| \chapter{The Python Debugger} |
| \label{module-pdb} |
| \stmodindex{pdb} |
| \index{debugging} |
| |
| \setindexsubitem{(in module pdb)} |
| |
| The module \code{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 a class |
| \code{Pdb}. This is currently undocumented but easily understood by |
| reading the source. The extension interface uses the (also |
| undocumented) modules \code{bdb} and \code{cmd}. |
| \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} |
| |
| The debugger's prompt is ``\code{(Pdb) }''. |
| Typical usage to run a program under control of the debugger is: |
| |
| \begin{verbatim} |
| >>> import pdb |
| >>> import mymodule |
| >>> pdb.run('mymodule.test()') |
| > <string>(0)?() |
| (Pdb) continue |
| > <string>(1)?() |
| (Pdb) continue |
| NameError: 'spam' |
| > <string>(1)?() |
| (Pdb) |
| \end{verbatim} |
| % |
| \code{pdb.py} can also be invoked as |
| a script to debug other scripts. For example: |
| \code{python /usr/local/lib/python1.4/pdb.py myscript.py} |
| |
| Typical usage to inspect a crashed program is: |
| |
| \begin{verbatim} |
| >>> import pdb |
| >>> import mymodule |
| >>> mymodule.test() |
| Traceback (innermost last): |
| File "<stdin>", line 1, in ? |
| 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) |
| \end{verbatim} |
| % |
| 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} |
| |
| \section{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 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. |
| |
| \begin{description} |
| |
| \item[h(elp) \optional{\var{command}}] |
| |
| Without argument, print the list of available commands. With a |
| \var{command} as argument, print help about that command. \samp{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, |
| \samp{help exec} must be entered to get help on the \samp{!} 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) \optional{\var{lineno}{\Large\code{|}}\var{function}% |
| \optional{, \code{'}\var{condition}\code{'}}}] |
| |
| 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. |
| If a second argument is present, it is a string (included in string |
| quotes!) specifying an expression which must evaluate to true before |
| the breakpoint is honored. |
| |
| \item[cl(ear) \optional{\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) \optional{\var{first\optional{, 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[\optional{!}\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} |
| |
| \section{How It Works} |
| |
| Some changes were made to the interpreter: |
| |
| \begin{itemize} |
| \item \code{sys.settrace(\var{func})} sets the global trace function |
| \item there can also a local trace function (see later) |
| \end{itemize} |
| |
| Trace functions have three arguments: (\var{frame}, \var{event}, \var{arg}) |
| |
| \begin{description} |
| |
| \item[\var{frame}] is the current stack frame |
| |
| \item[\var{event}] is a string: \code{'call'}, \code{'line'}, \code{'return'} |
| or \code{'exception'} |
| |
| \item[\var{arg}] is dependent on the event type |
| |
| \end{description} |
| |
| The global trace function is invoked (with \var{event} set to |
| \code{'call'}) whenever a new local scope is entered; it should return |
| a reference to the local trace function to be used that scope, or |
| \code{None} if the scope shouldn't be traced. |
| |
| The local trace function should return a reference to itself (or to |
| another function for further tracing in that scope), or \code{None} to |
| turn off tracing in that scope. |
| |
| Instance methods are accepted (and very useful!) as trace functions. |
| |
| The events have the following meaning: |
| |
| \begin{description} |
| |
| \item[\code{'call'}] |
| A function is called (or some other code block entered). The global |
| trace function is called; arg is the argument list to the function; |
| the return value specifies the local trace function. |
| |
| \item[\code{'line'}] |
| The interpreter is about to execute a new line of code (sometimes |
| multiple line events on one line exist). The local trace function is |
| called; arg in None; the return value specifies the new local trace |
| function. |
| |
| \item[\code{'return'}] |
| A function (or other code block) is about to return. The local trace |
| function is called; arg is the value that will be returned. The trace |
| function's return value is ignored. |
| |
| \item[\code{'exception'}] |
| An exception has occurred. The local trace function is called; arg is |
| a triple (exception, value, traceback); the return value specifies the |
| new local trace function |
| |
| \end{description} |
| |
| Note that as an exception is propagated down the chain of callers, an |
| \code{'exception'} event is generated at each level. |
| |
| Stack frame objects have the following read-only attributes: |
| |
| \begin{description} |
| \item[f_code] the code object being executed |
| \item[f_lineno] the current line number (\code{-1} for \code{'call'} events) |
| \item[f_back] the stack frame of the caller, or None |
| \item[f_locals] dictionary containing local name bindings |
| \item[f_globals] dictionary containing global name bindings |
| \end{description} |
| |
| Code objects have the following read-only attributes: |
| |
| \begin{description} |
| \item[co_code] the code string |
| \item[co_names] the list of names used by the code |
| \item[co_consts] the list of (literal) constants used by the code |
| \item[co_filename] the filename from which the code was compiled |
| \end{description} |