"pdb" — The Python Debugger
***************************

**Source code:** Lib/pdb.py

======================================================================

The module "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.

The debugger is extensible – it is actually defined as the class
"Pdb". This is currently undocumented but easily understood by reading
the source.  The extension interface uses the modules "bdb" and "cmd".

See also:

  Module "faulthandler"
     Used to dump Python tracebacks explicitly, on a fault, after a
     timeout, or on a user signal.

  Module "traceback"
     Standard interface to extract, format and print stack traces of
     Python programs.

The typical usage to break into the debugger is to insert:

   import pdb; pdb.set_trace()

Or:

   breakpoint()

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 "continue"
command.

New in version 3.7: The built-in "breakpoint()", when called with
defaults, can be used instead of "import pdb; pdb.set_trace()".

   def double(x):
      breakpoint()
      return x * 2
   val = 3
   print(f"{val} * 2 is {double(val)}")

The debugger’s prompt is "(Pdb)", which is the indicator that you are
in debug mode:

   > ...(3)double()
   -> return x * 2
   (Pdb) p x
   3
   (Pdb) continue
   3 * 2 is 6

Changed in version 3.3: Tab-completion via the "readline" module is
available for commands and command arguments, e.g. the current global
and local names are offered as arguments of the "p" command.

You can also invoke "pdb" from the command line to debug other
scripts.  For example:

   python -m pdb myscript.py

When invoked as a module, 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.

New in version 3.2: "-c" option is introduced to execute commands as
if given in a ".pdbrc" file, see Debugger Commands.

New in version 3.7: "-m" option is introduced to execute modules
similar to the way "python -m" does. As with a script, the debugger
will pause execution just before the first line of the module.

Typical usage to execute a statement under control of the debugger is:

   >>> import pdb
   >>> def f(x):
   ...     print(1 / x)
   >>> pdb.run("f(2)")
   > <string>(1)<module>()
   (Pdb) continue
   0.5
   >>>

The typical usage to inspect a crashed program is:

   >>> import pdb
   >>> def f(x):
   ...     print(1 / x)
   ...
   >>> f(0)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "<stdin>", line 2, in f
   ZeroDivisionError: division by zero
   >>> pdb.pm()
   > <stdin>(2)f()
   (Pdb) p x
   0
   (Pdb)

The module defines the following functions; each enters the debugger
in a slightly different way:

pdb.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 "continue", or you can
   step through the statement using "step" or "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 "__main__" is used.  (See the
   explanation of the built-in "exec()" or "eval()" functions.)

pdb.runeval(expression, globals=None, locals=None)

   Evaluate the *expression* (given as a string or a code object)
   under debugger control.  When "runeval()" returns, it returns the
   value of the *expression*.  Otherwise this function is similar to
   "run()".

pdb.runcall(function, *args, **kwds)

   Call the *function* (a function or method object, not a string)
   with the given arguments.  When "runcall()" returns, it returns
   whatever the function call returned.  The debugger prompt appears
   as soon as the function is entered.

pdb.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.

   Changed in version 3.7: The keyword-only argument *header*.

pdb.post_mortem(traceback=None)

   Enter post-mortem debugging of the given *traceback* object.  If no
   *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).

pdb.pm()

   Enter post-mortem debugging of the traceback found in
   "sys.last_traceback".

The "run*" functions and "set_trace()" are aliases for instantiating
the "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.Pdb(completekey='tab', stdin=None, stdout=None, skip=None, nosigint=False, readrc=True)

   "Pdb" is the debugger class.

   The *completekey*, *stdin* and *stdout* arguments are passed to the
   underlying "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 "Ctrl-C" on the console) when you give a
   "continue" command. This allows you to break into the debugger
   again by pressing "Ctrl-C".  If you 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()

   Raises an auditing event "pdb.Pdb" with no arguments.

   New in version 3.1: The *skip* argument.

   New in version 3.2: The *nosigint* argument.  Previously, a SIGINT
   handler was never set by Pdb.

   Changed in version 3.6: The *readrc* argument.

   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
=================

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 "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 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 "";"";"".

To set a temporary global variable, use a *convenience variable*. A
*convenience variable* is a variable whose name starts with "$".  For
example, "$foo = 1" sets a global variable "$foo" which you can use in
the debugger session.  The *convenience variables* are cleared when
the program resumes execution so it’s less likely to interfere with
your program compared to using normal variables like "foo = 1".

There are three preset *convenience variables*:

* "$_frame": the current frame you are debugging

* "$_retval": the return value if the frame is returning

* "$_exception": the exception if the frame is raising an exception

New in version 3.12.

If a 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.

Changed in version 3.11: ".pdbrc" is now read with "'utf-8'" encoding.
Previously, it was read with the system locale encoding.

Changed in version 3.2: ".pdbrc" can now contain commands that
continue debugging, such as "continue" or "next".  Previously, these
commands had no effect.

h(elp) [command]

   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 "pdb"
   module).  Since the *command* argument must be an identifier, "help
   exec" must be entered to get help on the "!" command.

w(here)

   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.

d(own) [count]

   Move the current frame *count* (default one) levels down in the
   stack trace (to a newer frame).

u(p) [count]

   Move the current frame *count* (default one) levels up in the stack
   trace (to an older frame).

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 "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.

tbreak [([filename:]lineno | function) [, condition]]

   Temporary breakpoint, which is removed automatically when it is
   first hit. The arguments are the same as for "break".

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).

disable bpnumber [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.

enable bpnumber [bpnumber ...]

   Enable the breakpoints specified.

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.

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.

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
      (com) p some_variable
      (com) end
      (Pdb)

   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 "continue" command, or "step", or any other command
   that resumes execution.

   Specifying any command resuming execution (currently "continue",
   "step", "next", "return", "jump", "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.

s(tep)

   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).

n(ext)

   Continue execution until the next line in the current function is
   reached or it returns.  (The difference between "next" and "step"
   is that "step" stops inside a called function, while "next"
   executes called functions at (nearly) full speed, only stopping at
   the next line in the current function.)

unt(il) [lineno]

   Without argument, continue execution until the line with a number
   greater than the current one is reached.

   With *lineno*, continue execution until a line with a number
   greater or equal to *lineno* is reached.  In both cases, also stop
   when the current frame returns.

   Changed in version 3.2: Allow giving an explicit line number.

r(eturn)

   Continue execution until the current function returns.

c(ont(inue))

   Continue execution, only stop when a breakpoint is encountered.

j(ump) lineno

   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 "for" loop or out of a
   "finally" clause.

l(ist) [first[, last]]

   List source code for the current file.  Without arguments, list 11
   lines 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.

   New in version 3.2: The ">>" marker.

ll | longlist

   List all source code for the current function or frame.
   Interesting lines are marked as for "list".

   New in version 3.2.

a(rgs)

   Print the arguments of the current function and their current
   values.

p expression

   Evaluate *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 "print()" function.

pp expression

   Like the "p" command, except the value of *expression* is pretty-
   printed using the "pprint" module.

whatis expression

   Print the type of *expression*.

source expression

   Try to get source code of *expression* and display it.

   New in version 3.2.

display [expression]

   Display the value of *expression* if it changed, each time
   execution stops in the current frame.

   Without *expression*, list all display expressions for the current
   frame.

   Note:

     Display evaluates *expression* and compares to the result of the
     previous evaluation of *expression*, so when the result is
     mutable, display may not be able to pick up the changes.

   Example:

      lst = []
      breakpoint()
      pass
      lst.append(1)
      print(lst)

   Display won’t realize "lst" has been changed because the result of
   evaluation is modified in place by "lst.append(1)" before being
   compared:

      > example.py(3)<module>()
      -> pass
      (Pdb) display lst
      display lst: []
      (Pdb) n
      > example.py(4)<module>()
      -> lst.append(1)
      (Pdb) n
      > example.py(5)<module>()
      -> print(lst)
      (Pdb)

   You can do some tricks with copy mechanism to make it work:

      > example.py(3)<module>()
      -> pass
      (Pdb) display lst[:]
      display lst[:]: []
      (Pdb) n
      > example.py(4)<module>()
      -> lst.append(1)
      (Pdb) n
      > example.py(5)<module>()
      -> print(lst)
      display lst[:]: [1]  [old: []]
      (Pdb)

   New in version 3.2.

undisplay [expression]

   Do not display *expression* anymore in the current frame.  Without
   *expression*, clear all display expressions for the current frame.

   New in version 3.2.

interact

   Start an interactive interpreter (using the "code" module) whose
   global namespace contains all the (global and local) names found in
   the current scope.

   New in version 3.2.

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 *command* is omitted, 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 ".pdbrc" file):

      # Print instance variables (usage "pi classInst")
      alias pi for k in %1.__dict__.keys(): print(f"%1.{k} = {%1.__dict__[k]}")
      # Print instance variables in self
      alias ps pi self

unalias name

   Delete the specified alias *name*.

! 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, e.g.:

      (Pdb) ! n=42
      (Pdb)

   To set a global variable, you can prefix the assignment command
   with a "global" statement on the same line, e.g.:

      (Pdb) global list_options; list_options = ['-l']
      (Pdb)

run [args ...]
restart [args ...]

   Restart the debugged Python program.  If *args* is supplied, it is
   split with "shlex" and the result is used as the new "sys.argv".
   History, breakpoints, actions and debugger options are preserved.
   "restart" is an alias for "run".

q(uit)

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

debug code

   Enter a recursive debugger that steps through *code* (which is an
   arbitrary expression or statement to be executed in the current
   environment).

retval

   Print the return value for the last return of the current function.

-[ Footnotes ]-

[1] Whether a frame is considered to originate in a certain module is
    determined by the "__name__" in the frame globals.
