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. If no
113 *traceback* is given, it uses the one of the exception that is currently
114 being handled (an exception must be being handled if the default is to be
120 Enter post-mortem debugging of the traceback found in ``sys.last_traceback``.
123 .. _debugger-commands:
128 The debugger recognizes the following commands. Most commands can be
129 abbreviated to one or two letters; e.g. ``h(elp)`` means that either ``h`` or
130 ``help`` can be used to enter the help command (but not ``he`` or ``hel``, nor
131 ``H`` or ``Help`` or ``HELP``). Arguments to commands must be separated by
132 whitespace (spaces or tabs). Optional arguments are enclosed in square brackets
133 (``[]``) in the command syntax; the square brackets must not be typed.
134 Alternatives in the command syntax are separated by a vertical bar (``|``).
136 Entering a blank line repeats the last command entered. Exception: if the last
137 command was a ``list`` command, the next 11 lines are listed.
139 Commands that the debugger doesn't recognize are assumed to be Python statements
140 and are executed in the context of the program being debugged. Python
141 statements can also be prefixed with an exclamation point (``!``). This is a
142 powerful way to inspect the program being debugged; it is even possible to
143 change a variable or call a function. When an exception occurs in such a
144 statement, the exception name is printed but the debugger's state is not
147 Multiple commands may be entered on a single line, separated by ``;;``. (A
148 single ``;`` is not used as it is the separator for multiple commands in a line
149 that is passed to the Python parser.) No intelligence is applied to separating
150 the commands; the input is split at the first ``;;`` pair, even if it is in the
151 middle of a quoted string.
153 The debugger supports aliases. Aliases can have parameters which allows one a
154 certain level of adaptability to the context under examination.
158 triple: debugger; configuration; file
160 If a file :file:`.pdbrc` exists in the user's home directory or in the current
161 directory, it is read in and executed as if it had been typed at the debugger
162 prompt. This is particularly useful for aliases. If both files exist, the one
163 in the home directory is read first and aliases defined there can be overridden
167 Without argument, print the list of available commands. With a *command* as
168 argument, print help about that command. ``help pdb`` displays the full
169 documentation file; if the environment variable :envvar:`PAGER` is defined, the
170 file is piped through that command instead. Since the *command* argument must
171 be an identifier, ``help exec`` must be entered to get help on the ``!``
175 Print a stack trace, with the most recent frame at the bottom. An arrow
176 indicates the current frame, which determines the context of most commands.
179 Move the current frame one level down in the stack trace (to a newer frame).
182 Move the current frame one level up in the stack trace (to an older frame).
184 b(reak) [[*filename*:]\ *lineno* | *function*\ [, *condition*]]
185 With a *lineno* argument, set a break there in the current file. With a
186 *function* argument, set a break at the first executable statement within that
187 function. The line number may be prefixed with a filename and a colon, to
188 specify a breakpoint in another file (probably one that hasn't been loaded yet).
189 The file is searched on ``sys.path``. Note that each breakpoint is assigned a
190 number to which all the other breakpoint commands refer.
192 If a second argument is present, it is an expression which must evaluate to true
193 before the breakpoint is honored.
195 Without argument, list all breaks, including for each breakpoint, the number of
196 times that breakpoint has been hit, the current ignore count, and the associated
199 tbreak [[*filename*:]\ *lineno* | *function*\ [, *condition*]]
200 Temporary breakpoint, which is removed automatically when it is first hit. The
201 arguments are the same as break.
203 cl(ear) [*bpnumber* [*bpnumber ...*]]
204 With a space separated list of breakpoint numbers, clear those breakpoints.
205 Without argument, clear all breaks (but first ask confirmation).
207 disable [*bpnumber* [*bpnumber ...*]]
208 Disables the breakpoints given as a space separated list of breakpoint numbers.
209 Disabling a breakpoint means it cannot cause the program to stop execution, but
210 unlike clearing a breakpoint, it remains in the list of breakpoints and can be
213 enable [*bpnumber* [*bpnumber ...*]]
214 Enables the breakpoints specified.
216 ignore *bpnumber* [*count*]
217 Sets the ignore count for the given breakpoint number. If count is omitted, the
218 ignore count is set to 0. A breakpoint becomes active when the ignore count is
219 zero. When non-zero, the count is decremented each time the breakpoint is
220 reached and the breakpoint is not disabled and any associated condition
223 condition *bpnumber* [*condition*]
224 Condition is an expression which must evaluate to true before the breakpoint is
225 honored. If condition is absent, any existing condition is removed; i.e., the
226 breakpoint is made unconditional.
228 commands [*bpnumber*]
229 Specify a list of commands for breakpoint number *bpnumber*. The commands
230 themselves appear on the following lines. Type a line containing just 'end' to
231 terminate the commands. An example::
234 (com) print some_variable
238 To remove all commands from a breakpoint, type commands and follow it
239 immediately with end; that is, give no commands.
241 With no *bpnumber* argument, commands refers to the last breakpoint set.
243 You can use breakpoint commands to start your program up again. Simply use the
244 continue command, or step, or any other command that resumes execution.
246 Specifying any command resuming execution (currently continue, step, next,
247 return, jump, quit and their abbreviations) terminates the command list (as if
248 that command was immediately followed by end). This is because any time you
249 resume execution (even with a simple next or step), you may encounter another
250 breakpoint--which could have its own command list, leading to ambiguities about
251 which list to execute.
253 If you use the 'silent' command in the command list, the usual message about
254 stopping at a breakpoint is not printed. This may be desirable for breakpoints
255 that are to print a specific message and then continue. If none of the other
256 commands print anything, you see no sign that the breakpoint was reached.
258 .. versionadded:: 2.5
261 Execute the current line, stop at the first possible occasion (either in a
262 function that is called or on the next line in the current function).
265 Continue execution until the next line in the current function is reached or it
266 returns. (The difference between ``next`` and ``step`` is that ``step`` stops
267 inside a called function, while ``next`` executes called functions at (nearly)
268 full speed, only stopping at the next line in the current function.)
271 Continue execution until the line with the the line number greater than the
272 current one is reached or when returning from current frame.
274 .. versionadded:: 2.6
277 Continue execution until the current function returns.
280 Continue execution, only stop when a breakpoint is encountered.
283 Set the next line that will be executed. Only available in the bottom-most
284 frame. This lets you jump back and execute code again, or jump forward to skip
285 code that you don't want to run.
287 It should be noted that not all jumps are allowed --- for instance it is not
288 possible to jump into the middle of a :keyword:`for` loop or out of a
289 :keyword:`finally` clause.
291 l(ist) [*first*\ [, *last*]]
292 List source code for the current file. Without arguments, list 11 lines around
293 the current line or continue the previous listing. With one argument, list 11
294 lines around at that line. With two arguments, list the given range; if the
295 second argument is less than the first, it is interpreted as a count.
298 Print the argument list of the current function.
301 Evaluate the *expression* in the current context and print its value.
305 ``print`` can also be used, but is not a debugger command --- this executes the
306 Python :keyword:`print` statement.
309 Like the ``p`` command, except the value of the expression is pretty-printed
310 using the :mod:`pprint` module.
312 alias [*name* [command]]
313 Creates an alias called *name* that executes *command*. The command must *not*
314 be enclosed in quotes. Replaceable parameters can be indicated by ``%1``,
315 ``%2``, and so on, while ``%*`` is replaced by all the parameters. If no
316 command is given, the current alias for *name* is shown. If no arguments are
317 given, all aliases are listed.
319 Aliases may be nested and can contain anything that can be legally typed at the
320 pdb prompt. Note that internal pdb commands *can* be overridden by aliases.
321 Such a command is then hidden until the alias is removed. Aliasing is
322 recursively applied to the first word of the command line; all other words in
323 the line are left alone.
325 As an example, here are two useful aliases (especially when placed in the
326 :file:`.pdbrc` file)::
328 #Print instance variables (usage "pi classInst")
329 alias pi for k in %1.__dict__.keys(): print "%1.",k,"=",%1.__dict__[k]
330 #Print instance variables in self
334 Deletes the specified alias.
337 Execute the (one-line) *statement* in the context of the current stack frame.
338 The exclamation point can be omitted unless the first word of the statement
339 resembles a debugger command. To set a global variable, you can prefix the
340 assignment command with a ``global`` command on the same line, e.g.::
342 (Pdb) global list_options; list_options = ['-l']
346 Restart the debugged python program. If an argument is supplied, it is split
347 with "shlex" and the result is used as the new sys.argv. History, breakpoints,
348 actions and debugger options are preserved. "restart" is an alias for "run".
350 .. versionadded:: 2.6
353 Quit from the debugger. The program being executed is aborted.