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.

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.

Since pdb is not a valid identifier for pip and easy_install, the package is named pdbpp:

$ pip install pdbpp

-- OR --

$ easy_install pdbpp

pdb is also available via conda:

$ conda install -c conda-forge pdbpp

Alternatively, you can just put pdb.py somewhere inside your PYTHONPATH.

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().

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

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 interative interpreter whose global namespace contains all the names found in the current scope.

track EXPRESSION

Display a graph showing which objects are the value of the expression refers to and are referred by. This command requires the pypy source code to be importable.

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.

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 loose 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 prefering 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]

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

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(object):
    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(object):
    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

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 = Color.turquoise

The color to use for line numbers.

filename_color = Color.yellow

The color to use for file names when printing the stack entries.

current_line_color = "39;49;7"

The SGR parameters for the ANSI escape sequence to highlight the current line. This is set inside 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. The default uses the default foreground (39) and background (49) colors, inversed (7).

use_pygments = True

If pygments is installed and highlight == True, apply syntax highlight to the source code when showing the longlist of a function or when in sticky mode.

bg = 'dark'

Passed directly to the pygments.formatters.TerminalFormatter constructor. Selects the color scheme to use, depending on the background color of your terminal. If you have a light background color, try to set it to 'light'.

colorscheme = None

Passed directly to the pygments.formatters.TerminalFormatter constructor. It expects a dictionary that maps token types to (lightbg, darkbg) color names or None (default: None = use builtin colorscheme).

editor = '${EDITOR:-vi}'

The command to invoke when using the edit command. By default, it uses $EDITOR if set, else vi. The command must support the standard notation COMMAND n filename to open filename at line n. emacs and vi are known to support this.

truncate_long_lines = True

Truncate lines which exceed the terminal width.

exec_if_unfocused = None

Shell command to execute when starting the pdb prompt and the terminal window is not focused. Useful to e.g. play a sound to alert the user that the execution of the program stopped. It requires the wmctrl module.

disable_pytest_capturing = False

Old versions of pytest crash when you execute pdb.set_trace() in a test, but the standard output is captured (i.e., without the -s option, which is the default behavior). When this option is on, the stdout capturing is automatically disabled before showing the interactive prompt.

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.

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 excercise 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 that cases it is fine to do a commmit 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 behaviour 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