3 :mod:`pdb` --- The Python Debugger
4 ==================================
7 :synopsis: The Python debugger for interactive interpreters.
10 .. index:: single: debugging
12 The module :mod:`pdb` defines an interactive source code debugger for Python
13 programs. It supports setting (conditional) breakpoints and single stepping at
14 the source line level, inspection of stack frames, source code listing, and
15 evaluation of arbitrary Python code in the context of any stack frame. It also
16 supports post-mortem debugging and can be called under program control.
19 single: Pdb (class in pdb)
23 The debugger is extensible --- it is actually defined as the class :class:`Pdb`.
24 This is currently undocumented but easily understood by reading the source. The
25 extension interface uses the modules :mod:`bdb` (undocumented) and :mod:`cmd`.
27 The debugger's prompt is ``(Pdb)``. Typical usage to run a program under control
32 >>> pdb.run('mymodule.test()')
41 :file:`pdb.py` can also be invoked as a script to debug other scripts. For
44 python -m pdb myscript.py
46 When invoked as a script, pdb will automatically enter post-mortem debugging if
47 the program being debugged exits abnormally. After post-mortem debugging (or
48 after normal exit of the program), pdb will restart the program. Automatic
49 restarting preserves pdb's state (such as breakpoints) and in most cases is more
50 useful than quitting the debugger upon program's exit.
53 Restarting post-mortem behavior added.
55 The typical usage to break into the debugger from a running program is to
58 import pdb; pdb.set_trace()
60 at the location you want to break into the debugger. You can then step through
61 the code following this statement, and continue running without the debugger using
64 The typical usage to inspect a crashed program is::
69 Traceback (most recent call last):
70 File "<stdin>", line 1, in ?
71 File "./mymodule.py", line 4, in test
73 File "./mymodule.py", line 3, in test2
77 > ./mymodule.py(3)test2()
82 The module defines the following functions; each enters the debugger in a
83 slightly different way:
85 .. function:: run(statement[, globals[, locals]])
87 Execute the *statement* (given as a string) under debugger control. The
88 debugger prompt appears before any code is executed; you can set breakpoints and
89 type ``continue``, or you can step through the statement using ``step`` or
90 ``next`` (all these commands are explained below). The optional *globals* and
91 *locals* arguments specify the environment in which the code is executed; by
92 default the dictionary of the module :mod:`__main__` is used. (See the
93 explanation of the :keyword:`exec` statement or the :func:`eval` built-in
97 .. function:: runeval(expression[, globals[, locals]])
99 Evaluate the *expression* (given as a string) under debugger control. When
100 :func:`runeval` returns, it returns the value of the expression. Otherwise this
101 function is similar to :func:`run`.
104 .. function:: runcall(function[, argument, ...])
106 Call the *function* (a function or method object, not a string) with the given
107 arguments. When :func:`runcall` returns, it returns whatever the function call
108 returned. The debugger prompt appears as soon as the function is entered.
111 .. function:: set_trace()
113 Enter the debugger at the calling stack frame. This is useful to hard-code a
114 breakpoint at a given point in a program, even if the code is not otherwise
115 being debugged (e.g. when an assertion fails).
118 .. function:: post_mortem([traceback])
120 Enter post-mortem debugging of the given *traceback* object. If no
121 *traceback* is given, it uses the one of the exception that is currently
122 being handled (an exception must be being handled if the default is to be
128 Enter post-mortem debugging of the traceback found in
129 :data:`sys.last_traceback`.
132 The ``run_*`` functions and :func:`set_trace` are aliases for instantiating the
133 :class:`Pdb` class and calling the method of the same name. If you want to
134 access further features, you have to do this yourself:
136 .. class:: Pdb(completekey='tab', stdin=None, stdout=None, skip=None)
138 :class:`Pdb` is the debugger class.
140 The *completekey*, *stdin* and *stdout* arguments are passed to the
141 underlying :class:`cmd.Cmd` class; see the description there.
143 The *skip* argument, if given, must be an iterable of glob-style module name
144 patterns. The debugger will not step into frames that originate in a module
145 that matches one of these patterns. [1]_
147 Example call to enable tracing with *skip*::
149 import pdb; pdb.Pdb(skip=['django.*']).set_trace()
151 .. versionadded:: 2.7
154 .. method:: run(statement[, globals[, locals]])
155 runeval(expression[, globals[, locals]])
156 runcall(function[, argument, ...])
159 See the documentation for the functions explained above.
162 .. _debugger-commands:
167 The debugger recognizes the following commands. Most commands can be
168 abbreviated to one or two letters; e.g. ``h(elp)`` means that either ``h`` or
169 ``help`` can be used to enter the help command (but not ``he`` or ``hel``, nor
170 ``H`` or ``Help`` or ``HELP``). Arguments to commands must be separated by
171 whitespace (spaces or tabs). Optional arguments are enclosed in square brackets
172 (``[]``) in the command syntax; the square brackets must not be typed.
173 Alternatives in the command syntax are separated by a vertical bar (``|``).
175 Entering a blank line repeats the last command entered. Exception: if the last
176 command was a ``list`` command, the next 11 lines are listed.
178 Commands that the debugger doesn't recognize are assumed to be Python statements
179 and are executed in the context of the program being debugged. Python
180 statements can also be prefixed with an exclamation point (``!``). This is a
181 powerful way to inspect the program being debugged; it is even possible to
182 change a variable or call a function. When an exception occurs in such a
183 statement, the exception name is printed but the debugger's state is not
186 Multiple commands may be entered on a single line, separated by ``;;``. (A
187 single ``;`` is not used as it is the separator for multiple commands in a line
188 that is passed to the Python parser.) No intelligence is applied to separating
189 the commands; the input is split at the first ``;;`` pair, even if it is in the
190 middle of a quoted string.
192 The debugger supports aliases. Aliases can have parameters which allows one a
193 certain level of adaptability to the context under examination.
197 triple: debugger; configuration; file
199 If a file :file:`.pdbrc` exists in the user's home directory or in the current
200 directory, it is read in and executed as if it had been typed at the debugger
201 prompt. This is particularly useful for aliases. If both files exist, the one
202 in the home directory is read first and aliases defined there can be overridden
206 Without argument, print the list of available commands. With a *command* as
207 argument, print help about that command. ``help pdb`` displays the full
208 documentation file; if the environment variable :envvar:`PAGER` is defined, the
209 file is piped through that command instead. Since the *command* argument must
210 be an identifier, ``help exec`` must be entered to get help on the ``!``
214 Print a stack trace, with the most recent frame at the bottom. An arrow
215 indicates the current frame, which determines the context of most commands.
218 Move the current frame one level down in the stack trace (to a newer frame).
221 Move the current frame one level up in the stack trace (to an older frame).
223 b(reak) [[*filename*:]\ *lineno* | *function*\ [, *condition*]]
224 With a *lineno* argument, set a break there in the current file. With a
225 *function* argument, set a break at the first executable statement within that
226 function. The line number may be prefixed with a filename and a colon, to
227 specify a breakpoint in another file (probably one that hasn't been loaded yet).
228 The file is searched on ``sys.path``. Note that each breakpoint is assigned a
229 number to which all the other breakpoint commands refer.
231 If a second argument is present, it is an expression which must evaluate to true
232 before the breakpoint is honored.
234 Without argument, list all breaks, including for each breakpoint, the number of
235 times that breakpoint has been hit, the current ignore count, and the associated
238 tbreak [[*filename*:]\ *lineno* | *function*\ [, *condition*]]
239 Temporary breakpoint, which is removed automatically when it is first hit. The
240 arguments are the same as break.
242 cl(ear) [*bpnumber* [*bpnumber ...*]]
243 With a space separated list of breakpoint numbers, clear those breakpoints.
244 Without argument, clear all breaks (but first ask confirmation).
246 disable [*bpnumber* [*bpnumber ...*]]
247 Disables the breakpoints given as a space separated list of breakpoint numbers.
248 Disabling a breakpoint means it cannot cause the program to stop execution, but
249 unlike clearing a breakpoint, it remains in the list of breakpoints and can be
252 enable [*bpnumber* [*bpnumber ...*]]
253 Enables the breakpoints specified.
255 ignore *bpnumber* [*count*]
256 Sets the ignore count for the given breakpoint number. If count is omitted, the
257 ignore count is set to 0. A breakpoint becomes active when the ignore count is
258 zero. When non-zero, the count is decremented each time the breakpoint is
259 reached and the breakpoint is not disabled and any associated condition
262 condition *bpnumber* [*condition*]
263 Condition is an expression which must evaluate to true before the breakpoint is
264 honored. If condition is absent, any existing condition is removed; i.e., the
265 breakpoint is made unconditional.
267 commands [*bpnumber*]
268 Specify a list of commands for breakpoint number *bpnumber*. The commands
269 themselves appear on the following lines. Type a line containing just 'end' to
270 terminate the commands. An example::
273 (com) print some_variable
277 To remove all commands from a breakpoint, type commands and follow it
278 immediately with end; that is, give no commands.
280 With no *bpnumber* argument, commands refers to the last breakpoint set.
282 You can use breakpoint commands to start your program up again. Simply use the
283 continue command, or step, or any other command that resumes execution.
285 Specifying any command resuming execution (currently continue, step, next,
286 return, jump, quit and their abbreviations) terminates the command list (as if
287 that command was immediately followed by end). This is because any time you
288 resume execution (even with a simple next or step), you may encounter another
289 breakpoint--which could have its own command list, leading to ambiguities about
290 which list to execute.
292 If you use the 'silent' command in the command list, the usual message about
293 stopping at a breakpoint is not printed. This may be desirable for breakpoints
294 that are to print a specific message and then continue. If none of the other
295 commands print anything, you see no sign that the breakpoint was reached.
297 .. versionadded:: 2.5
300 Execute the current line, stop at the first possible occasion (either in a
301 function that is called or on the next line in the current function).
304 Continue execution until the next line in the current function is reached or it
305 returns. (The difference between ``next`` and ``step`` is that ``step`` stops
306 inside a called function, while ``next`` executes called functions at (nearly)
307 full speed, only stopping at the next line in the current function.)
310 Continue execution until the line with the line number greater than the
311 current one is reached or when returning from current frame.
313 .. versionadded:: 2.6
316 Continue execution until the current function returns.
319 Continue execution, only stop when a breakpoint is encountered.
322 Set the next line that will be executed. Only available in the bottom-most
323 frame. This lets you jump back and execute code again, or jump forward to skip
324 code that you don't want to run.
326 It should be noted that not all jumps are allowed --- for instance it is not
327 possible to jump into the middle of a :keyword:`for` loop or out of a
328 :keyword:`finally` clause.
330 l(ist) [*first*\ [, *last*]]
331 List source code for the current file. Without arguments, list 11 lines around
332 the current line or continue the previous listing. With one argument, list 11
333 lines around at that line. With two arguments, list the given range; if the
334 second argument is less than the first, it is interpreted as a count.
337 Print the argument list of the current function.
340 Evaluate the *expression* in the current context and print its value.
344 ``print`` can also be used, but is not a debugger command --- this executes the
345 Python :keyword:`print` statement.
348 Like the ``p`` command, except the value of the expression is pretty-printed
349 using the :mod:`pprint` module.
351 alias [*name* [command]]
352 Creates an alias called *name* that executes *command*. The command must *not*
353 be enclosed in quotes. Replaceable parameters can be indicated by ``%1``,
354 ``%2``, and so on, while ``%*`` is replaced by all the parameters. If no
355 command is given, the current alias for *name* is shown. If no arguments are
356 given, all aliases are listed.
358 Aliases may be nested and can contain anything that can be legally typed at the
359 pdb prompt. Note that internal pdb commands *can* be overridden by aliases.
360 Such a command is then hidden until the alias is removed. Aliasing is
361 recursively applied to the first word of the command line; all other words in
362 the line are left alone.
364 As an example, here are two useful aliases (especially when placed in the
365 :file:`.pdbrc` file)::
367 #Print instance variables (usage "pi classInst")
368 alias pi for k in %1.__dict__.keys(): print "%1.",k,"=",%1.__dict__[k]
369 #Print instance variables in self
373 Deletes the specified alias.
376 Execute the (one-line) *statement* in the context of the current stack frame.
377 The exclamation point can be omitted unless the first word of the statement
378 resembles a debugger command. To set a global variable, you can prefix the
379 assignment command with a ``global`` command on the same line, e.g.::
381 (Pdb) global list_options; list_options = ['-l']
385 Restart the debugged Python program. If an argument is supplied, it is split
386 with "shlex" and the result is used as the new sys.argv. History, breakpoints,
387 actions and debugger options are preserved. "restart" is an alias for "run".
389 .. versionadded:: 2.6
392 Quit from the debugger. The program being executed is aborted.
395 .. rubric:: Footnotes
397 .. [1] Whether a frame is considered to originate in a certain module
398 is determined by the ``__name__`` in the frame globals.