26.2. "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".

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)

"pdb.py" can also be invoked as a script to debug other scripts.  For
example:

   python -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.

New in version 2.4: Restarting post-mortem behavior added.

The typical usage to break into the debugger from a running program is
to insert

   import pdb; pdb.set_trace()

at the location you want to break into the debugger.  You can then
step through the code following this statement, and continue running
without the debugger using the "c" command.

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
       print spam
   NameError: spam
   >>> pdb.pm()
   > ./mymodule.py(3)test2()
   -> print spam
   (Pdb)

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

pdb.run(statement[, globals[, locals]])

   Execute the *statement* (given as a string) 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 "exec" statement or the "eval()" built-in function.)

pdb.runeval(expression[, globals[, locals]])

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

pdb.runcall(function[, argument, ...])

   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()

   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).

pdb.post_mortem([traceback])

   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)

   "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]

   Example call to enable tracing with *skip*:

      import pdb; pdb.Pdb(skip=['django.*']).set_trace()

   New in version 2.7: The *skip* argument.

   run(statement[, globals[, locals]])
   runeval(expression[, globals[, locals]])
   runcall(function[, argument, ...])
   set_trace()

      See the documentation for the functions explained above.


26.3. Debugger Commands
***********************

The debugger recognizes the following commands.  Most commands can be
abbreviated to one or two letters; 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.

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.

The debugger supports aliases.  Aliases can have parameters which
allows one a certain level of adaptability to the context under
examination.

If a file ".pdbrc"  exists in the user’s home directory or in the
current directory, it is read in 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.

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 file; if the environment variable
   "PAGER" is defined, the file is piped through that command instead.
   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)
   Move the current frame one level down in the stack trace (to a
   newer frame).

u(p)
   Move the current frame one level 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 break.

cl(ear) [*filename:lineno* | *bpnumber* [*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 …*]]
   Disables 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 …*]]
   Enables the breakpoints specified.

ignore *bpnumber* [*count*]
   Sets 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*]
   Condition is 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) print 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.

   New in version 2.5.

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)
   Continue execution until the line with the line number greater than
   the current one is reached or when returning from current frame.

   New in version 2.6.

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 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.

a(rgs)
   Print the argument list of the current function.

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 "print" statement.

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

alias [*name* [command]]
   Creates 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 ".pdbrc" file):

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

unalias *name*
   Deletes the specified alias.

[!]*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 "global"
   command on the same line, e.g.:

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

run [*args* …]
   Restart the debugged Python program. If an argument 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”.

   New in version 2.6.

q(uit)
   Quit from the debugger. The program being executed is aborted.

-[ Footnotes ]-

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