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/Doc/library/pdb.rst

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  1
  2.. _debugger:
  3
  4:mod:`pdb` --- The Python Debugger
  5==================================
  6
  7.. module:: pdb
  8   :synopsis: The Python debugger for interactive interpreters.
  9
 10
 11.. index:: single: debugging
 12
 13The module :mod:`pdb` defines an interactive source code debugger for Python
 14programs.  It supports setting (conditional) breakpoints and single stepping at
 15the source line level, inspection of stack frames, source code listing, and
 16evaluation of arbitrary Python code in the context of any stack frame.  It also
 17supports post-mortem debugging and can be called under program control.
 18
 19.. index::
 20   single: Pdb (class in pdb)
 21   module: bdb
 22   module: cmd
 23
 24The debugger is extensible --- it is actually defined as the class :class:`Pdb`.
 25This is currently undocumented but easily understood by reading the source.  The
 26extension interface uses the modules :mod:`bdb` (undocumented) and :mod:`cmd`.
 27
 28The debugger's prompt is ``(Pdb)``. Typical usage to run a program under control
 29of the debugger is::
 30
 31   >>> import pdb
 32   >>> import mymodule
 33   >>> pdb.run('mymodule.test()')
 34   > <string>(0)?()
 35   (Pdb) continue
 36   > <string>(1)?()
 37   (Pdb) continue
 38   NameError: 'spam'
 39   > <string>(1)?()
 40   (Pdb)
 41
 42:file:`pdb.py` can also be invoked as a script to debug other scripts.  For
 43example::
 44
 45   python -m pdb myscript.py
 46
 47When invoked as a script, pdb will automatically enter post-mortem debugging if
 48the program being debugged exits abnormally. After post-mortem debugging (or
 49after normal exit of the program), pdb will restart the program. Automatic
 50restarting preserves pdb's state (such as breakpoints) and in most cases is more
 51useful than quitting the debugger upon program's exit.
 52
 53.. versionadded:: 2.4
 54   Restarting post-mortem behavior added.
 55
 56Typical usage to inspect a crashed program is::
 57
 58   >>> import pdb
 59   >>> import mymodule
 60   >>> mymodule.test()
 61   Traceback (most recent call last):
 62     File "<stdin>", line 1, in ?
 63     File "./mymodule.py", line 4, in test
 64       test2()
 65     File "./mymodule.py", line 3, in test2
 66       print spam
 67   NameError: spam
 68   >>> pdb.pm()
 69   > ./mymodule.py(3)test2()
 70   -> print spam
 71   (Pdb)
 72
 73The module defines the following functions; each enters the debugger in a
 74slightly different way:
 75
 76
 77.. function:: run(statement[, globals[, locals]])
 78
 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
 86   function.)
 87
 88
 89.. function:: runeval(expression[, globals[, locals]])
 90
 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`.
 94
 95
 96.. function:: runcall(function[, argument, ...])
 97
 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.
101
102
103.. function:: set_trace()
104
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).
108
109
110.. function:: post_mortem([traceback])
111
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
115   used).
116
117
118.. function:: pm()
119
120   Enter post-mortem debugging of the traceback found in ``sys.last_traceback``.
121
122
123.. _debugger-commands:
124
125Debugger Commands
126=================
127
128The debugger recognizes the following commands.  Most commands can be
129abbreviated 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
132whitespace (spaces or tabs).  Optional arguments are enclosed in square brackets
133(``[]``) in the command syntax; the square brackets must not be typed.
134Alternatives in the command syntax are separated by a vertical bar (``|``).
135
136Entering a blank line repeats the last command entered.  Exception: if the last
137command was a ``list`` command, the next 11 lines are listed.
138
139Commands that the debugger doesn't recognize are assumed to be Python statements
140and are executed in the context of the program being debugged.  Python
141statements can also be prefixed with an exclamation point (``!``).  This is a
142powerful way to inspect the program being debugged; it is even possible to
143change a variable or call a function.  When an exception occurs in such a
144statement, the exception name is printed but the debugger's state is not
145changed.
146
147Multiple commands may be entered on a single line, separated by ``;;``.  (A
148single ``;`` is not used as it is the separator for multiple commands in a line
149that is passed to the Python parser.) No intelligence is applied to separating
150the commands; the input is split at the first ``;;`` pair, even if it is in the
151middle of a quoted string.
152
153The debugger supports aliases.  Aliases can have parameters which allows one a
154certain level of adaptability to the context under examination.
155
156.. index::
157   pair: .pdbrc; file
158   triple: debugger; configuration; file
159
160If a file :file:`.pdbrc`  exists in the user's home directory or in the current
161directory, it is read in and executed as if it had been typed at the debugger
162prompt. This is particularly useful for aliases.  If both files exist, the one
163in the home directory is read first and aliases defined there can be overridden
164by the local file.
165
166h(elp) [*command*]
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 ``!``
172   command.
173
174w(here)
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.
177
178d(own)
179   Move the current frame one level down in the stack trace (to a newer frame).
180
181u(p)
182   Move the current frame one level up in the stack trace (to an older frame).
183
184b(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.
191
192   If a second argument is present, it is an expression which must evaluate to true
193   before the breakpoint is honored.
194
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
197   condition if any.
198
199tbreak [[*filename*:]\ *lineno* | *function*\ [, *condition*]]
200   Temporary breakpoint, which is removed automatically when it is first hit.  The
201   arguments are the same as break.
202
203cl(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).
206
207disable [*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
211   (re-)enabled.
212
213enable [*bpnumber* [*bpnumber ...*]]
214   Enables the breakpoints specified.
215
216ignore *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
221   evaluates to true.
222
223condition *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.
227
228commands [*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::
232
233      (Pdb) commands 1
234      (com) print some_variable
235      (com) end
236      (Pdb)
237
238   To remove all commands from a breakpoint, type commands and follow it
239   immediately with  end; that is, give no commands.
240
241   With no *bpnumber* argument, commands refers to the last breakpoint set.
242
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.
245
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.
252
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.
257
258   .. versionadded:: 2.5
259
260s(tep)
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).
263
264n(ext)
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.)
269
270unt(il)
271   Continue execution until the line with the line number greater than the
272   current one is reached or when returning from current frame.
273
274   .. versionadded:: 2.6
275
276r(eturn)
277   Continue execution until the current function returns.
278
279c(ont(inue))
280   Continue execution, only stop when a breakpoint is encountered.
281
282j(ump) *lineno*
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.
286
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.
290
291l(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.
296
297a(rgs)
298   Print the argument list of the current function.
299
300p *expression*
301   Evaluate the *expression* in the current context and print its value.
302
303   .. note::
304
305      ``print`` can also be used, but is not a debugger command --- this executes the
306      Python :keyword:`print` statement.
307
308pp *expression*
309   Like the ``p`` command, except the value of the expression is pretty-printed
310   using the :mod:`pprint` module.
311
312alias [*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.
318
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.
324
325   As an example, here are two useful aliases (especially when placed in the
326   :file:`.pdbrc` file)::
327
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
331      alias ps pi self
332
333unalias *name*
334   Deletes the specified alias.
335
336[!]\ *statement*
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.::
341
342      (Pdb) global list_options; list_options = ['-l']
343      (Pdb)
344
345run [*args* ...]
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".
349
350   .. versionadded:: 2.6
351
352q(uit)
353   Quit from the debugger. The program being executed is aborted.