| \chapter{The Python Debugger} |
| \stmodindex{pdb} |
| \index{debugging} |
| |
| \renewcommand{\indexsubitem}{(in module pdb)} |
| |
| The module \code{pdb} defines an interactive source code debugger for |
| Python programs. It supports setting 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 but |
| easily understood by reading the source. |
| \ttindex{Pdb} |
| \ttindex{bdb} |
| \ttindex{cmd} |
| |
| A primitive windowing version of the debugger also exists --- this is |
| module \code{wdb}, which requires STDWIN (see the chapter on STDWIN |
| specific modules). |
| \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} (given as a string) under debugger |
| control. The debugger prompt appears before any code is executed; you |
| can set breakpoints and type \code{continue}, or you can step through |
| the statement using \code{step} or \code{next} (all these commands are |
| explained below). 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} (given as a 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} (a function or method object, not a string) |
| with the given arguments. When \code{runcall()} returns, it returns |
| whatever the function call returned. 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 a program, even if the code |
| is not otherwise being debugged (e.g. when an assertion fails). |
| \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 of 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}'' must be entered to get 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) [\var{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 (nearly) 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 interpreted as 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. (Note: \code{print} can also be used, but is not a debugger |
| command --- this executes the Python \code{print} statement.) |
| |
| \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} |