognizes the following commands. Most commands can be
126 abbreviated to one or two letters; e.g. ``h(elp)`` means that either ``h`` or
127 ``help`` can be used to enter the help command (but not ``he`` or ``hel``, nor
128 ``H`` or ``Help`` or ``HELP``). Arguments to commands must be separated by
129 whitespace (spaces or tabs). Optional arguments are enclosed in square brackets
130 (``[]``) in the command syntax; the square brackets must not be typed.
131 Alternatives in the command syntax are separated by a vertical bar (``|``).
133 Entering a blank line repeats the last command entered. Exception: if the last
134 command was a ``list`` command, the next 11 lines are listed.
136 Commands that the debugger doesn't recognize are assumed to be Python statements
137 and are executed in the context of the program being debugged. Python
138 statements can also be prefixed with an exclamation point (``!``). This is a
139 powerful way to inspect the program being debugged; it is even possible to
140 change a variable or call a function. When an exception occurs in such a
141 statement, the exception name is printed but the debugger's state is not
144 Multiple commands may be entered on a single line, separated by ``;;``. (A
145 single ``;`` is not used as it is the separator for multiple commands in a line
146 that is passed to the Python parser.) No intelligence is applied to separating
147 the commands; the input is split at the first ``;;`` pair, even if it is in the
148 middle of a quoted string.
150 The debugger supports aliases. Aliases can have parameters which allows one a
151 certain level of adaptability to the context under examination.
155 triple: debugger; configuration; file
157 If a file :file:`.pdbrc` exists in the user's home directory or in the current
158 directory, it is read in and executed as if it had been typed at the debugger
159 prompt. This is particularly useful for aliases. If both files exist, the one
160 in the home directory is read first and aliases defined there can be overridden
164 Without argument, print the list of available commands. With a *command* as
165 argument, print help about that command. ``help pdb`` displays the full
166 documentation file; if the environment variable :envvar:`PAGER` is defined, the
167 file is piped through that command instead. Since the *command* argument must
168 be an identifier, ``help exec`` must be entered to get help on the ``!``
172 Print a stack trace, with the most recent frame at the bottom. An arrow
173 indicates the current frame, which determines the context of most commands.
176 Move the current frame one level down in the stack trace (to a newer frame).
179 Move the current frame one level up in the stack trace (to an older frame).
181 b(reak) [[*filename*:]\ *lineno* | *function*\ [, *condition*]]
182 With a *lineno* argument, set a break there in the current file. With a
183 *function* argument, set a break at the first executable statement within that
184 function. The line number may be prefixed with a filename and a colon, to
185 specify a breakpoint in another file (probably one that hasn't been loaded yet).
186 The file is searched on ``sys.path``. Note that each breakpoint is assigned a
187 number to which all the other breakpoint commands refer.
189 If a second argument is present, it is an expression which must evaluate to true
190 before the breakpoint is honored.
192 Without argument, list all breaks, including for each breakpoint, the number of
193 times that breakpoint has been hit, the current ignore count, and the associated
196 tbreak [[*filename*:]\ *lineno* | *function*\ [, *condition*]]
197 Temporary breakpoint, which is removed automatically when it is first hit. The
198 arguments are the same as break.
200 cl(ear) [*bpnumber* [*bpnumber ...*]]
201 With a space separated list of breakpoint numbers, clear those breakpoints.
202 Without argument, clear all breaks (but first ask confirmation).
204 disable [*bpnumber* [*bpnumber ...*]]
205 Disables the breakpoints given as a space separated list of breakpoint numbers.
206 Disabling a breakpoint means it cannot cause the program to stop execution, but
207 unlike clearing a breakpoint, it remains in the list of breakpoints and can be
210 enable [*bpnumber* [*bpnumber ...*]]
211 Enables the breakpoints specified.
213 ignore *bpnumber* [*count*]
214 Sets the ignore count for the given breakpoint number. If count is omitted, the
215 ignore count is set to 0. A breakpoint becomes active when the ignore count is
216 zero. When non-zero, the count is decremented each time the breakpoint is
217 reached and the breakpoint is not disabled and any associated condition
220 condition *bpnumber* [*condition*]
221 Condition is an expression which must evaluate to true before the breakpoint is
222 honored. If condition is absent, any existing condition is removed; i.e., the
223 breakpoint is made unconditional.
225 commands [*bpnumber*]
226 Specify a list of commands for breakpoint number *bpnumber*. The commands
227 themselves appear on the following lines. Type a line containing just 'end' to
228 terminate the commands. An example::
231 (com) print some_variable
235 To remove all commands from a breakpoint, type commands and follow it
236 immediately with end; that is, give no commands.
238 With no *bpnumber* argument, commands refers to the last breakpoint set.
240 You can use breakpoint commands to start your program up again. Simply use the
241 continue command, or step, or any other command that resumes execution.
243 Specifying any command resuming execution (currently continue, step, next,
244 return, jump, quit and their abbreviations) terminates the command list (as if
245 that command was immediately followed by end). This is because any time you
246 resume execution (even with a simple next or step), you may encounter another
247 breakpoint--which could have its own command list, leading to ambiguities about
248 which list to execute.
250 If you use the 'silent' command in the command list, the usual message about
251 stopping at a breakpoint is not printed. This may be desirable for breakpoints
252 that are to print a specific message and then continue. If none of the other
253 commands print anything, you see no sign that the breakpoint was reached.
255 .. versionadded:: 2.5
258 Execute the current line, stop at the first possible occasion (either in a
259 function that is called or on the next line in the current function).
262 Continue execution until the next line in the current function is reached or it
263 returns. (The difference between ``next`` and ``step`` is that ``step`` stops
264 inside a called function, while ``next`` executes called functions at (nearly)
265 full speed, only stopping at the next line in the current function.)
268 Continue execution until the current function returns.
271 Continue execution, only stop when a breakpoint is encountered.
274 Set the next line that will be executed. Only available in the bottom-most
275 frame. This lets you jump back and execute code again, or jump forward to skip
276 code that you don't want to run.
278 It should be noted that not all jumps are allowed --- for instance it is not
279 possible to jump into the middle of a :keyword:`for` loop or out of a
280 :keyword:`finally` clause.
282 l(ist) [*first*\ [, *last*]]
283 List source code for the current file. Without arguments, list 11 lines around
284 the current line or continue the previous listing. With one argument, list 11
285 lines around at that line. With two arguments, list the given range; if the
286 second argument is less than the first, it is interpreted as a count.
289 Print the argument list of the current function.
292 Evaluate the *expression* in the current context and print its value.
296 ``print`` can also be used, but is not a debugger command --- this executes the
297 Python :keyword:`print` statement.
300 Like the ``p`` command, except the value of the expression is pretty-printed
301 using the :mod:`pprint` module.
303 alias [*name* [command]]
304 Creates an alias called *name* that executes *command*. The command must *not*
305 be enclosed in quotes. Replaceable parameters can be indicated by ``%1``,
306 ``%2``, and so on, while ``%*`` is replaced by all the parameters. If no
307 command is given, the current alias for *name* is shown. If no arguments are
308 given, all aliases are listed.
310 Aliases may be nested and can contain anything that can be legally typed at the
311 pdb prompt. Note that internal pdb commands *can* be overridden by aliases.
312 Such a command is then hidden until the alias is removed. Aliasing is
313 recursively applied to the first word of the command line; all other words in
314 the line are left alone.
316 As an example, here are two useful aliases (especially when placed in the
317 :file:`.pdbrc` file)::
319 #Print instance variables (usage "pi classInst")
320 alias pi for k in %1.__dict__.keys(): print "%1.",k,"=",%1.__dict__[k]
321 #Print instance variables in self
325 Deletes the specified alias.
328 Execute the (one-line) *statement* in the context of the current stack frame.
329 The exclamation point can be omitted unless the first word of the statement
330 resembles a debugger command. To set a global variable, you can prefix the
331 assignment command with a ``global`` command on the same line, e.g.::
333 (Pdb) global list_options; list_options = ['-l']
337 Restart the debugged python program. If an argument is supplied, it is split
338 with "shlex" and the result is used as the new sys.argv. History, breakpoints,
339 actions and debugger options are preserved. "restart" is an alias for "run".
341 .. versionadded:: 2.6
344 Quit from the debugger. The program being executed is aborted.
352 Some changes were made to the interpreter:
354 * ``sys.settrace(func)`` sets the global trace function
356 * there can also a local trace function (see later)
358 Trace functions have three arguments: *frame*, *event*, and *arg*. *frame* is
359 the current stack frame. *event* is a string: ``'call'``, ``'line'``,
360 ``'return'``, ``'exception'``, ``'c_call'``, ``'c_return'``, or
361 ``'c_exception'``. *arg* depends on the event type.
363 The global trace function is invoked (with *event* set to ``'call'``) whenever a
364 new local scope is entered; it should return a reference to the local trace
365 function to be used that scope, or ``None`` if the scope shouldn't be traced.
367 The local trace function should return a reference to itself (or to another
368 function for further tracing in that scope), or ``None`` to turn off tracing in
371 Instance methods are accepted (and very useful!) as trace functions.
373 The events have the following meaning:
376 A function is called (or some other code block entered). The global trace
377 function is called; *arg* is ``None``; the return value specifies the local
381 The interpreter is about to execute a new line of code (sometimes multiple line
382 events on one line exist). The local trace function is called; *arg* is
383 ``None``; the return value specifies the new local trace function.
386 A function (or other code block) is about to return. The local trace function
387 is called; *arg* is the value that will be returned. The trace function's
388 return value is ignored.
391 An exception has occurred. The local trace function is called; *arg* is a
392 triple ``(exception, value, traceback)``; the return value specifies the new
393 local trace function.
396 A C function is about to be called. This may be an extension function or a
397 builtin. *arg* is the C function object.
400 A C function has returned. *arg* is ``None``.
403 A C function has thrown an exception. *arg* is ``None``.
405 Note that as an exception is propagated down the chain of callers, an
406 ``'exception'`` event is generated at each level.
408 For more information on code and frame objects, refer to :ref:`types`.