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 

    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.

    18 

    19 .. index::

    20    single: Pdb (class in pdb)

    21    module: bdb

    22    module: cmd

    23 

    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`.

    27 

    28 The debugger's prompt is ``(Pdb)``. Typical usage to run a program under control

    29 of 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

    43 example::

    44 

    45    python -m pdb myscript.py

    46 

    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.

    52 

    53 .. versionadded:: 2.4

    54    Restarting post-mortem behavior added.

    55 

    56 Typical 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 

    73 The module defines the following functions; each enters the debugger in a

    74 slightly 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 

   125 Debugger Commands

   126 =================

   127 

   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 (``|``).

   135 

   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.

   138 

   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

   145 changed.

   146 

   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.

   152 

   153 The debugger supports aliases.  Aliases can have parameters which allows one a

   154 certain level of adaptability to the context under examination.

   155 

   156 .. index::

   157    pair: .pdbrc; file

   158    triple: debugger; configuration; file

   159 

   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

   164 by the local file.

   165 

   166 h(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 

   174 w(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 

   178 d(own)

   179    Move the current frame one level down in the stack trace (to a newer frame).

   180 

   181 u(p)

   182    Move the current frame one level up in the stack trace (to an older frame).

   183 

   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.

   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 

   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.

   202 

   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).

   206 

   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

   211    (re-)enabled.

   212 

   213 enable [*bpnumber* [*bpnumber ...*]]

   214    Enables the breakpoints specified.

   215 

   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

   221    evaluates to true.

   222 

   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.

   227 

   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::

   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 

   260 s(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 

   264 n(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 

   270 unt(il)

   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.

   273 

   274    .. versionadded:: 2.6

   275 

   276 r(eturn)

   277    Continue execution until the current function returns.

   278 

   279 c(ont(inue))

   280    Continue execution, only stop when a breakpoint is encountered.

   281 

   282 j(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 

   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.

   296 

   297 a(rgs)

   298    Print the argument list of the current function.

   299 

   300 p *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 

   308 pp *expression*

   309    Like the ``p`` command, except the value of the expression is pretty-printed

   310    using the :mod:`pprint` module.

   311 

   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.

   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 

   333 unalias *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 

   345 run [*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 

   352 q(uit)

   353    Quit from the debugger. The program being executed is aborted.

   354 

   355 

   356 .. _debugger-hooks:

   357 

   358 How It Works

   359 ============

   360 

   361 Some changes were made to the interpreter:

   362 

   363 * ``sys.settrace(func)`` sets the global trace function

   364 

   365 * there can also a local trace function (see later)

   366 

   367 Trace functions have three arguments: *frame*, *event*, and *arg*. *frame* is

   368 the current stack frame.  *event* is a string: ``'call'``, ``'line'``,

   369 ``'return'``, ``'exception'``, ``'c_call'``, ``'c_return'``, or

   370 ``'c_exception'``. *arg* depends on the event type.

   371 

   372 The global trace function is invoked (with *event* set to ``'call'``) whenever a

   373 new local scope is entered; it should return a reference to the local trace

   374 function to be used that scope, or ``None`` if the scope shouldn't be traced.

   375 

   376 The local trace function should return a reference to itself (or to another

   377 function for further tracing in that scope), or ``None`` to turn off tracing in

   378 that scope.

   379 

   380 Instance methods are accepted (and very useful!) as trace functions.

   381 

   382 The events have the following meaning:

   383 

   384 ``'call'``

   385    A function is called (or some other code block entered).  The global trace

   386    function is called; *arg* is ``None``; the return value specifies the local

   387    trace function.

   388 

   389 ``'line'``

   390    The interpreter is about to execute a new line of code (sometimes multiple line

   391    events on one line exist).  The local trace function is called; *arg* is

   392    ``None``; the return value specifies the new local trace function.

   393 

   394 ``'return'``

   395    A function (or other code block) is about to return.  The local trace function

   396    is called; *arg* is the value that will be returned.  The trace function's

   397    return value is ignored.

   398 

   399 ``'exception'``

   400    An exception has occurred.  The local trace function is called; *arg* is a

   401    triple ``(exception, value, traceback)``; the return value specifies the new

   402    local trace function.

   403 

   404 ``'c_call'``

   405    A C function is about to be called.  This may be an extension function or a

   406    builtin.  *arg* is the C function object.

   407 

   408 ``'c_return'``

   409    A C function has returned. *arg* is ``None``.

   410 

   411 ``'c_exception'``

   412    A C function has thrown an exception.  *arg* is ``None``.

   413 

   414 Note that as an exception is propagated down the chain of callers, an

   415 ``'exception'`` event is generated at each level.

   416 

   417 For more information on code and frame objects, refer to :ref:`types`.

   418