\section{Standard module \sectcode{pdb}}
\stmodindex{pdb}
\index{debugging}

This module defines an interactive source code debugger for Python
programs.  It supports 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 a class
\code{Pdb}.  The extension interface uses the (also undocumented)
modules \code{bdb} and \code{cmd}; it is currently undocumented.
\ttindex{Pdb}
\ttindex{bdb}
\ttindex{cmd}

A primitive windowing version of the debugger also exists --- this is
module \code{wdb}, which requires STDWIN.
\index{stdwin}
\ttindex{wdb}

Typical usage to run a program under control of the debugger is:

\begin{verbatim}
>>> import pdb
>>> import mymodule
>>> pdb.run('mymodule.test()')
(Pdb)
\end{verbatim}

Typical usage to inspect a crashed program is:

\begin{verbatim}
>>> import pdb
>>> import mymodule
>>> mymodule.test()
(crashes with a stack trace)
>>> pdb.pm()
(Pdb)
\end{verbatim}

The debugger's prompt is ``\code{(Pdb) }''.

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

\begin{funcdesc}{run}{statement\optional{\, globals\optional{\, locals}}}
Execute the \var{statement} (which should be a string) under debugger
control.  The debugger prompt appears before any code is executed; you
can set breakpoint and type \code{continue}, or you can step through
the statement using \code{step} or \code{next}.  The optional
\var{globals} and \var{locals} arguments specify the environment in
which the code is executed; by default the dictionary of the module
\code{__main__} is used.  (See the explanation of the \code{exec}
statement or the \code{eval()} built-in function.)
\end{funcdesc}

\begin{funcdesc}{runeval}{expression\optional{\, globals\optional{\, locals}}}
Evaluate the \var{expression} (which should be a string) under
debugger control.  When \code{runeval()} returns, it returns the value
of the expression.  Otherwise this function is similar to
\code{run()}.
\end{funcdesc}

\begin{funcdesc}{runcall}{function\optional{\, argument\, ...}}
Call the \var{function} (which should be a callable Python object, not
a string) with the given arguments.  When \code{runcall()} returns, it
returns the return value of the function call.  The debugger prompt
appears as soon as the function is entered.
\end{funcdesc}

\begin{funcdesc}{set_trace}{}
Enter the debugger at the calling stack frame.  This is useful to
hard-code a breakpoint at a given point in code, even if the code is
not otherwise being debugged.
\end{funcdesc}

\begin{funcdesc}{post_mortem}{traceback}
Enter post-mortem debugging of the given \var{traceback} object.
\end{funcdesc}

\begin{funcdesc}{pm}{}
Enter post-mortem debugging based on the traceback found in
\code{sys.last_traceback}.
\end{funcdesc}

\subsection{Debugger Commands}

The debugger recognizes the following commands.  Most commands can be
abbreviated to one or two letters; e.g. ``\code{h(elp)}'' means that
either ``\code{h}'' or ``\code{help}'' can be used to enter the help
command (but not ``\code{he}'' or ``\code{hel}'', nor ``\code{H}'' or
``\code{Help} or ``\code{HELP}'').  Arguments to commands must be
separated by whitespace (spaces or tabs).  Optional arguments are
enclosed in square brackets (``\code{[]}'')in the command syntax; the
square brackets must not be typed.  Alternatives in the command syntax
are separated by a vertical bar (``\code{|}'').

Entering a blank line repeats the last command entered.  Exception: if
the last command was a ``\code{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 (``\code{!}'').  This is a powerful way to inspect the program
being debugged; it is even possible to change variables.  When an
exception occurs in such a statement, the exception name is printed
but the debugger's state is not changed.

\begin{description}

\item[{h(elp) [\var{command}]}]

Without argument, print the list of available commands.
With a \var{command} as argument, print help about that command.
``\code{help pdb}'' displays the full documentation file; if the
environment variable \code{PAGER} is defined, the file is piped
through that command instead.  Since the var{command} argument must be
an identifier, ``\code{help exec}'' gives help on the ``\code{!}''
command.

\item[{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.

\item[{d(own)}]

Move the current frame one level down in the stack trace
(to an older frame).

\item[{u(p)}]

Move the current frame one level up in the stack trace
(to a newer frame).

\item[{b(reak) [\var{lineno} \code{|} \var{function}]}]

With a \var{lineno} argument, set a break there in the current
file.  With a \var{function} argument, set a break at the entry of
that function.  Without argument, list all breaks.

\item[{cl(ear) [lineno]}]

With a \var{lineno} argument, clear that break in the current file.
Without argument, clear all breaks (but first ask confirmation).

\item[{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).

\item[{n(ext)}]

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

\item[{r(eturn)}]

Continue execution until the current function returns.

\item[{c(ont(inue))}]

Continue execution, only stop when a breakpoint is encountered.

\item[{l(ist) [\var{first} [, \var{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 a count.

\item[{a(rgs)}]

Print the argument list of the current function.

\item[{p \var{expression}}]

Evaluate the \var{expression} in the current context and print its
value.

\item[{[!] \var{statement}}]

Execute the (one-line) \var{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 ``\code{global}'' command on the same line, e.g.:
\begin{verbatim}
(Pdb) global list_options; list_options = ['-l']
(Pdb)
\end{verbatim}

\item[{q(uit)}]

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

\end{description}