Doc/lib/libpdb.tex

   1 \chapter{The Python Debugger \label{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 -m pdb myscript.py
  43 \end{verbatim}
  44
  45 When invoked as a script, pdb will automatically enter post-mortem debugging
  46 if the program being debugged exits abnormally. After post-mortem debugging
  47 (or after normal exit of the program), pdb will restart the program.
  48 Automatic restarting preserves pdb's state (such as breakpoints) and in most
  49 cases is more useful than quitting the debugger upon program's exit.
  50 \versionadded[Restarting post-mortem behavior added]{2.4}
  51
  52 Typical usage to inspect a crashed program is:
  53
  54 \begin{verbatim}
  55 >>> import pdb
  56 >>> import mymodule
  57 >>> mymodule.test()
  58 Traceback (most recent call last):
  59   File "<stdin>", line 1, in ?
  60   File "./mymodule.py", line 4, in test
  61     test2()
  62   File "./mymodule.py", line 3, in test2
  63     print spam
  64 NameError: spam
  65 >>> pdb.pm()
  66 > ./mymodule.py(3)test2()
  67 -> print spam
  68 (Pdb)
  69 \end{verbatim}
  70
  71 The module defines the following functions; each enters the debugger
  72 in a slightly different way:
  73
  74 \begin{funcdesc}{run}{statement\optional{, globals\optional{, locals}}}
  75 Execute the \var{statement} (given as a string) under debugger
  76 control.  The debugger prompt appears before any code is executed; you
  77 can set breakpoints and type \samp{continue}, or you can step through
  78 the statement using \samp{step} or \samp{next} (all these commands are
  79 explained below).  The optional \var{globals} and \var{locals}
  80 arguments specify the environment in which the code is executed; by
  81 default the dictionary of the module \refmodule[main]{__main__} is
  82 used.  (See the explanation of the \keyword{exec} statement or the
  83 \function{eval()} built-in function.)
  84 \end{funcdesc}
  85
  86 \begin{funcdesc}{runeval}{expression\optional{, globals\optional{, locals}}}
  87 Evaluate the \var{expression} (given as a string) under debugger
  88 control.  When \function{runeval()} returns, it returns the value of the
  89 expression.  Otherwise this function is similar to
  90 \function{run()}.
  91 \end{funcdesc}
  92
  93 \begin{funcdesc}{runcall}{function\optional{, argument, ...}}
  94 Call the \var{function} (a function or method object, not a string)
  95 with the given arguments.  When \function{runcall()} returns, it returns
  96 whatever the function call returned.  The debugger prompt appears as
  97 soon as the function is entered.
  98 \end{funcdesc}
  99
 100 \begin{funcdesc}{set_trace}{}
 101 Enter the debugger at the calling stack frame.  This is useful to
 102 hard-code a breakpoint at a given point in a program, even if the code
 103 is not otherwise being debugged (e.g. when an assertion fails).
 104 \end{funcdesc}
 105
 106 \begin{funcdesc}{post_mortem}{traceback}
 107 Enter post-mortem debugging of the given \var{traceback} object.
 108 \end{funcdesc}
 109
 110 \begin{funcdesc}{pm}{}
 111 Enter post-mortem debugging of the traceback found in
 112 \code{sys.last_traceback}.
 113 \end{funcdesc}
 114
 115
 116 \section{Debugger Commands \label{debugger-commands}}
 117
 118 The debugger recognizes the following commands.  Most commands can be
 119 abbreviated to one or two letters; e.g. \samp{h(elp)} means that
 120 either \samp{h} or \samp{help} can be used to enter the help
 121 command (but not \samp{he} or \samp{hel}, nor \samp{H} or
 122 \samp{Help} or \samp{HELP}).  Arguments to commands must be
 123 separated by whitespace (spaces or tabs).  Optional arguments are
 124 enclosed in square brackets (\samp{[]}) in the command syntax; the
 125 square brackets must not be typed.  Alternatives in the command syntax
 126 are separated by a vertical bar (\samp{|}).
 127
 128 Entering a blank line repeats the last command entered.  Exception: if
 129 the last command was a \samp{list} command, the next 11 lines are
 130 listed.
 131
 132 Commands that the debugger doesn't recognize are assumed to be Python
 133 statements and are executed in the context of the program being
 134 debugged.  Python statements can also be prefixed with an exclamation
 135 point (\samp{!}).  This is a powerful way to inspect the program
 136 being debugged; it is even possible to change a variable or call a
 137 function.  When an
 138 exception occurs in such a statement, the exception name is printed
 139 but the debugger's state is not changed.
 140
 141 Multiple commands may be entered on a single line, separated by
 142 \samp{;;}.  (A single \samp{;} is not used as it is
 143 the separator for multiple commands in a line that is passed to
 144 the Python parser.)
 145 No intelligence is applied to separating the commands;
 146 the input is split at the first \samp{;;} pair, even if it is in
 147 the middle of a quoted string.
 148
 149 The debugger supports aliases.  Aliases can have parameters which
 150 allows one a certain level of adaptability to the context under
 151 examination.
 152
 153 If a file \file{.pdbrc}
 154 \indexii{.pdbrc}{file}\indexiii{debugger}{configuration}{file}
 155 exists in the user's home directory or in the current directory, it is
 156 read in and executed as if it had been typed at the debugger prompt.
 157 This is particularly useful for aliases.  If both files exist, the one
 158 in the home directory is read first and aliases defined there can be
 159 overridden by the local file.
 160
 161 \begin{description}
 162
 163 \item[h(elp) \optional{\var{command}}]
 164
 165 Without argument, print the list of available commands.  With a
 166 \var{command} as argument, print help about that command.  \samp{help
 167 pdb} displays the full documentation file; if the environment variable
 168 \envvar{PAGER} is defined, the file is piped through that command
 169 instead.  Since the \var{command} argument must be an identifier,
 170 \samp{help exec} must be entered to get help on the \samp{!} command.
 171
 172 \item[w(here)]
 173
 174 Print a stack trace, with the most recent frame at the bottom.  An
 175 arrow indicates the current frame, which determines the context of
 176 most commands.
 177
 178 \item[d(own)]
 179
 180 Move the current frame one level down in the stack trace
 181 (to an newer frame).
 182
 183 \item[u(p)]
 184
 185 Move the current frame one level up in the stack trace
 186 (to a older frame).
 187
 188 \item[b(reak) \optional{\optional{\var{filename}:}\var{lineno}\code{\Large{|}}\var{function}\optional{, \var{condition}}}]
 189
 190 With a \var{lineno} argument, set a break there in the current
 191 file.  With a \var{function} argument, set a break at the first
 192 executable statement within that function.
 193 The line number may be prefixed with a filename and a colon,
 194 to specify a breakpoint in another file (probably one that
 195 hasn't been loaded yet).  The file is searched on \code{sys.path}.
 196 Note that each breakpoint is assigned a number to which all the other
 197 breakpoint commands refer.
 198
 199 If a second argument is present, it is an expression which must
 200 evaluate to true before the breakpoint is honored.
 201
 202 Without argument, list all breaks, including for each breakpoint,
 203 the number of times that breakpoint has been hit, the current
 204 ignore count, and the associated condition if any.
 205
 206 \item[tbreak \optional{\optional{\var{filename}:}\var{lineno}\code{\Large{|}}\var{function}\optional{, \var{condition}}}]
 207
 208 Temporary breakpoint, which is removed automatically when it is
 209 first hit.  The arguments are the same as break.
 210
 211 \item[cl(ear) \optional{\var{bpnumber} \optional{\var{bpnumber ...}}}]
 212
 213 With a space separated list of breakpoint numbers, clear those
 214 breakpoints.  Without argument, clear all breaks (but first
 215 ask confirmation).
 216
 217 \item[disable \optional{\var{bpnumber} \optional{\var{bpnumber ...}}}]
 218
 219 Disables the breakpoints given as a space separated list of
 220 breakpoint numbers.  Disabling a breakpoint means it cannot cause
 221 the program to stop execution, but unlike clearing a breakpoint, it
 222 remains in the list of breakpoints and can be (re-)enabled.
 223
 224 \item[enable \optional{\var{bpnumber} \optional{\var{bpnumber ...}}}]
 225
 226 Enables the breakpoints specified.
 227
 228 \item[ignore \var{bpnumber} \optional{\var{count}}]
 229
 230 Sets the ignore count for the given breakpoint number.  If
 231 count is omitted, the ignore count is set to 0.  A breakpoint
 232 becomes active when the ignore count is zero.  When non-zero,
 233 the count is decremented each time the breakpoint is reached
 234 and the breakpoint is not disabled and any associated condition
 235 evaluates to true.
 236
 237 \item[condition \var{bpnumber} \optional{\var{condition}}]
 238
 239 Condition is an expression which must evaluate to true before
 240 the breakpoint is honored.  If condition is absent, any existing
 241 condition is removed; i.e., the breakpoint is made unconditional.
 242
 243 \item[s(tep)]
 244
 245 Execute the current line, stop at the first possible occasion
 246 (either in a function that is called or on the next line in the
 247 current function).
 248
 249 \item[n(ext)]
 250
 251 Continue execution until the next line in the current function
 252 is reached or it returns.  (The difference between \samp{next} and
 253 \samp{step} is that \samp{step} stops inside a called function, while
 254 \samp{next} executes called functions at (nearly) full speed, only
 255 stopping at the next line in the current function.)
 256
 257 \item[r(eturn)]
 258
 259 Continue execution until the current function returns.
 260
 261 \item[c(ont(inue))]
 262
 263 Continue execution, only stop when a breakpoint is encountered.
 264
 265 \item[j(ump) \var{lineno}]
 266
 267 Set the next line that will be executed.  Only available in the
 268 bottom-most frame.  This lets you jump back and execute code
 269 again, or jump forward to skip code that you don't want to run.
 270
 271 It should be noted that not all jumps are allowed --- for instance it
 272 is not possible to jump into the middle of a \keyword{for} loop or out
 273 of a \keyword{finally} clause.
 274
 275 \item[l(ist) \optional{\var{first}\optional{, \var{last}}}]
 276
 277 List source code for the current file.  Without arguments, list 11
 278 lines around the current line or continue the previous listing.  With
 279 one argument, list 11 lines around at that line.  With two arguments,
 280 list the given range; if the second argument is less than the first,
 281 it is interpreted as a count.
 282
 283 \item[a(rgs)]
 284
 285 Print the argument list of the current function.
 286
 287 \item[p \var{expression}]
 288
 289 Evaluate the \var{expression} in the current context and print its
 290 value.  \note{\samp{print} can also be used, but is not a debugger
 291 command --- this executes the Python \keyword{print} statement.}
 292
 293 \item[pp \var{expression}]
 294
 295 Like the \samp{p} command, except the value of the expression is
 296 pretty-printed using the \module{pprint} module.
 297
 298 \item[alias \optional{\var{name} \optional{command}}]
 299
 300 Creates an alias called \var{name} that executes \var{command}.  The
 301 command must \emph{not} be enclosed in quotes.  Replaceable parameters
 302 can be indicated by \samp{\%1}, \samp{\%2}, and so on, while \samp{\%*} is
 303 replaced by all the parameters.  If no command is given, the current
 304 alias for \var{name} is shown. If no arguments are given, all
 305 aliases are listed.
 306
 307 Aliases may be nested and can contain anything that can be
 308 legally typed at the pdb prompt.  Note that internal pdb commands
 309 \emph{can} be overridden by aliases.  Such a command is
 310 then hidden until the alias is removed.  Aliasing is recursively
 311 applied to the first word of the command line; all other words
 312 in the line are left alone.
 313
 314 As an example, here are two useful aliases (especially when placed
 315 in the \file{.pdbrc} file):
 316
 317 \begin{verbatim}
 318 #Print instance variables (usage "pi classInst")
 319 alias pi for k in %1.__dict__.keys(): print "%1.",k,"=",%1.__dict__[k]
 320 #Print instance variables in self
 321 alias ps pi self
 322 \end{verbatim}
 323
 324 \item[unalias \var{name}]
 325
 326 Deletes the specified alias.
 327
 328 \item[\optional{!}\var{statement}]
 329
 330 Execute the (one-line) \var{statement} in the context of
 331 the current stack frame.
 332 The exclamation point can be omitted unless the first word
 333 of the statement resembles a debugger command.
 334 To set a global variable, you can prefix the assignment
 335 command with a \samp{global} command on the same line, e.g.:
 336
 337 \begin{verbatim}
 338 (Pdb) global list_options; list_options = ['-l']
 339 (Pdb)
 340 \end{verbatim}
 341
 342 \item[q(uit)]
 343
 344 Quit from the debugger.
 345 The program being executed is aborted.
 346
 347 \end{description}
 348
 349 \section{How It Works \label{debugger-hooks}}
 350
 351 Some changes were made to the interpreter:
 352
 353 \begin{itemize}
 354 \item \code{sys.settrace(\var{func})} sets the global trace function
 355 \item there can also a local trace function (see later)
 356 \end{itemize}
 357
 358 Trace functions have three arguments: \var{frame}, \var{event}, and
 359 \var{arg}. \var{frame} is the current stack frame.  \var{event} is a
 360 string: \code{'call'}, \code{'line'}, \code{'return'}, \code{'exception'},
 361  \code{'c_call'}, \code{'c_return'}, or \code{'c_exception'}. \var{arg}
 362  depends on the event type.
 363
 364 The global trace function is invoked (with \var{event} set to
 365 \code{'call'}) whenever a new local scope is entered; it should return
 366 a reference to the local trace function to be used that scope, or
 367 \code{None} if the scope shouldn't be traced.
 368
 369 The local trace function should return a reference to itself (or to
 370 another function for further tracing in that scope), or \code{None} to
 371 turn off tracing in that scope.
 372
 373 Instance methods are accepted (and very useful!) as trace functions.
 374
 375 The events have the following meaning:
 376
 377 \begin{description}
 378
 379 \item[\code{'call'}]
 380 A function is called (or some other code block entered).  The global
 381 trace function is called; \var{arg} is \code{None};
 382 the return value specifies the local trace function.
 383
 384 \item[\code{'line'}]
 385 The interpreter is about to execute a new line of code (sometimes
 386 multiple line events on one line exist).  The local trace function is
 387 called; \var{arg} is \code{None}; the return value specifies the new
 388 local trace function.
 389
 390 \item[\code{'return'}]
 391 A function (or other code block) is about to return.  The local trace
 392 function is called; \var{arg} is the value that will be returned.  The
 393 trace function's return value is ignored.
 394
 395 \item[\code{'exception'}]
 396 An exception has occurred.  The local trace function is called;
 397 \var{arg} is a triple \code{(\var{exception}, \var{value},
 398 \var{traceback})}; the return value specifies the new local trace
 399 function.
 400
 401 \item[\code{'c_call'}]
 402 A C function is about to be called.  This may be an extension function
 403 or a builtin.  \var{arg} is the C function object.
 404
 405 \item[\code{'c_return'}]
 406 A C function has returned. \var{arg} is \code{None}.
 407
 408 \item[\code{'c_exception'}]
 409 A C function has thrown an exception.  \var{arg} is \code{None}.
 410
 411 \end{description}
 412
 413 Note that as an exception is propagated down the chain of callers, an
 414 \code{'exception'} event is generated at each level.
 415
 416 For more information on code and frame objects, refer to the
 417 \citetitle[../ref/ref.html]{Python Reference Manual}.