Skip to content
Snippets Groups Projects
pdb.rst 20.2 KiB
Newer Older
  • Learn to ignore specific revisions
  • :mod:`pdb` --- The Python Debugger
    ==================================
    
    
    .. module:: pdb
       :synopsis: The Python debugger for interactive interpreters.
    
    
    **Source code:** :source:`Lib/pdb.py`
    
    
    .. index:: single: debugging
    
    
    The module :mod:`pdb` defines an interactive source code debugger for Python
    programs.  It supports setting (conditional) breakpoints and single stepping at
    the source line level, inspection of stack frames, source code listing, and
    evaluation of arbitrary Python code in the context of any stack frame.  It also
    supports post-mortem debugging and can be called under program control.
    
    .. index::
       single: Pdb (class in pdb)
       module: bdb
       module: cmd
    
    
    The debugger is extensible -- it is actually defined as the class :class:`Pdb`.
    
    This is currently undocumented but easily understood by reading the source.  The
    
    extension interface uses the modules :mod:`bdb` and :mod:`cmd`.
    
    
    The debugger's prompt is ``(Pdb)``. Typical usage to run a program under control
    of the debugger is::
    
       >>> import pdb
       >>> import mymodule
       >>> pdb.run('mymodule.test()')
       > <string>(0)?()
       (Pdb) continue
       > <string>(1)?()
       (Pdb) continue
       NameError: 'spam'
       > <string>(1)?()
    
       (Pdb)
    
    .. versionchanged:: 3.3
       Tab-completion via the :mod:`readline` module is available for commands and
       command arguments, e.g. the current global and local names are offered as
    
    :file:`pdb.py` can also be invoked as a script to debug other scripts.  For
    example::
    
    
       python3 -m pdb myscript.py
    
    
    When invoked as a script, pdb will automatically enter post-mortem debugging if
    
    the program being debugged exits abnormally.  After post-mortem debugging (or
    after normal exit of the program), pdb will restart the program.  Automatic
    
    restarting preserves pdb's state (such as breakpoints) and in most cases is more
    useful than quitting the debugger upon program's exit.
    
    
    .. versionadded:: 3.2
       :file:`pdb.py` now accepts a ``-c`` option that executes commands as if given
       in a :file:`.pdbrc` file, see :ref:`debugger-commands`.
    
    
    .. versionadded:: 3.7
       :file:`pdb.py` now accepts a ``-m`` option that execute modules similar to the way
       ``python3 -m`` does. As with a script, the debugger will pause execution just
       before the first line of the module.
    
    
    
    The typical usage to break into the debugger is to insert::
    
    
       import pdb; pdb.set_trace()
    
    
    at the location you want to break into the debugger, and then run the program.
    You can then step through the code following this statement, and continue
    running without the debugger using the :pdbcmd:`continue` command.
    
    .. versionadded:: 3.7
       The built-in :func:`breakpoint()`, when called with defaults, can be used
       instead of ``import pdb; pdb.set_trace()``.
    
    
    The typical usage to inspect a crashed program is::
    
    
       >>> import pdb
       >>> import mymodule
       >>> mymodule.test()
       Traceback (most recent call last):
    
         File "<stdin>", line 1, in <module>
    
         File "./mymodule.py", line 4, in test
           test2()
         File "./mymodule.py", line 3, in test2
    
       NameError: spam
       >>> pdb.pm()
       > ./mymodule.py(3)test2()
    
       (Pdb)
    
    The module defines the following functions; each enters the debugger in a
    slightly different way:
    
    
    .. function:: run(statement, globals=None, locals=None)
    
       Execute the *statement* (given as a string or a code object) under debugger
       control.  The debugger prompt appears before any code is executed; you can
       set breakpoints and type :pdbcmd:`continue`, or you can step through the
       statement using :pdbcmd:`step` or :pdbcmd:`next` (all these commands are
       explained below).  The optional *globals* and *locals* arguments specify the
       environment in which the code is executed; by default the dictionary of the
       module :mod:`__main__` is used.  (See the explanation of the built-in
       :func:`exec` or :func:`eval` functions.)
    
    .. function:: runeval(expression, globals=None, locals=None)
    
       Evaluate the *expression* (given as a string or a code object) under debugger
       control.  When :func:`runeval` returns, it returns the value of the
       expression.  Otherwise this function is similar to :func:`run`.
    
    .. function:: runcall(function, *args, **kwds)
    
       Call the *function* (a function or method object, not a string) with the
       given arguments.  When :func:`runcall` returns, it returns whatever the
       function call returned.  The debugger prompt appears as soon as the function
       is entered.
    
    .. function:: set_trace(*, header=None)
    
       Enter the debugger at the calling stack frame.  This is useful to hard-code
       a breakpoint at a given point in a program, even if the code is not
       otherwise being debugged (e.g. when an assertion fails).  If given,
    
       *header* is printed to the console just before debugging begins.
    
       .. versionchanged:: 3.7
          The keyword-only argument *header*.
    
    .. function:: post_mortem(traceback=None)
    
       Enter post-mortem debugging of the given *traceback* object.  If no
    
    Christian Heimes's avatar
    Christian Heimes committed
       *traceback* is given, it uses the one of the exception that is currently
       being handled (an exception must be being handled if the default is to be
       used).
    
       Enter post-mortem debugging of the traceback found in
       :data:`sys.last_traceback`.
    
    
    
    The ``run*`` functions and :func:`set_trace` are aliases for instantiating the
    
    :class:`Pdb` class and calling the method of the same name.  If you want to
    access further features, you have to do this yourself:
    
    
    .. class:: Pdb(completekey='tab', stdin=None, stdout=None, skip=None, \
    
                   nosigint=False, readrc=True)
    
    
       :class:`Pdb` is the debugger class.
    
       The *completekey*, *stdin* and *stdout* arguments are passed to the
       underlying :class:`cmd.Cmd` class; see the description there.
    
       The *skip* argument, if given, must be an iterable of glob-style module name
       patterns.  The debugger will not step into frames that originate in a module
       that matches one of these patterns. [1]_
    
    
       By default, Pdb sets a handler for the SIGINT signal (which is sent when the
    
       user presses :kbd:`Ctrl-C` on the console) when you give a ``continue`` command.
       This allows you to break into the debugger again by pressing :kbd:`Ctrl-C`.  If you
    
    Terry Jan Reedy's avatar
    Terry Jan Reedy committed
       want Pdb not to touch the SIGINT handler, set *nosigint* to true.
    
       The *readrc* argument defaults to true and controls whether Pdb will load
    
       .pdbrc files from the filesystem.
    
    
       Example call to enable tracing with *skip*::
    
          import pdb; pdb.Pdb(skip=['django.*']).set_trace()
    
    
       .. audit-event:: pdb.Pdb "" pdb.Pdb
    
       .. versionadded:: 3.1
    
          The *skip* argument.
    
    
       .. versionadded:: 3.2
          The *nosigint* argument.  Previously, a SIGINT handler was never set by
          Pdb.
    
    
       .. versionchanged:: 3.6
    
       .. method:: run(statement, globals=None, locals=None)
                   runeval(expression, globals=None, locals=None)
                   runcall(function, *args, **kwds)
    
                   set_trace()
    
          See the documentation for the functions explained above.
    
    
    
    .. _debugger-commands:
    
    Debugger Commands
    
    The commands recognized by the debugger are listed below.  Most commands can be
    abbreviated to one or two letters as indicated; e.g. ``h(elp)`` means that
    either ``h`` or ``help`` can be used to enter the help command (but not ``he``
    or ``hel``, nor ``H`` or ``Help`` or ``HELP``).  Arguments to commands must be
    separated by whitespace (spaces or tabs).  Optional arguments are enclosed in
    square brackets (``[]``) in the command syntax; the square brackets must not be
    typed.  Alternatives in the command syntax are separated by a vertical bar
    (``|``).
    
    
    Entering a blank line repeats the last command entered.  Exception: if the last
    
    command was a :pdbcmd:`list` command, the next 11 lines are listed.
    
    
    Commands that the debugger doesn't recognize are assumed to be Python statements
    and are executed in the context of the program being debugged.  Python
    statements can also be prefixed with an exclamation point (``!``).  This is a
    powerful way to inspect the program being debugged; it is even possible to
    change a variable or call a function.  When an exception occurs in such a
    statement, the exception name is printed but the debugger's state is not
    changed.
    
    
    The debugger supports :ref:`aliases <debugger-aliases>`.  Aliases can have
    parameters which allows one a certain level of adaptability to the context under
    examination.
    
    
    Multiple commands may be entered on a single line, separated by ``;;``.  (A
    single ``;`` is not used as it is the separator for multiple commands in a line
    
    that is passed to the Python parser.)  No intelligence is applied to separating
    
    the commands; the input is split at the first ``;;`` pair, even if it is in the
    
    middle of a quoted string. A workaround for strings with double semicolons
    is to use implicit string concatenation ``';'';'`` or ``";"";"``.
    
    
    .. index::
       pair: .pdbrc; file
       triple: debugger; configuration; file
    
    
    If a file :file:`.pdbrc` exists in the user's home directory or in the current
    
    directory, it is read with ``'utf-8'`` encoding and executed as if it had been
    typed at the debugger prompt.  This is particularly useful for aliases.  If both
    files exist, the one in the home directory is read first and aliases defined there
    can be overridden by the local file.
    
    .. versionchanged:: 3.11
       :file:`.pdbrc` is now read with ``'utf-8'`` encoding. Previously, it was read
       with the system locale encoding.
    
    .. versionchanged:: 3.2
       :file:`.pdbrc` can now contain commands that continue debugging, such as
       :pdbcmd:`continue` or :pdbcmd:`next`.  Previously, these commands had no
       effect.
    
    
       Without argument, print the list of available commands.  With a *command* as
       argument, print help about that command.  ``help pdb`` displays the full
    
       documentation (the docstring of the :mod:`pdb` module).  Since the *command*
       argument must be an identifier, ``help exec`` must be entered to get help on
       the ``!`` command.
    
    
       Print a stack trace, with the most recent frame at the bottom.  An arrow
       indicates the current frame, which determines the context of most commands.
    
    
       Move the current frame *count* (default one) levels down in the stack trace
       (to a newer frame).
    
    .. pdbcommand:: u(p) [count]
    
       Move the current frame *count* (default one) levels up in the stack trace (to
       an older frame).
    
    .. pdbcommand:: b(reak) [([filename:]lineno | function) [, condition]]
    
    
       With a *lineno* argument, set a break there in the current file.  With a
    
       *function* argument, set a break at the first executable statement within
       that function.  The line number may be prefixed with a filename and a colon,
       to specify a breakpoint in another file (probably one that hasn't been loaded
       yet).  The file is searched on :data:`sys.path`.  Note that each breakpoint
       is assigned a number to which all the other breakpoint commands refer.
    
       If a second argument is present, it is an expression which must evaluate to
       true before the breakpoint is honored.
    
       Without argument, list all breaks, including for each breakpoint, the number
       of times that breakpoint has been hit, the current ignore count, and the
       associated condition if any.
    
    .. pdbcommand:: tbreak [([filename:]lineno | function) [, condition]]
    
       Temporary breakpoint, which is removed automatically when it is first hit.
       The arguments are the same as for :pdbcmd:`break`.
    
    
    .. pdbcommand:: cl(ear) [filename:lineno | bpnumber ...]
    
       With a *filename:lineno* argument, clear all the breakpoints at this line.
    
       With a space separated list of breakpoint numbers, clear those breakpoints.
       Without argument, clear all breaks (but first ask confirmation).
    
    
    .. pdbcommand:: disable [bpnumber ...]
    
    
       Disable the breakpoints given as a space separated list of breakpoint
       numbers.  Disabling a breakpoint means it cannot cause the program to stop
       execution, but unlike clearing a breakpoint, it remains in the list of
       breakpoints and can be (re-)enabled.
    
    
    .. pdbcommand:: enable [bpnumber ...]
    
    .. pdbcommand:: ignore bpnumber [count]
    
       Set the ignore count for the given breakpoint number.  If count is omitted,
       the ignore count is set to 0.  A breakpoint becomes active when the ignore
       count is zero.  When non-zero, the count is decremented each time the
       breakpoint is reached and the breakpoint is not disabled and any associated
       condition evaluates to true.
    
    .. pdbcommand:: condition bpnumber [condition]
    
       Set a new *condition* for the breakpoint, an expression which must evaluate
       to true before the breakpoint is honored.  If *condition* is absent, any
       existing condition is removed; i.e., the breakpoint is made unconditional.
    
    .. pdbcommand:: commands [bpnumber]
    
    
       Specify a list of commands for breakpoint number *bpnumber*.  The commands
    
       themselves appear on the following lines.  Type a line containing just
       ``end`` to terminate the commands. An example::
    
    
          (Pdb) commands 1
    
       To remove all commands from a breakpoint, type ``commands`` and follow it
    
       immediately with ``end``; that is, give no commands.
    
       With no *bpnumber* argument, ``commands`` refers to the last breakpoint set.
    
       You can use breakpoint commands to start your program up again.  Simply use
    
       the :pdbcmd:`continue` command, or :pdbcmd:`step`,
       or any other command that resumes execution.
    
       Specifying any command resuming execution
       (currently :pdbcmd:`continue`, :pdbcmd:`step`, :pdbcmd:`next`,
       :pdbcmd:`return`, :pdbcmd:`jump`, :pdbcmd:`quit` and their abbreviations)
    
       terminates the command list (as if
    
       that command was immediately followed by end). This is because any time you
    
       resume execution (even with a simple next or step), you may encounter another
    
       breakpoint—which could have its own command list, leading to ambiguities about
    
       which list to execute.
    
       If you use the 'silent' command in the command list, the usual message about
       stopping at a breakpoint is not printed.  This may be desirable for breakpoints
       that are to print a specific message and then continue.  If none of the other
       commands print anything, you see no sign that the breakpoint was reached.
    
    
       Execute the current line, stop at the first possible occasion (either in a
       function that is called or on the next line in the current function).
    
    
    .. pdbcommand:: n(ext)
    
       Continue execution until the next line in the current function is reached or
       it returns.  (The difference between :pdbcmd:`next` and :pdbcmd:`step` is
       that :pdbcmd:`step` stops inside a called function, while :pdbcmd:`next`
       executes called functions at (nearly) full speed, only stopping at the next
       line in the current function.)
    
    
    .. pdbcommand:: unt(il) [lineno]
    
       Without argument, continue execution until the line with a number greater
       than the current one is reached.
    
       With a line number, continue execution until a line with a number greater or
       equal to that is reached.  In both cases, also stop when the current frame
       returns.
    
       .. versionchanged:: 3.2
          Allow giving an explicit line number.
    
    
       Continue execution until the current function returns.
    
    
       Continue execution, only stop when a breakpoint is encountered.
    
    
       Set the next line that will be executed.  Only available in the bottom-most
    
       frame.  This lets you jump back and execute code again, or jump forward to
       skip code that you don't want to run.
    
       It should be noted that not all jumps are allowed -- for instance it is not
    
       possible to jump into the middle of a :keyword:`for` loop or out of a
       :keyword:`finally` clause.
    
    
    .. pdbcommand:: l(ist) [first[, last]]
    
       List source code for the current file.  Without arguments, list 11 lines
    
    Georg Brandl's avatar
    Georg Brandl committed
       around the current line or continue the previous listing.  With ``.`` as
       argument, list 11 lines around the current line.  With one argument,
    
       list 11 lines around at that line.  With two arguments, list the given range;
       if the second argument is less than the first, it is interpreted as a count.
    
    
       The current line in the current frame is indicated by ``->``.  If an
       exception is being debugged, the line where the exception was originally
       raised or propagated is indicated by ``>>``, if it differs from the current
       line.
    
       .. versionadded:: 3.2
          The ``>>`` marker.
    
    
       List all source code for the current function or frame.  Interesting lines
       are marked as for :pdbcmd:`list`.
    
    
       Print the argument list of the current function.
    
    
    .. pdbcommand:: p expression
    
       Evaluate the *expression* in the current context and print its value.
    
    
       .. note::
    
          ``print()`` can also be used, but is not a debugger command --- this executes the
          Python :func:`print` function.
    
    
    
       Like the :pdbcmd:`p` command, except the value of the expression is
    
       pretty-printed using the :mod:`pprint` module.
    
    .. pdbcommand:: source expression
    
       Try to get source code for the given object and display it.
    
       .. versionadded:: 3.2
    
    
    .. pdbcommand:: display [expression]
    
       Display the value of the expression if it changed, each time execution stops
       in the current frame.
    
       Without expression, list all display expressions for the current frame.
    
       .. versionadded:: 3.2
    
    .. pdbcommand:: undisplay [expression]
    
       Do not display the expression any more in the current frame.  Without
       expression, clear all display expressions for the current frame.
    
       .. versionadded:: 3.2
    
    
    .. pdbcommand:: interact
    
    
       Start an interactive interpreter (using the :mod:`code` module) whose global
    
       namespace contains all the (global and local) names found in the current
       scope.
    
       .. versionadded:: 3.2
    
    
    .. _debugger-aliases:
    
    .. pdbcommand:: alias [name [command]]
    
       Create an alias called *name* that executes *command*.  The command must
       *not* be enclosed in quotes.  Replaceable parameters can be indicated by
       ``%1``, ``%2``, and so on, while ``%*`` is replaced by all the parameters.
       If no command is given, the current alias for *name* is shown. If no
       arguments are given, all aliases are listed.
    
       Aliases may be nested and can contain anything that can be legally typed at
       the pdb prompt.  Note that internal pdb commands *can* be overridden by
       aliases.  Such a command is then hidden until the alias is removed.  Aliasing
       is recursively applied to the first word of the command line; all other words
       in the line are left alone.
    
    
       As an example, here are two useful aliases (especially when placed in the
       :file:`.pdbrc` file)::
    
    
          # Print instance variables (usage "pi classInst")
    
          alias pi for k in %1.__dict__.keys(): print("%1.",k,"=",%1.__dict__[k])
    
          alias ps pi self
    
    
    .. pdbcommand:: unalias name
    
       Delete the specified alias.
    
    .. pdbcommand:: ! statement
    
    
       Execute the (one-line) *statement* in the context of the current stack frame.
       The exclamation point can be omitted unless the first word of the statement
    
       resembles a debugger command.  To set a global variable, you can prefix the
       assignment command with a :keyword:`global` statement on the same line,
       e.g.::
    
    
          (Pdb) global list_options; list_options = ['-l']
          (Pdb)
    
    
    .. pdbcommand:: run [args ...]
                    restart [args ...]
    
       Restart the debugged Python program.  If an argument is supplied, it is split
       with :mod:`shlex` and the result is used as the new :data:`sys.argv`.
       History, breakpoints, actions and debugger options are preserved.
       :pdbcmd:`restart` is an alias for :pdbcmd:`run`.
    
    .. pdbcommand:: q(uit)
    
       Quit from the debugger.  The program being executed is aborted.
    
    .. pdbcommand:: debug code
    
       Enter a recursive debugger that steps through the code
       argument (which is an arbitrary expression or statement to be
       executed in the current environment).
    
    .. pdbcommand:: retval
    
       Print the return value for the last return of a function.
    
    
    .. rubric:: Footnotes
    
    .. [1] Whether a frame is considered to originate in a certain module
           is determined by the ``__name__`` in the frame globals.