Doc/lib/libinspect.tex

   1 \section{\module{inspect} ---
   2          Inspect live objects}
   3
   4 \declaremodule{standard}{inspect}
   5 \modulesynopsis{Extract information and source code from live objects.}
   6 \moduleauthor{Ka-Ping Yee}{ping@lfw.org}
   7 \sectionauthor{Ka-Ping Yee}{ping@lfw.org}
   8
   9 \versionadded{2.1}
  10
  11 The \module{inspect} module provides several useful functions
  12 to help get information about live objects such as modules,
  13 classes, methods, functions, tracebacks, frame objects, and
  14 code objects.  For example, it can help you examine the
  15 contents of a class, retrieve the source code of a method,
  16 extract and format the argument list for a function, or
  17 get all the information you need to display a detailed traceback.
  18
  19 There are four main kinds of services provided by this module:
  20 type checking, getting source code, inspecting classes
  21 and functions, and examining the interpreter stack.
  22
  23 \subsection{Types and members
  24             \label{inspect-types}}
  25
  26 The \function{getmembers()} function retrieves the members
  27 of an object such as a class or module.
  28 The eleven functions whose names begin with ``is'' are mainly
  29 provided as convenient choices for the second argument to
  30 \function{getmembers()}.  They also help you determine when
  31 you can expect to find the following special attributes:
  32
  33 \begin{tableiv}{c|l|l|c}{}{Type}{Attribute}{Description}{Notes}
  34   \lineiv{module}{__doc__}{documentation string}{}
  35   \lineiv{}{__file__}{filename (missing for built-in modules)}{}
  36   \hline
  37   \lineiv{class}{__doc__}{documentation string}{}
  38   \lineiv{}{__module__}{name of module in which this class was defined}{}
  39   \hline
  40   \lineiv{method}{__doc__}{documentation string}{}
  41   \lineiv{}{__name__}{name with which this method was defined}{}
  42   \lineiv{}{im_class}{class object that asked for this method}{(1)}
  43   \lineiv{}{im_func}{function object containing implementation of method}{}
  44   \lineiv{}{im_self}{instance to which this method is bound, or \code{None}}{}
  45   \hline
  46   \lineiv{function}{__doc__}{documentation string}{}
  47   \lineiv{}{__name__}{name with which this function was defined}{}
  48   \lineiv{}{func_code}{code object containing compiled function bytecode}{}
  49   \lineiv{}{func_defaults}{tuple of any default values for arguments}{}
  50   \lineiv{}{func_doc}{(same as __doc__)}{}
  51   \lineiv{}{func_globals}{global namespace in which this function was defined}{}
  52   \lineiv{}{func_name}{(same as __name__)}{}
  53   \hline
  54   \lineiv{traceback}{tb_frame}{frame object at this level}{}
  55   \lineiv{}{tb_lasti}{index of last attempted instruction in bytecode}{}
  56   \lineiv{}{tb_lineno}{current line number in Python source code}{}
  57   \lineiv{}{tb_next}{next inner traceback object (called by this level)}{}
  58   \hline
  59   \lineiv{frame}{f_back}{next outer frame object (this frame's caller)}{}
  60   \lineiv{}{f_builtins}{built-in namespace seen by this frame}{}
  61   \lineiv{}{f_code}{code object being executed in this frame}{}
  62   \lineiv{}{f_exc_traceback}{traceback if raised in this frame, or \code{None}}{}
  63   \lineiv{}{f_exc_type}{exception type if raised in this frame, or \code{None}}{}
  64   \lineiv{}{f_exc_value}{exception value if raised in this frame, or \code{None}}{}
  65   \lineiv{}{f_globals}{global namespace seen by this frame}{}
  66   \lineiv{}{f_lasti}{index of last attempted instruction in bytecode}{}
  67   \lineiv{}{f_lineno}{current line number in Python source code}{}
  68   \lineiv{}{f_locals}{local namespace seen by this frame}{}
  69   \lineiv{}{f_restricted}{0 or 1 if frame is in restricted execution mode}{}
  70   \lineiv{}{f_trace}{tracing function for this frame, or \code{None}}{}
  71   \hline
  72   \lineiv{code}{co_argcount}{number of arguments (not including * or ** args)}{}
  73   \lineiv{}{co_code}{string of raw compiled bytecode}{}
  74   \lineiv{}{co_consts}{tuple of constants used in the bytecode}{}
  75   \lineiv{}{co_filename}{name of file in which this code object was created}{}
  76   \lineiv{}{co_firstlineno}{number of first line in Python source code}{}
  77   \lineiv{}{co_flags}{bitmap: 1=optimized \code{|} 2=newlocals \code{|} 4=*arg \code{|} 8=**arg}{}
  78   \lineiv{}{co_lnotab}{encoded mapping of line numbers to bytecode indices}{}
  79   \lineiv{}{co_name}{name with which this code object was defined}{}
  80   \lineiv{}{co_names}{tuple of names of local variables}{}
  81   \lineiv{}{co_nlocals}{number of local variables}{}
  82   \lineiv{}{co_stacksize}{virtual machine stack space required}{}
  83   \lineiv{}{co_varnames}{tuple of names of arguments and local variables}{}
  84   \hline
  85   \lineiv{builtin}{__doc__}{documentation string}{}
  86   \lineiv{}{__name__}{original name of this function or method}{}
  87   \lineiv{}{__self__}{instance to which a method is bound, or \code{None}}{}
  88 \end{tableiv}
  89
  90 \noindent
  91 Note:
  92 \begin{description}
  93 \item[(1)]
  94 \versionchanged[\member{im_class} used to refer to the class that
  95                 defined the method]{2.2}
  96 \end{description}
  97
  98
  99 \begin{funcdesc}{getmembers}{object\optional{, predicate}}
 100   Return all the members of an object in a list of (name, value) pairs
 101   sorted by name.  If the optional \var{predicate} argument is supplied,
 102   only members for which the predicate returns a true value are included.
 103 \end{funcdesc}
 104
 105 \begin{funcdesc}{getmoduleinfo}{path}
 106   Return a tuple of values that describe how Python will interpret the
 107   file identified by \var{path} if it is a module, or \code{None} if
 108   it would not be identified as a module.  The return tuple is
 109   \code{(\var{name}, \var{suffix}, \var{mode}, \var{mtype})}, where
 110   \var{name} is the name of the module without the name of any
 111   enclosing package, \var{suffix} is the trailing part of the file
 112   name (which may not be a dot-delimited extension), \var{mode} is the
 113   \function{open()} mode that would be used (\code{'r'} or
 114   \code{'rb'}), and \var{mtype} is an integer giving the type of the
 115   module.  \var{mtype} will have a value which can be compared to the
 116   constants defined in the \refmodule{imp} module; see the
 117   documentation for that module for more information on module types.
 118 \end{funcdesc}
 119
 120 \begin{funcdesc}{getmodulename}{path}
 121   Return the name of the module named by the file \var{path}, without
 122   including the names of enclosing packages.  This uses the same
 123   algorithm as the interpreter uses when searching for modules.  If
 124   the name cannot be matched according to the interpreter's rules,
 125   \code{None} is returned.
 126 \end{funcdesc}
 127
 128 \begin{funcdesc}{ismodule}{object}
 129   Return true if the object is a module.
 130 \end{funcdesc}
 131
 132 \begin{funcdesc}{isclass}{object}
 133   Return true if the object is a class.
 134 \end{funcdesc}
 135
 136 \begin{funcdesc}{ismethod}{object}
 137   Return true if the object is a method.
 138 \end{funcdesc}
 139
 140 \begin{funcdesc}{isfunction}{object}
 141   Return true if the object is a Python function or unnamed (lambda) function.
 142 \end{funcdesc}
 143
 144 \begin{funcdesc}{istraceback}{object}
 145   Return true if the object is a traceback.
 146 \end{funcdesc}
 147
 148 \begin{funcdesc}{isframe}{object}
 149   Return true if the object is a frame.
 150 \end{funcdesc}
 151
 152 \begin{funcdesc}{iscode}{object}
 153   Return true if the object is a code.
 154 \end{funcdesc}
 155
 156 \begin{funcdesc}{isbuiltin}{object}
 157   Return true if the object is a built-in function.
 158 \end{funcdesc}
 159
 160 \begin{funcdesc}{isroutine}{object}
 161   Return true if the object is a user-defined or built-in function or method.
 162 \end{funcdesc}
 163
 164 \begin{funcdesc}{ismethoddescriptor}{object}
 165   Return true if the object is a method descriptor, but not if ismethod() or
 166   isclass() or isfunction() are true.
 167
 168   This is new as of Python 2.2, and, for example, is true of int.__add__.
 169   An object passing this test has a __get__ attribute but not a __set__
 170   attribute, but beyond that the set of attributes varies.  __name__ is
 171   usually sensible, and __doc__ often is.
 172
 173   Methods implemented via descriptors that also pass one of the other
 174   tests return false from the ismethoddescriptor() test, simply because
 175   the other tests promise more -- you can, e.g., count on having the
 176   im_func attribute (etc) when an object passes ismethod().
 177 \end{funcdesc}
 178
 179 \begin{funcdesc}{isdatadescriptor}{object}
 180   Return true if the object is a data descriptor.
 181
 182   Data descriptors have both a __get__ and a __set__ attribute.  Examples are
 183   properties (defined in Python) and getsets and members (defined in C).
 184   Typically, data descriptors will also have __name__ and __doc__ attributes
 185   (properties, getsets, and members have both of these attributes), but this
 186   is not guaranteed.
 187 \versionadded{2.3}
 188 \end{funcdesc}
 189
 190 \subsection{Retrieving source code
 191             \label{inspect-source}}
 192
 193 \begin{funcdesc}{getdoc}{object}
 194   Get the documentation string for an object.
 195   All tabs are expanded to spaces.  To clean up docstrings that are
 196   indented to line up with blocks of code, any whitespace than can be
 197   uniformly removed from the second line onwards is removed.
 198 \end{funcdesc}
 199
 200 \begin{funcdesc}{getcomments}{object}
 201   Return in a single string any lines of comments immediately preceding
 202   the object's source code (for a class, function, or method), or at the
 203   top of the Python source file (if the object is a module).
 204 \end{funcdesc}
 205
 206 \begin{funcdesc}{getfile}{object}
 207   Return the name of the (text or binary) file in which an object was
 208   defined.  This will fail with a \exception{TypeError} if the object
 209   is a built-in module, class, or function.
 210 \end{funcdesc}
 211
 212 \begin{funcdesc}{getmodule}{object}
 213   Try to guess which module an object was defined in.
 214 \end{funcdesc}
 215
 216 \begin{funcdesc}{getsourcefile}{object}
 217   Return the name of the Python source file in which an object was
 218   defined.  This will fail with a \exception{TypeError} if the object
 219   is a built-in module, class, or function.
 220 \end{funcdesc}
 221
 222 \begin{funcdesc}{getsourcelines}{object}
 223   Return a list of source lines and starting line number for an object.
 224   The argument may be a module, class, method, function, traceback, frame,
 225   or code object.  The source code is returned as a list of the lines
 226   corresponding to the object and the line number indicates where in the
 227   original source file the first line of code was found.  An
 228   \exception{IOError} is raised if the source code cannot be retrieved.
 229 \end{funcdesc}
 230
 231 \begin{funcdesc}{getsource}{object}
 232   Return the text of the source code for an object.
 233   The argument may be a module, class, method, function, traceback, frame,
 234   or code object.  The source code is returned as a single string.  An
 235   \exception{IOError} is raised if the source code cannot be retrieved.
 236 \end{funcdesc}
 237
 238 \subsection{Classes and functions
 239             \label{inspect-classes-functions}}
 240
 241 \begin{funcdesc}{getclasstree}{classes\optional{, unique}}
 242   Arrange the given list of classes into a hierarchy of nested lists.
 243   Where a nested list appears, it contains classes derived from the class
 244   whose entry immediately precedes the list.  Each entry is a 2-tuple
 245   containing a class and a tuple of its base classes.  If the \var{unique}
 246   argument is true, exactly one entry appears in the returned structure
 247   for each class in the given list.  Otherwise, classes using multiple
 248   inheritance and their descendants will appear multiple times.
 249 \end{funcdesc}
 250
 251 \begin{funcdesc}{getargspec}{func}
 252   Get the names and default values of a function's arguments.
 253   A tuple of four things is returned: \code{(\var{args},
 254     \var{varargs}, \var{varkw}, \var{defaults})}.
 255   \var{args} is a list of the argument names (it may contain nested lists).
 256   \var{varargs} and \var{varkw} are the names of the \code{*} and
 257   \code{**} arguments or \code{None}.
 258   \var{defaults} is a tuple of default argument values or None if there are no
 259   default arguments; if this tuple has \var{n} elements, they correspond to
 260   the last \var{n} elements listed in \var{args}.
 261 \end{funcdesc}
 262
 263 \begin{funcdesc}{getargvalues}{frame}
 264   Get information about arguments passed into a particular frame.
 265   A tuple of four things is returned: \code{(\var{args},
 266     \var{varargs}, \var{varkw}, \var{locals})}.
 267   \var{args} is a list of the argument names (it may contain nested
 268   lists).
 269   \var{varargs} and \var{varkw} are the names of the \code{*} and
 270   \code{**} arguments or \code{None}.
 271   \var{locals} is the locals dictionary of the given frame.
 272 \end{funcdesc}
 273
 274 \begin{funcdesc}{formatargspec}{args\optional{, varargs, varkw, defaults,
 275       argformat, varargsformat, varkwformat, defaultformat}}
 276
 277   Format a pretty argument spec from the four values returned by
 278   \function{getargspec()}.  The other four arguments are the
 279   corresponding optional formatting functions that are called to turn
 280   names and values into strings.
 281 \end{funcdesc}
 282
 283 \begin{funcdesc}{formatargvalues}{args\optional{, varargs, varkw, locals,
 284       argformat, varargsformat, varkwformat, valueformat}}
 285   Format a pretty argument spec from the four values returned by
 286   \function{getargvalues()}.  The other four arguments are the
 287   corresponding optional formatting functions that are called to turn
 288   names and values into strings.
 289 \end{funcdesc}
 290
 291 \begin{funcdesc}{getmro}{cls}
 292   Return a tuple of class cls's base classes, including cls, in
 293   method resolution order.  No class appears more than once in this tuple.
 294   Note that the method resolution order depends on cls's type.  Unless a
 295   very peculiar user-defined metatype is in use, cls will be the first
 296   element of the tuple.
 297 \end{funcdesc}
 298
 299 \subsection{The interpreter stack
 300             \label{inspect-stack}}
 301
 302 When the following functions return ``frame records,'' each record
 303 is a tuple of six items: the frame object, the filename,
 304 the line number of the current line, the function name, a list of
 305 lines of context from the source code, and the index of the current
 306 line within that list.
 307
 308 \begin{notice}[warning]
 309 Keeping references to frame objects, as found in
 310 the first element of the frame records these functions return, can
 311 cause your program to create reference cycles.  Once a reference cycle
 312 has been created, the lifespan of all objects which can be accessed
 313 from the objects which form the cycle can become much longer even if
 314 Python's optional cycle detector is enabled.  If such cycles must be
 315 created, it is important to ensure they are explicitly broken to avoid
 316 the delayed destruction of objects and increased memory consumption
 317 which occurs.
 318
 319 Though the cycle detector will catch these, destruction of the frames
 320 (and local variables) can be made deterministic by removing the cycle
 321 in a \keyword{finally} clause.  This is also important if the cycle
 322 detector was disabled when Python was compiled or using
 323 \function{\refmodule{gc}.disable()}.  For example:
 324
 325 \begin{verbatim}
 326 def handle_stackframe_without_leak():
 327     frame = inspect.currentframe()
 328     try:
 329         # do something with the frame
 330     finally:
 331         del frame
 332 \end{verbatim}
 333 \end{notice}
 334
 335 The optional \var{context} argument supported by most of these
 336 functions specifies the number of lines of context to return, which
 337 are centered around the current line.
 338
 339 \begin{funcdesc}{getframeinfo}{frame\optional{, context}}
 340   Get information about a frame or traceback object.  A 5-tuple
 341   is returned, the last five elements of the frame's frame record.
 342 \end{funcdesc}
 343
 344 \begin{funcdesc}{getouterframes}{frame\optional{, context}}
 345   Get a list of frame records for a frame and all outer frames.  These
 346   frames represent the calls that lead to the creation of \var{frame}.
 347   The first entry in the returned list represents \var{frame}; the
 348   last entry represents the outermost call on \var{frame}'s stack.
 349 \end{funcdesc}
 350
 351 \begin{funcdesc}{getinnerframes}{traceback\optional{, context}}
 352   Get a list of frame records for a traceback's frame and all inner
 353   frames.  These frames represent calls made as a consequence of
 354   \var{frame}.  The first entry in the list represents
 355   \var{traceback}; the last entry represents where the exception was
 356   raised.
 357 \end{funcdesc}
 358
 359 \begin{funcdesc}{currentframe}{}
 360   Return the frame object for the caller's stack frame.
 361 \end{funcdesc}
 362
 363 \begin{funcdesc}{stack}{\optional{context}}
 364   Return a list of frame records for the caller's stack.  The first
 365   entry in the returned list represents the caller; the last entry
 366   represents the outermost call on the stack.
 367 \end{funcdesc}
 368
 369 \begin{funcdesc}{trace}{\optional{context}}
 370   Return a list of frame records for the stack between the current
 371   frame and the frame in which an exception currently being handled
 372   was raised in.  The first entry in the list represents the caller;
 373   the last entry represents where the exception was raised.
 374 \end{funcdesc}