pdb++, a drop-in replacement for pdb

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What is it?

This module is an extension of the pdb_ module of the standard library. It is
meant to be fully compatible with its predecessor, yet it introduces a number
of new features to make your debugging experience as nice as possible.

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pdb++ features include:

colorful TAB completion of Python expressions (through fancycompleter_)
optional syntax highlighting of code listings (through Pygments_)
sticky mode_
several new commands to be used from the interactive (Pdb++) prompt
smart command parsing_ (hint: have you ever typed r or c at the
prompt to print the value of some variable?)
additional convenience functions in the pdb module, to be used from
your program

pdb++ is meant to be a drop-in replacement for pdb. If you find some
unexpected behavior, please report it as a bug.

.. _pdb: http://docs.python.org/library/pdb.html
.. _fancycompleter: https://github.com/bretello/fancycompleter
.. _Pygments: http://pygments.org/

Installation

$ pip install pdbpp

Usage

Note that the module is called pdb.py so that pdb++ will automatically
be used in all places that do import pdb (e.g. pytest --pdb will
give you a pdb++ prompt).

The old pdb module is still available by doing e.g. import pdb; pdb.pdb.set_trace().

New interactive commands

The following are new commands that you can use from the interactive
(Pdb++) prompt.

.. _sticky mode:

sticky [start end]
Toggle sticky mode. When in this mode, every time the current position
changes, the screen is repainted and the whole function shown. Thus, when
doing step-by-step execution you can easily follow the flow of the
execution. If start and end are given, sticky mode is enabled and
only lines within that range (extremes included) will be displayed.

longlist (ll)
List source code for the current function. Different from the normal pdb
list command, longlist displays the whole function. The current
line is marked with ->. In case of post-mortem debugging, the line
which actually raised the exception is marked with >>. If the
highlight config option_ is set and Pygments_ is installed, the source
code is highlighted.

interact
Start an interactive interpreter whose global namespace contains all the
names found in the current scope.

display EXPRESSION
Add an expression to the display list; expressions in this list are
evaluated at each step, and printed every time its value changes.
WARNING: since these expressions are evaluated multiple times, make sure
not to put expressions with side-effects in the display list.

undisplay EXPRESSION:
Remove EXPRESSION from the display list.

source EXPRESSION
Show the source code for the given function/method/class.

edit EXPRESSION
Open the editor in the right position to edit the given
function/method/class. The editor used is specified in a config option_.

hf_unhide, hf_hide, hf_list
Some frames might be marked as "hidden" by e.g. using the @pdb.hideframe_
function decorator. By default, hidden frames are not shown in the stack
trace, and cannot be reached using up and down. You can use
hf_unhide to tell pdb to ignore the hidden status (i.e., to treat hidden
frames as normal ones), and hf_hide to hide them again. hf_list
prints a list of hidden frames.
The config option enable_hidden_frames can be used to disable handling
of hidden frames in general.

Smart command parsing

By default, pdb tries hard to interpret what you enter at the command prompt
as one of its builtin commands. However, this is inconvenient if you want to
just print the value of a local variable which happens to have the same name
as one of the commands. E.g.::

(Pdb) list
  1
  2     def fn():
  3         c = 42
  4         import pdb;pdb.set_trace()
  5  ->     return c
(Pdb) c

In the example above, instead of printing 42 pdb interprets the input as the
command continue, and then you lose your prompt. It's even worse than
that, because it happens even if you type e.g. c.__class__.

pdb++ fixes this unfriendly (from the author's point of view, of course :-))
behavior by always preferring the variable in scope, if it exists. If you really
want to execute the corresponding command, you can prefix it with !!.
Thus, the example above becomes::

(Pdb++) list
  1
  2     def fn():
  3         c = 42
  4         import pdb;pdb.set_trace()
  5  ->     return c
(Pdb++) c
42
(Pdb++) !!c

Note that the "smart" behavior takes place only when there is ambiguity, i.e.
if there exists a variable with the same name as a command: in all other
cases, everything works as usual.

Regarding the list command itself, using list(… is a special case
that gets handled as the Python builtin::

(Pdb++) list([1, 2])
[1, 2]

Additional functions in the `pdb` module

The pdb module that comes with pdb++ includes all the functions and
classes that are in the module from the standard library. If you find any
difference, please report it as a bug.

In addition, there are some new convenience functions that are unique to
pdb++.

pdb.xpm()
eXtended Post Mortem: it is equivalent to
pdb.post_mortem(sys.exc_info()[2]). If used inside an except
clause, it will start a post-mortem pdb prompt from the line that raised the
exception being caught.

pdb.disable()
Disable pdb.set_trace(): any subsequent call to it will be ignored.

pdb.enable()
Re-enable pdb.set_trace(), in case it was disabled by pdb.disable().

.. _@pdb.hideframe:

@pdb.hideframe
A function decorator that tells pdb++ to hide the frame corresponding to the
function. Hidden frames do not show up when using interactive commands such
as up, down or where, unless hf_unhide is invoked.

@pdb.break_on_setattr(attrname, condition=always)
class decorator: break the execution of the program every time the
attribute attrname is set on any instance of the class. condition is
a callable that takes the target object of the setattr and the actual value;
by default, it breaks every time the attribute is set. E.g.::

  @break_on_setattr('bar')
  class Foo:
      pass
  f = Foo()
  f.bar = 42    # the program breaks here

If can be used even after the class has already been created, e.g. if we
want to break when some attribute of a particular object is set::

  class Foo:
      pass
  a = Foo()
  b = Foo()

  def break_if_a(obj, value):
      return obj is a

  break_on_setattr('bar', condition=break_if_a)(Foo)
  b.bar = 10   # no break
  a.bar = 42   # the program breaks here

This can be used after pdb.set_trace() also::

  (Pdb++) import pdb
  (Pdb++) pdb.break_on_setattr('tree_id')(obj.__class__)
  (Pdb++) continue

Configuration and customization

.. _config option:

To customize pdb++, you can put a file named .pdbrc.py in your home
directory. The file must contain a class named Config inheriting from
pdb.DefaultConfig and override the desired values.

The following is a list of the options you can customize, together with their
default value:

prompt = '(Pdb++) '
The prompt to show when in interactive mode.

highlight = True
Highlight line numbers and the current line when showing the longlist of
a function or when in sticky mode.

encoding = 'utf-8'
File encoding. Useful when there are international characters in your string
literals or comments.

sticky_by_default = False
Determine whether pdb++ starts in sticky mode or not.

line_number_color = pdb.Color.turquoise
The color to use for line numbers.
See Notes on color options_.

filename_color = pdb.Color.yellow
The color to use for file names when printing the stack entries.
See Notes on color options_.

current_line_color = "39;49;7"
The SGR parameters for the ANSI escape sequence to highlight the current
line. The default uses the default foreground (39) and background
(49) colors, inversed (7).
See Notes on color options_.

editor = None
The command to invoke when using the edit command. By default, it uses $EDITOR
if set, else vim or vi (if found). If only the editor command is specified, the emacs and
vi notation will be used to specify the line number: COMMAND +n filename. It's
otherwise possible to use another syntax by using the placeholders {filename} and
{lineno}. For example with sublime text, specify editor = "subl {filename}:{lineno}".

truncate_long_lines = True
Truncate lines which exceed the terminal width.

enable_hidden_frames = True
Certain frames can be hidden by default.
If enabled, the commands hf_unhide, hf_hide, and hf_list can be
used to control display of them.

show_hidden_frames_count = True
If enable_hidden_frames is True this controls if the number of
hidden frames gets displayed.

def setup(self, pdb): pass
This method is called during the initialization of the Pdb class. Useful
to do complex setup.

show_traceback_on_error = True
Display tracebacks for errors via Pdb.error, that come from
Pdb.default (i.e. the execution of an unrecognized pdb command),
and are not a direct cause of the expression itself (e.g. NameError
with a command like doesnotexist).

With this option disabled only *** exception string gets printed, which
often misses useful context.

show_traceback_on_error_limit = None
This option sets the limit to be used with traceback.format_exception,
when show_traceback_on_error is enabled.

Options relevant for source code highlighting (using Pygments)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

use_pygments = None
By default Pygments_ is used for syntax highlighting of source code when it
can be imported, e.g. when showing the longlist of a function or when in
sticky mode.

pygments_formatter_class = None

You can configure the Pygments formatter to use via the
pygments_formatter_class config setting as a string (dotted path).
This should be one of the following typically:
"pygments.formatters.Terminal256Formatter",
"pygments.formatters.TerminalTrueColorFormatter", or
"pygments.formatters.TerminalFormatter".

The default is to auto-detect the best formatter based on the $TERM
variable, e.g. it uses Terminal256Formatter if the $TERM variable
contains "256color" (e.g. xterm-256color), but also knows about
e.g. "xterm-kitty" to support true colors (TerminalTrueColorFormatter).
TerminalFormatter gets used as a fallback.

pygments_formatter_kwargs = {}

A dictionary of keyword arguments to pass to the formatter's constructor.

The default arguments (updated with this setting) are::

  {
      "style": "default",
      "bg": self.config.bg,
      "colorscheme": self.config.colorscheme,
  }

``style = 'default'``

 The style to use, can be a string or a Pygments Style subclass.
 E.g. ``"solarized-dark"``.
 See http://pygments.org/docs/styles/.

bg = 'dark'

 Selects a different palette for dark/light backgrounds.
 Only used by ``TerminalFormatter``.

colorscheme = None

 A dictionary mapping token types to (lightbg, darkbg) color names or
 ``None`` (default: ``None`` = use builtin colorscheme).
 Only used by ``TerminalFormatter``.

Example::

class Config(pdb.DefaultConfig):
    pygments_formatter_class = "pygments.formatters.TerminalTrueColorFormatter"
    pygments_formatter_kwargs = {"style": "solarized-light"}

.. _SGR parameters: https://en.wikipedia.org/wiki/ANSI_escape_code#SGR_parameters

Notes on color options
^^^^^^^^^^^^^^^^^^^^^^

The values for color options will be used inside of the SGR escape sequence
\e[%sm where \e is the ESC character and %s the given value.
See SGR parameters_.

The following means "reset all colors" (0), set foreground color to 18
(48;5;18), and background to 21: "0;48;5;18;38;5;21".

Constants are available via pdb.Color, e.g. pdb.Color.red
("31;01"), but in general any string can be used here.

Coding guidelines

pdb++ is developed using Test Driven Development, and we try to keep test
coverage high.

As a general rule, every commit should come with its own test. If it's a new
feature, it should come with one or many tests which exercise it. If it's a
bug fix, the test should fail before the fix, and pass after.

The goal is to make refactoring easier in the future: if you wonder why a
certain line of code does something, in principle it should be possible to
comment it out and see which tests fail.

In exceptional cases, the test might be too hard or impossible to write: in
such cases it is fine to do a commit without a test, but you should explain
very precisely in the commit message why it is hard to write a test and how to
reproduce the buggy behavior by hand.

It is fine NOT to write a test in the following cases:

typos, documentation, and in general any non-coding commit
code refactorings which do not add any feature
commits which fix an already failing test
commits to silence warnings
purely cosmetic changes, such as change the color of the output

Dependency	Optional
fancycompleter	false
pygments	false
ipython	true
pexpect	true
pytest	true
pytest-cov	true

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