Doc/lib/libpdb.tex

   1 \chapter{The Python Debugger}
   2
   3 \declaremodule{standard}{pdb}
   4 \modulesynopsis{The Python debugger for interactive interpreters.}
   5
   6
   7 The module \module{pdb} defines an interactive source code
   8 debugger\index{debugging} for Python programs.  It supports setting
   9 (conditional) breakpoints and single stepping at the source line
  10 level, inspection of stack frames, source code listing, and evaluation
  11 of arbitrary Python code in the context of any stack frame.  It also
  12 supports post-mortem debugging and can be called under program
  13 control.
  14
  15 The debugger is extensible --- it is actually defined as the class
  16 \class{Pdb}\withsubitem{(class in pdb)}{\ttindex{Pdb}}.
  17 This is currently undocumented but easily understood by reading the
  18 source.  The extension interface uses the modules
  19 \module{bdb}\refstmodindex{bdb} (undocumented) and
  20 \refmodule{cmd}\refstmodindex{cmd}.
  21
  22 The debugger's prompt is \samp{(Pdb) }.
  23 Typical usage to run a program under control of the debugger is:
  24
  25 \begin{verbatim}
  26 >>> import pdb
  27 >>> import mymodule
  28 >>> pdb.run('mymodule.test()')
  29 > <string>(0)?()
  30 (Pdb) continue
  31 > <string>(1)?()
  32 (Pdb) continue
  33 NameError: 'spam'
  34 > <string>(1)?()
  35 (Pdb)
  36 \end{verbatim}
  37
  38 \file{pdb.py} can also be invoked as
  39 a script to debug other scripts.  For example:
  40
  41 \begin{verbatim}
  42 python /usr/local/lib/python1.5/pdb.py myscript.py
  43 \end{verbatim}
  44
  45 Typical usage to inspect a crashed program is:
  46
  47 \begin{verbatim}
  48 >>> import pdb
  49 >>> import mymodule
  50 >>> mymodule.test()
  51 Traceback (most recent call last):
  52   File "<stdin>", line 1, in ?
  53   File "./mymodule.py", line 4, in test
  54     test2()
  55   File "./mymodule.py", line 3, in test2
  56     print spam
  57 NameError: spam
  58 >>> pdb.pm()
  59 > ./mymodule.py(3)test2()
  60 -> print spam
  61 (Pdb)
  62 \end{verbatim}
  63
  64 The module defines the following functions; each enters the debugger
  65 in a slightly different way:
  66
  67 \begin{funcdesc}{run}{statement\optional{, globals\optional{, locals}}}
  68 Execute the \var{statement} (given as a string) under debugger
  69 control.  The debugger prompt appears before any code is executed; you
  70 can set breakpoints and type \samp{continue}, or you can step through
  71 the statement using \samp{step} or \samp{next} (all these commands are
  72 explained below).  The optional \var{globals} and \var{locals}
  73 arguments specify the environment in which the code is executed; by
  74 default the dictionary of the module \refmodule[main]{__main__} is
  75 used.  (See the explanation of the \keyword{exec} statement or the
  76 \function{eval()} built-in function.)
  77 \end{funcdesc}
  78
  79 \begin{funcdesc}{runeval}{expression\optional{, globals\optional{, locals}}}
  80 Evaluate the \var{expression} (given as a a string) under debugger
  81 control.  When \function{runeval()} returns, it returns the value of the
  82 expression.  Otherwise this function is similar to
  83 \function{run()}.
  84 \end{funcdesc}
  85
  86 \begin{funcdesc}{runcall}{function\optional{, argument, ...}}
  87 Call the \var{function} (a function or method object, not a string)
  88 with the given arguments.  When \function{runcall()} returns, it returns
  89 whatever the function call returned.  The debugger prompt appears as
  90 soon as the function is entered.
  91 \end{funcdesc}
  92
  93 \begin{funcdesc}{set_trace}{}
  94 Enter the debugger at the calling stack frame.  This is useful to
  95 hard-code a breakpoint at a given point in a program, even if the code
  96 is not otherwise being debugged (e.g. when an assertion fails).
  97 \end{funcdesc}
  98
  99 \begin{funcdesc}{post_mortem}{traceback}
 100 Enter post-mortem debugging of the given \var{traceback} object.
 101 \end{funcdesc}
 102
 103 \begin{funcdesc}{pm}{}
 104 Enter post-mortem debugging of the traceback found in
 105 \code{sys.last_traceback}.
 106 \end{funcdesc}
 107
 108
 109 \section{Debugger Commands \label{debugger-commands}}
 110
 111 The debugger recognizes the following commands.  Most commands can be
 112 abbreviated to one or two letters; e.g. \samp{h(elp)} means that
 113 either \samp{h} or \samp{help} can be used to enter the help
 114 command (but not \samp{he} or \samp{hel}, nor \samp{H} or
 115 \samp{Help} or \samp{HELP}).  Arguments to commands must be
 116 separated by whitespace (spaces or tabs).  Optional arguments are
 117 enclosed in square brackets (\samp{[]}) in the command syntax; the
 118 square brackets must not be typed.  Alternatives in the command syntax
 119 are separated by a vertical bar (\samp{|}).
 120
 121 Entering a blank line repeats the last command entered.  Exception: if
 122 the last command was a \samp{list} command, the next 11 lines are
 123 listed.
 124
 125 Commands that the debugger doesn't recognize are assumed to be Python
 126 statements and are executed in the context of the program being
 127 debugged.  Python statements can also be prefixed with an exclamation
 128 point (\samp{!}).  This is a powerful way to inspect the program
 129 being debugged; it is even possible to change a variable or call a
 130 function.  When an
 131 exception occurs in such a statement, the exception name is printed
 132 but the debugger's state is not changed.
 133
 134 Multiple commands may be entered on a single line, separated by
 135 \samp{;;}.  (A single \samp{;} is not used as it is
 136 the separator for multiple commands in a line that is passed to
 137 the Python parser.)
 138 No intelligence is applied to separating the commands;
 139 the input is split at the first \samp{;;} pair, even if it is in
 140 the middle of a quoted string.
 141
 142 The debugger supports aliases.  Aliases can have parameters which
 143 allows one a certain level of adaptability to the context under
 144 examination.
 145
 146 If a file \file{.pdbrc}
 147 \indexii{.pdbrc}{file}\indexiii{debugger}{configuration}{file}
 148 exists in the user's home directory or in the current directory, it is
 149 read in and executed as if it had been typed at the debugger prompt.
 150 This is particularly useful for aliases.  If both files exist, the one
 151 in the home directory is read first and aliases defined there can be
 152 overridden by the local file.
 153
 154 \begin{description}
 155
 156 \item[h(elp) \optional{\var{command}}]
 157
 158 Without argument, print the list of available commands.  With a
 159 \var{command} as argument, print help about that command.  \samp{help
 160 pdb} displays the full documentation file; if the environment variable
 161 \envvar{PAGER} is defined, the file is piped through that command
 162 instead.  Since the \var{command} argument must be an identifier,
 163 \samp{help exec} must be entered to get help on the \samp{!} command.
 164
 165 \item[w(here)]
 166
 167 Print a stack trace, with the most recent frame at the bottom.  An
 168 arrow indicates the current frame, which determines the context of
 169 most commands.
 170
 171 \item[d(own)]
 172
 173 Move the current frame one level down in the stack trace
 174 (to an newer frame).
 175
 176 \item[u(p)]
 177
 178 Move the current frame one level up in the stack trace
 179 (to a older frame).
 180
 181 \item[b(reak) \optional{\optional{\var{filename}:}\var{lineno}\code{\Large{|}}\var{function}\optional{, \var{condition}}}]
 182
 183 With a \var{lineno} argument, set a break there in the current
 184 file.  With a \var{function} argument, set a break at the first
 185 executable statement within that function.
 186 The line number may be prefixed with a filename and a colon,
 187 to specify a breakpoint in another file (probably one that
 188 hasn't been loaded yet).  The file is searched on \code{sys.path}.
 189 Note that each breakpoint is assigned a number to which all the other
 190 breakpoint commands refer.
 191
 192 If a second argument is present, it is an expression which must
 193 evaluate to true before the breakpoint is honored.
 194
 195 Without argument, list all breaks, including for each breakpoint,
 196 the number of times that breakpoint has been hit, the current
 197 ignore count, and the associated condition if any.
 198
 199 \item[tbreak \optional{\optional{\var{filename}:}\var{lineno}\code{\Large{|}}\var{function}\optional{, \var{condition}}}]
 200
 201 Temporary breakpoint, which is removed automatically when it is
 202 first hit.  The arguments are the same as break.
 203
 204 \item[cl(ear) \optional{\var{bpnumber} \optional{\var{bpnumber ...}}}]
 205
 206 With a space separated list of breakpoint numbers, clear those
 207 breakpoints.  Without argument, clear all breaks (but first
 208 ask confirmation).
 209
 210 \item[disable \optional{\var{bpnumber} \optional{\var{bpnumber ...}}}]
 211
 212 Disables the breakpoints given as a space separated list of
 213 breakpoint numbers.  Disabling a breakpoint means it cannot cause
 214 the program to stop execution, but unlike clearing a breakpoint, it
 215 remains in the list of breakpoints and can be (re-)enabled.
 216
 217 \item[enable \optional{\var{bpnumber} \optional{\var{bpnumber ...}}}]
 218
 219 Enables the breakpoints specified.
 220
 221 \item[ignore \var{bpnumber} \optional{\var{count}}]
 222
 223 Sets the ignore count for the given breakpoint number.  If
 224 count is omitted, the ignore count is set to 0.  A breakpoint
 225 becomes active when the ignore count is zero.  When non-zero,
 226 the count is decremented each time the breakpoint is reached
 227 and the breakpoint is not disabled and any associated condition
 228 evaluates to true.
 229
 230 \item[condition \var{bpnumber} \optional{\var{condition}}]
 231
 232 Condition is an expression which must evaluate to true before
 233 the breakpoint is honored.  If condition is absent, any existing
 234 condition is removed; i.e., the breakpoint is made unconditional.
 235
 236 \item[s(tep)]
 237
 238 Execute the current line, stop at the first possible occasion
 239 (either in a function that is called or on the next line in the
 240 current function).
 241
 242 \item[n(ext)]
 243
 244 Continue execution until the next line in the current function
 245 is reached or it returns.  (The difference between \samp{next} and
 246 \samp{step} is that \samp{step} stops inside a called function, while
 247 \samp{next} executes called functions at (nearly) full speed, only
 248 stopping at the next line in the current function.)
 249
 250 \item[r(eturn)]
 251
 252 Continue execution until the current function returns.
 253
 254 \item[c(ont(inue))]
 255
 256 Continue execution, only stop when a breakpoint is encountered.
 257
 258 \item[l(ist) \optional{\var{first\optional{, last}}}]
 259
 260 List source code for the current file.  Without arguments, list 11
 261 lines around the current line or continue the previous listing.  With
 262 one argument, list 11 lines around at that line.  With two arguments,
 263 list the given range; if the second argument is less than the first,
 264 it is interpreted as a count.
 265
 266 \item[a(rgs)]
 267
 268 Print the argument list of the current function.
 269
 270 \item[p \var{expression}]
 271
 272 Evaluate the \var{expression} in the current context and print its
 273 value.  (Note: \samp{print} can also be used, but is not a debugger
 274 command --- this executes the Python \keyword{print} statement.)
 275
 276 \item[alias \optional{\var{name} \optional{command}}]
 277
 278 Creates an alias called \var{name} that executes \var{command}.  The
 279 command must \emph{not} be enclosed in quotes.  Replaceable parameters
 280 can be indicated by \samp{\%1}, \samp{\%2}, and so on, while \samp{\%*} is
 281 replaced by all the parameters.  If no command is given, the current
 282 alias for \var{name} is shown. If no arguments are given, all
 283 aliases are listed.
 284
 285 Aliases may be nested and can contain anything that can be
 286 legally typed at the pdb prompt.  Note that internal pdb commands
 287 \emph{can} be overridden by aliases.  Such a command is
 288 then hidden until the alias is removed.  Aliasing is recursively
 289 applied to the first word of the command line; all other words
 290 in the line are left alone.
 291
 292 As an example, here are two useful aliases (especially when placed
 293 in the \file{.pdbrc} file):
 294
 295 \begin{verbatim}
 296 #Print instance variables (usage "pi classInst")
 297 alias pi for k in %1.__dict__.keys(): print "%1.",k,"=",%1.__dict__[k]
 298 #Print instance variables in self
 299 alias ps pi self
 300 \end{verbatim}
 301
 302 \item[unalias \var{name}]
 303
 304 Deletes the specified alias.
 305
 306 \item[\optional{!}\var{statement}]
 307
 308 Execute the (one-line) \var{statement} in the context of
 309 the current stack frame.
 310 The exclamation point can be omitted unless the first word
 311 of the statement resembles a debugger command.
 312 To set a global variable, you can prefix the assignment
 313 command with a \samp{global} command on the same line, e.g.:
 314
 315 \begin{verbatim}
 316 (Pdb) global list_options; list_options = ['-l']
 317 (Pdb)
 318 \end{verbatim}
 319
 320 \item[q(uit)]
 321
 322 Quit from the debugger.
 323 The program being executed is aborted.
 324
 325 \end{description}
 326
 327 \section{How It Works}
 328
 329 Some changes were made to the interpreter:
 330
 331 \begin{itemize}
 332 \item \code{sys.settrace(\var{func})} sets the global trace function
 333 \item there can also a local trace function (see later)
 334 \end{itemize}
 335
 336 Trace functions have three arguments: \var{frame}, \var{event}, and
 337 \var{arg}. \var{frame} is the current stack frame.  \var{event} is a
 338 string: \code{'call'}, \code{'line'}, \code{'return'} or
 339 \code{'exception'}.  \var{arg} depends on the event type.
 340
 341 The global trace function is invoked (with \var{event} set to
 342 \code{'call'}) whenever a new local scope is entered; it should return
 343 a reference to the local trace function to be used that scope, or
 344 \code{None} if the scope shouldn't be traced.
 345
 346 The local trace function should return a reference to itself (or to
 347 another function for further tracing in that scope), or \code{None} to
 348 turn off tracing in that scope.
 349
 350 Instance methods are accepted (and very useful!) as trace functions.
 351
 352 The events have the following meaning:
 353
 354 \begin{description}
 355
 356 \item[\code{'call'}]
 357 A function is called (or some other code block entered).  The global
 358 trace function is called; arg is the argument list to the function;
 359 the return value specifies the local trace function.
 360
 361 \item[\code{'line'}]
 362 The interpreter is about to execute a new line of code (sometimes
 363 multiple line events on one line exist).  The local trace function is
 364 called; arg in None; the return value specifies the new local trace
 365 function.
 366
 367 \item[\code{'return'}]
 368 A function (or other code block) is about to return.  The local trace
 369 function is called; arg is the value that will be returned.  The trace
 370 function's return value is ignored.
 371
 372 \item[\code{'exception'}]
 373 An exception has occurred.  The local trace function is called; arg is
 374 a triple (exception, value, traceback); the return value specifies the
 375 new local trace function
 376
 377 \end{description}
 378
 379 Note that as an exception is propagated down the chain of callers, an
 380 \code{'exception'} event is generated at each level.
 381
 382 For more information on code and frame objects, refer to the
 383 \citetitle[../ref/ref.html]{Python Reference Manual}.