\chapter{The Python Debugger \label{debugger}} \declaremodule{standard}{pdb}\modulesynopsis{The Python debugger for interactive interpreters.}  The module \module{pdb} defines an interactive source codedebugger\index{debugging} for Python programs.  It supports setting(conditional) breakpoints and single stepping at the source linelevel, inspection of stack frames, source code listing, and evaluationof arbitrary Python code in the context of any stack frame.  It alsosupports post-mortem debugging and can be called under programcontrol. The debugger is extensible --- it is actually defined as the class\class{Pdb}\withsubitem{(class in pdb)}{\ttindex{Pdb}}.This is currently undocumented but easily understood by reading thesource.  The extension interface uses the modules\module{bdb}\refstmodindex{bdb} (undocumented) and\refmodule{cmd}\refstmodindex{cmd}. The debugger's prompt is \samp{(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) continueNameError: 'spam'> <string>(1)?()(Pdb) \end{verbatim} \file{pdb.py} can also be invoked asa script to debug other scripts.  For example: \begin{verbatim}python -m pdb myscript.py\end{verbatim} When invoked as a script, pdb will automatically enter post-mortem debuggingif 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 mostcases is more useful than quitting the debugger upon program's exit.\versionadded[Restarting post-mortem behavior added]{2.4} Typical usage to inspect a crashed program is: \begin{verbatim}>>> import pdb>>> import mymodule>>> mymodule.test()Traceback (most recent call last):  File "<stdin>", line 1, in ?  File "./mymodule.py", line 4, in test    test2()  File "./mymodule.py", line 3, in test2    print spamNameError: spam>>> pdb.pm()> ./mymodule.py(3)test2()-> print spam(Pdb) \end{verbatim} The module defines the following functions; each enters the debuggerin a slightly different way: \begin{funcdesc}{run}{statement\optional{, globals\optional{, locals}}}Execute the \var{statement} (given as a string) under debuggercontrol.  The debugger prompt appears before any code is executed; youcan set breakpoints and type \samp{continue}, or you can step throughthe statement using \samp{step} or \samp{next} (all these commands areexplained below).  The optional \var{globals} and \var{locals}arguments specify the environment in which the code is executed; bydefault the dictionary of the module \refmodule[main]{__main__} isused.  (See the explanation of the \keyword{exec} statement or the\function{eval()} built-in function.)\end{funcdesc} \begin{funcdesc}{runeval}{expression\optional{, globals\optional{, locals}}}Evaluate the \var{expression} (given as a string) under debuggercontrol.  When \function{runeval()} returns, it returns the value of theexpression.  Otherwise this function is similar to\function{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 \function{runcall()} returns, it returnswhatever the function call returned.  The debugger prompt appears assoon as the function is entered.\end{funcdesc} \begin{funcdesc}{set_trace}{}Enter the debugger at the calling stack frame.  This is useful tohard-code a breakpoint at a given point in a program, even if the codeis 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 \label{debugger-commands}} The debugger recognizes the following commands.  Most commands can beabbreviated to one or two letters; e.g. \samp{h(elp)} means thateither \samp{h} or \samp{help} can be used to enter the helpcommand (but not \samp{he} or \samp{hel}, nor \samp{H} or\samp{Help} or \samp{HELP}).  Arguments to commands must beseparated by whitespace (spaces or tabs).  Optional arguments areenclosed in square brackets (\samp{[]}) in the command syntax; thesquare brackets must not be typed.  Alternatives in the command syntaxare separated by a vertical bar (\samp{|}). Entering a blank line repeats the last command entered.  Exception: ifthe last command was a \samp{list} command, the next 11 lines arelisted. Commands that the debugger doesn't recognize are assumed to be Pythonstatements and are executed in the context of the program beingdebugged.  Python statements can also be prefixed with an exclamationpoint (\samp{!}).  This is a powerful way to inspect the programbeing debugged; it is even possible to change a variable or call afunction.  When anexception occurs in such a statement, the exception name is printedbut the debugger's state is not changed. Multiple commands may be entered on a single line, separated by\samp{;;}.  (A single \samp{;} is not used as it isthe separator for multiple commands in a line that is passed tothe Python parser.)No intelligence is applied to separating the commands;the input is split at the first \samp{;;} pair, even if it is inthe middle of a quoted string. The debugger supports aliases.  Aliases can have parameters whichallows one a certain level of adaptability to the context underexamination. If a file \file{.pdbrc}\indexii{.pdbrc}{file}\indexiii{debugger}{configuration}{file}exists in the user's home directory or in the current directory, it isread in and executed as if it had been typed at the debugger prompt.This is particularly useful for aliases.  If both files exist, the onein the home directory is read first and aliases defined there can beoverridden by the local file. \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{helppdb} displays the full documentation file; if the environment variable\envvar{PAGER} is defined, the file is piped through that commandinstead.  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.  Anarrow indicates the current frame, which determines the context ofmost commands. \item[d(own)] Move the current frame one level down in the stack trace(to a newer frame). \item[u(p)] Move the current frame one level up in the stack trace(to an older frame). \item[b(reak) \optional{\optional{\var{filename}:}\var{lineno}\code{\Large{|}}\var{function}\optional{, \var{condition}}}] With a \var{lineno} argument, set a break there in the currentfile.  With a \var{function} argument, set a break at the firstexecutable 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 thathasn't been loaded yet).  The file is searched on \code{sys.path}.Note that each breakpoint is assigned a number to which all the otherbreakpoint commands refer. If a second argument is present, it is an expression which mustevaluate 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 currentignore count, and the associated condition if any. \item[tbreak \optional{\optional{\var{filename}:}\var{lineno}\code{\Large{|}}\var{function}\optional{, \var{condition}}}] Temporary breakpoint, which is removed automatically when it isfirst hit.  The arguments are the same as break. \item[cl(ear) \optional{\var{bpnumber} \optional{\var{bpnumber ...}}}] With a space separated list of breakpoint numbers, clear thosebreakpoints.  Without argument, clear all breaks (but firstask confirmation). \item[disable \optional{\var{bpnumber} \optional{\var{bpnumber ...}}}] Disables the breakpoints given as a space separated list ofbreakpoint numbers.  Disabling a breakpoint means it cannot causethe program to stop execution, but unlike clearing a breakpoint, itremains in the list of breakpoints and can be (re-)enabled. \item[enable \optional{\var{bpnumber} \optional{\var{bpnumber ...}}}] Enables the breakpoints specified. \item[ignore \var{bpnumber} \optional{\var{count}}] Sets the ignore count for the given breakpoint number.  Ifcount is omitted, the ignore count is set to 0.  A breakpointbecomes active when the ignore count is zero.  When non-zero,the count is decremented each time the breakpoint is reachedand the breakpoint is not disabled and any associated conditionevaluates to true. \item[condition \var{bpnumber} \optional{\var{condition}}] Condition is an expression which must evaluate to true beforethe breakpoint is honored.  If condition is absent, any existingcondition is removed; i.e., the breakpoint is made unconditional. \item[commands \optional{\var{bpnumber}}] Specify a list of commands for breakpoint number \var{bpnumber}.  Thecommands themselves appear on the following lines.  Type a linecontaining just 'end' to terminate the commands. An example: \begin{verbatim}(Pdb) commands 1(com) print some_variable(com) end(Pdb)\end{verbatim} To remove all commands from a breakpoint, type commands andfollow it immediately with  end; that is, give no commands. With no \var{bpnumber} argument, commands refers to the lastbreakpoint set. You can use breakpoint commands to start your program up again.Simply use the continue command, or step, or any othercommand that resumes execution. Specifying any command resuming execution (currently continue,step, next, return, jump, quit and their abbreviations) terminatesthe 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 toambiguities about which list to execute.    If you use the 'silent' command in the command list, theusual message about stopping at a breakpoint is not printed.  This maybe desirable for breakpoints that are to print a specific message andthen continue.  If none of the other commands print anything, yousee no sign that the breakpoint was reached. \versionadded{2.5} \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 thecurrent function). \item[n(ext)] Continue execution until the next line in the current functionis reached or it returns.  (The difference between \samp{next} and\samp{step} is that \samp{step} stops inside a called function, while\samp{next} executes called functions at (nearly) full speed, onlystopping 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[j(ump) \var{lineno}] Set the next line that will be executed.  Only available in thebottom-most frame.  This lets you jump back and execute codeagain, 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 itis not possible to jump into the middle of a \keyword{for} loop or outof a \keyword{finally} clause. \item[l(ist) \optional{\var{first}\optional{, \var{last}}}] List source code for the current file.  Without arguments, list 11lines around the current line or continue the previous listing.  Withone 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 itsvalue.  \note{\samp{print} can also be used, but is not a debuggercommand --- this executes the Python \keyword{print} statement.} \item[pp \var{expression}] Like the \samp{p} command, except the value of the expression ispretty-printed using the \module{pprint} module. \item[alias \optional{\var{name} \optional{command}}] Creates an alias called \var{name} that executes \var{command}.  Thecommand must \emph{not} be enclosed in quotes.  Replaceable parameterscan be indicated by \samp{\%1}, \samp{\%2}, and so on, while \samp{\%*} isreplaced by all the parameters.  If no command is given, the currentalias for \var{name} is shown. If no arguments are given, allaliases are listed. Aliases may be nested and can contain anything that can belegally typed at the pdb prompt.  Note that internal pdb commands\emph{can} be overridden by aliases.  Such a command isthen hidden until the alias is removed.  Aliasing is recursivelyapplied to the first word of the command line; all other wordsin the line are left alone. As an example, here are two useful aliases (especially when placedin the \file{.pdbrc} file): \begin{verbatim}#Print instance variables (usage "pi classInst")alias pi for k in %1.__dict__.keys(): print "%1.",k,"=",%1.__dict__[k]#Print instance variables in selfalias ps pi self\end{verbatim}                \item[unalias \var{name}] Deletes the specified alias. \item[\optional{!}\var{statement}] Execute the (one-line) \var{statement} in the context ofthe current stack frame.The exclamation point can be omitted unless the first wordof the statement resembles a debugger command.To set a global variable, you can prefix the assignmentcommand with a \samp{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 \label{debugger-hooks}} 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}, and\var{arg}. \var{frame} is the current stack frame.  \var{event} is astring: \code{'call'}, \code{'line'}, \code{'return'}, \code{'exception'}, \code{'c_call'}, \code{'c_return'}, or \code{'c_exception'}. \var{arg} depends on the event type. The global trace function is invoked (with \var{event} set to\code{'call'}) whenever a new local scope is entered; it should returna 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 toanother function for further tracing in that scope), or \code{None} toturn 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 globaltrace function is called; \var{arg} is \code{None};the return value specifies the local trace function. \item[\code{'line'}]The interpreter is about to execute a new line of code (sometimesmultiple line events on one line exist).  The local trace function iscalled; \var{arg} is \code{None}; the return value specifies the newlocal trace function. \item[\code{'return'}]A function (or other code block) is about to return.  The local tracefunction is called; \var{arg} is the value that will be returned.  Thetrace function's return value is ignored. \item[\code{'exception'}]An exception has occurred.  The local trace function is called;\var{arg} is a triple \code{(\var{exception}, \var{value},\var{traceback})}; the return value specifies the new local tracefunction. \item[\code{'c_call'}]A C function is about to be called.  This may be an extension functionor a builtin.  \var{arg} is the C function object. \item[\code{'c_return'}]A C function has returned. \var{arg} is \code{None}. \item[\code{'c_exception'}]A C function has thrown an exception.  \var{arg} is \code{None}. \end{description} Note that as an exception is propagated down the chain of callers, an\code{'exception'} event is generated at each level. For more information on code and frame objects, refer to the\citetitle[../ref/ref.html]{Python Reference Manual}.