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