cpython/Doc/api/utilities.tex

\chapter{Utilities \label{utilities}}

The functions in this chapter perform various utility tasks, ranging
from helping C code be more portable across platforms, using Python
modules from C, and parsing function arguments and constructing Python
values from C values.


\section{Operating System Utilities \label{os}}

\begin{cfuncdesc}{int}{Py_FdIsInteractive}{FILE *fp, char *filename}
  Return true (nonzero) if the standard I/O file \var{fp} with name
  \var{filename} is deemed interactive.  This is the case for files
  for which \samp{isatty(fileno(\var{fp}))} is true.  If the global
  flag \cdata{Py_InteractiveFlag} is true, this function also returns
  true if the \var{filename} pointer is \NULL{} or if the name is
  equal to one of the strings \code{'<stdin>'} or \code{'???'}.
\end{cfuncdesc}

\begin{cfuncdesc}{long}{PyOS_GetLastModificationTime}{char *filename}
  Return the time of last modification of the file \var{filename}.
  The result is encoded in the same way as the timestamp returned by
  the standard C library function \cfunction{time()}.
\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyOS_AfterFork}{}
  Function to update some internal state after a process fork; this
  should be called in the new process if the Python interpreter will
  continue to be used.  If a new executable is loaded into the new
  process, this function does not need to be called.
\end{cfuncdesc}

\begin{cfuncdesc}{int}{PyOS_CheckStack}{}
  Return true when the interpreter runs out of stack space.  This is a
  reliable check, but is only available when \constant{USE_STACKCHECK}
  is defined (currently on Windows using the Microsoft Visual \Cpp{}
  compiler and on the Macintosh).  \constant{USE_CHECKSTACK} will be
  defined automatically; you should never change the definition in
  your own code.
\end{cfuncdesc}

\begin{cfuncdesc}{PyOS_sighandler_t}{PyOS_getsig}{int i}
  Return the current signal handler for signal \var{i}.  This is a
  thin wrapper around either \cfunction{sigaction()} or
  \cfunction{signal()}.  Do not call those functions directly!
  \ctype{PyOS_sighandler_t} is a typedef alias for \ctype{void
  (*)(int)}.
\end{cfuncdesc}

\begin{cfuncdesc}{PyOS_sighandler_t}{PyOS_setsig}{int i, PyOS_sighandler_t h}
  Set the signal handler for signal \var{i} to be \var{h}; return the
  old signal handler.  This is a thin wrapper around either
  \cfunction{sigaction()} or \cfunction{signal()}.  Do not call those
  functions directly!  \ctype{PyOS_sighandler_t} is a typedef alias
  for \ctype{void (*)(int)}.
\end{cfuncdesc}


\section{Process Control \label{processControl}}

\begin{cfuncdesc}{void}{Py_FatalError}{char *message}
  Print a fatal error message and kill the process.  No cleanup is
  performed.  This function should only be invoked when a condition is
  detected that would make it dangerous to continue using the Python
  interpreter; e.g., when the object administration appears to be
  corrupted.  On \UNIX, the standard C library function
  \cfunction{abort()}\ttindex{abort()} is called which will attempt to
  produce a \file{core} file.
\end{cfuncdesc}

\begin{cfuncdesc}{void}{Py_Exit}{int status}
  Exit the current process.  This calls
  \cfunction{Py_Finalize()}\ttindex{Py_Finalize()} and then calls the
  standard C library function
  \code{exit(\var{status})}\ttindex{exit()}.
\end{cfuncdesc}

\begin{cfuncdesc}{int}{Py_AtExit}{void (*func) ()}
  Register a cleanup function to be called by
  \cfunction{Py_Finalize()}\ttindex{Py_Finalize()}.  The cleanup
  function will be called with no arguments and should return no
  value.  At most 32 \index{cleanup functions}cleanup functions can be
  registered.  When the registration is successful,
  \cfunction{Py_AtExit()} returns \code{0}; on failure, it returns
  \code{-1}.  The cleanup function registered last is called first.
  Each cleanup function will be called at most once.  Since Python's
  internal finallization will have completed before the cleanup
  function, no Python APIs should be called by \var{func}.
\end{cfuncdesc}


\section{Importing Modules \label{importing}}

\begin{cfuncdesc}{PyObject*}{PyImport_ImportModule}{char *name}
  This is a simplified interface to
  \cfunction{PyImport_ImportModuleEx()} below, leaving the
  \var{globals} and \var{locals} arguments set to \NULL.  When the
  \var{name} argument contains a dot (when it specifies a submodule of
  a package), the \var{fromlist} argument is set to the list
  \code{['*']} so that the return value is the named module rather
  than the top-level package containing it as would otherwise be the
  case.  (Unfortunately, this has an additional side effect when
  \var{name} in fact specifies a subpackage instead of a submodule:
  the submodules specified in the package's \code{__all__} variable
  are \index{package variable!\code{__all__}}
  \withsubitem{(package variable)}{\ttindex{__all__}}loaded.)  Return
  a new reference to the imported module, or \NULL{} with an exception
  set on failure (the module may still be created in this case ---
  examine \code{sys.modules} to find out).
  \withsubitem{(in module sys)}{\ttindex{modules}}
\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{PyImport_ImportModuleEx}{char *name,
                       PyObject *globals, PyObject *locals, PyObject *fromlist}
  Import a module.  This is best described by referring to the
  built-in Python function
  \function{__import__()}\bifuncindex{__import__}, as the standard
  \function{__import__()} function calls this function directly.

  The return value is a new reference to the imported module or
  top-level package, or \NULL{} with an exception set on failure (the
  module may still be created in this case).  Like for
  \function{__import__()}, the return value when a submodule of a
  package was requested is normally the top-level package, unless a
  non-empty \var{fromlist} was given.
\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{PyImport_Import}{PyObject *name}
  This is a higher-level interface that calls the current ``import
  hook function''.  It invokes the \function{__import__()} function
  from the \code{__builtins__} of the current globals.  This means
  that the import is done using whatever import hooks are installed in
  the current environment, e.g. by \module{rexec}\refstmodindex{rexec}
  or \module{ihooks}\refstmodindex{ihooks}.
\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{PyImport_ReloadModule}{PyObject *m}
  Reload a module.  This is best described by referring to the
  built-in Python function \function{reload()}\bifuncindex{reload}, as
  the standard \function{reload()} function calls this function
  directly.  Return a new reference to the reloaded module, or \NULL{}
  with an exception set on failure (the module still exists in this
  case).
\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{PyImport_AddModule}{char *name}
  Return the module object corresponding to a module name.  The
  \var{name} argument may be of the form \code{package.module}).
  First check the modules dictionary if there's one there, and if not,
  create a new one and insert in in the modules dictionary.
  \note{This function does not load or import the module; if the
  module wasn't already loaded, you will get an empty module object.
  Use \cfunction{PyImport_ImportModule()} or one of its variants to
  import a module.  Return \NULL{} with an exception set on failure.}
\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{PyImport_ExecCodeModule}{char *name, PyObject *co}
  Given a module name (possibly of the form \code{package.module}) and
  a code object read from a Python bytecode file or obtained from the
  built-in function \function{compile()}\bifuncindex{compile}, load
  the module.  Return a new reference to the module object, or \NULL{}
  with an exception set if an error occurred (the module may still be
  created in this case).  (This function would reload the module if it
  was already imported.)
\end{cfuncdesc}

\begin{cfuncdesc}{long}{PyImport_GetMagicNumber}{}
  Return the magic number for Python bytecode files
  (a.k.a. \file{.pyc} and \file{.pyo} files).  The magic number should
  be present in the first four bytes of the bytecode file, in
  little-endian byte order.
\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{PyImport_GetModuleDict}{}
  Return the dictionary used for the module administration
  (a.k.a.\ \code{sys.modules}).  Note that this is a per-interpreter
  variable.
\end{cfuncdesc}

\begin{cfuncdesc}{void}{_PyImport_Init}{}
  Initialize the import mechanism.  For internal use only.
\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyImport_Cleanup}{}
  Empty the module table.  For internal use only.
\end{cfuncdesc}

\begin{cfuncdesc}{void}{_PyImport_Fini}{}
  Finalize the import mechanism.  For internal use only.
\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{_PyImport_FindExtension}{char *, char *}
  For internal use only.
\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{_PyImport_FixupExtension}{char *, char *}
  For internal use only.
\end{cfuncdesc}

\begin{cfuncdesc}{int}{PyImport_ImportFrozenModule}{char *name}
  Load a frozen module named \var{name}.  Return \code{1} for success,
  \code{0} if the module is not found, and \code{-1} with an exception
  set if the initialization failed.  To access the imported module on
  a successful load, use \cfunction{PyImport_ImportModule()}.  (Note
  the misnomer --- this function would reload the module if it was
  already imported.)
\end{cfuncdesc}

\begin{ctypedesc}[_frozen]{struct _frozen}
  This is the structure type definition for frozen module descriptors,
  as generated by the \program{freeze}\index{freeze utility} utility
  (see \file{Tools/freeze/} in the Python source distribution).  Its
  definition, found in \file{Include/import.h}, is:

\begin{verbatim}
struct _frozen {
    char *name;
    unsigned char *code;
    int size;
};
\end{verbatim}
\end{ctypedesc}

\begin{cvardesc}{struct _frozen*}{PyImport_FrozenModules}
  This pointer is initialized to point to an array of \ctype{struct
  _frozen} records, terminated by one whose members are all \NULL{} or
  zero.  When a frozen module is imported, it is searched in this
  table.  Third-party code could play tricks with this to provide a
  dynamically created collection of frozen modules.
\end{cvardesc}

\begin{cfuncdesc}{int}{PyImport_AppendInittab}{char *name,
                                               void (*initfunc)(void)}
  Add a single module to the existing table of built-in modules.  This
  is a convenience wrapper around
  \cfunction{PyImport_ExtendInittab()}, returning \code{-1} if the
  table could not be extended.  The new module can be imported by the
  name \var{name}, and uses the function \var{initfunc} as the
  initialization function called on the first attempted import.  This
  should be called before \cfunction{Py_Initialize()}.
\end{cfuncdesc}

\begin{ctypedesc}[_inittab]{struct _inittab}
  Structure describing a single entry in the list of built-in
  modules.  Each of these structures gives the name and initialization
  function for a module built into the interpreter.  Programs which
  embed Python may use an array of these structures in conjunction
  with \cfunction{PyImport_ExtendInittab()} to provide additional
  built-in modules.  The structure is defined in
  \file{Include/import.h} as:

\begin{verbatim}
struct _inittab {
    char *name;
    void (*initfunc)(void);
};
\end{verbatim}
\end{ctypedesc}

\begin{cfuncdesc}{int}{PyImport_ExtendInittab}{struct _inittab *newtab}
  Add a collection of modules to the table of built-in modules.  The
  \var{newtab} array must end with a sentinel entry which contains
  \NULL{} for the \member{name} field; failure to provide the sentinel
  value can result in a memory fault.  Returns \code{0} on success or
  \code{-1} if insufficient memory could be allocated to extend the
  internal table.  In the event of failure, no modules are added to
  the internal table.  This should be called before
  \cfunction{Py_Initialize()}.
\end{cfuncdesc}


\section{Data marshalling support \label{marshalling-utils}}

These routines allow C code to work with serialized objects using the
same data format as the \module{marshal} module.  There are functions
to write data into the serialization format, and additional functions
that can be used to read the data back.  Files used to store marshalled
data must be opened in binary mode.

Numeric values are stored with the least significant byte first.

\begin{cfuncdesc}{void}{PyMarshal_WriteLongToFile}{long value, FILE *file}
  Marshal a \ctype{long} integer, \var{value}, to \var{file}.  This
  will only write the least-significant 32 bits of \var{value};
  regardless of the size of the native \ctype{long} type.
\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyMarshal_WriteShortToFile}{short value, FILE *file}
  Marshal a \ctype{short} integer, \var{value}, to \var{file}.
\end{cfuncdesc}

\begin{cfuncdesc}{void}{PyMarshal_WriteObjectToFile}{PyObject *value,
                                                     FILE *file}
  Marshal a Python object, \var{value}, to \var{file}.  This
  will only write the least-significant 16 bits of \var{value};
  regardless of the size of the native \ctype{short} type.
\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{PyMarshal_WriteObjectToString}{PyObject *value}
  Return a string object containing the marshalled representation of
  \var{value}.
\end{cfuncdesc}

The following functions allow marshalled values to be read back in.

XXX What about error detection?  It appears that reading past the end
of the file will always result in a negative numeric value (where
that's relevant), but it's not clear that negative values won't be
handled properly when there's no error.  What's the right way to tell?
Should only non-negative values be written using these routines?

\begin{cfuncdesc}{long}{PyMarshal_ReadLongFromFile}{FILE *file}
  Return a C \ctype{long} from the data stream in a \ctype{FILE*}
  opened for reading.  Only a 32-bit value can be read in using
  this function, regardless of the native size of \ctype{long}.
\end{cfuncdesc}

\begin{cfuncdesc}{int}{PyMarshal_ReadShortFromFile}{FILE *file}
  Return a C \ctype{short} from the data stream in a \ctype{FILE*}
  opened for reading.  Only a 16-bit value can be read in using
  this function, regardless of the native size of \ctype{long}.
\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{PyMarshal_ReadObjectFromFile}{FILE *file}
  Return a Python object from the data stream in a \ctype{FILE*}
  opened for reading.  On error, sets the appropriate exception
  (\exception{EOFError} or \exception{TypeError}) and returns \NULL.
\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{PyMarshal_ReadLastObjectFromFile}{FILE *file}
  Return a Python object from the data stream in a \ctype{FILE*}
  opened for reading.  Unlike
  \cfunction{PyMarshal_ReadObjectFromFile()}, this function assumes
  that no further objects will be read from the file, allowing it to
  aggressively load file data into memory so that the de-serialization
  can operate from data in memory rather than reading a byte at a time
  from the file.  Only use these variant if you are certain that you
  won't be reading anything else from the file.  On error, sets the
  appropriate exception (\exception{EOFError} or
  \exception{TypeError}) and returns \NULL.
\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{PyMarshal_ReadObjectFromString}{char *string,
                                                             int len}
  Return a Python object from the data stream in a character buffer
  containing \var{len} bytes pointed to by \var{string}.  On error,
  sets the appropriate exception (\exception{EOFError} or
  \exception{TypeError}) and returns \NULL.
\end{cfuncdesc}


\section{Parsing arguments and building values
         \label{arg-parsing}}

These functions are useful when creating your own extensions functions
and methods.  Additional information and examples are available in
\citetitle[../ext/ext.html]{Extending and Embedding the Python
Interpreter}.

\begin{cfuncdesc}{int}{PyArg_ParseTuple}{PyObject *args, char *format,
                                         \moreargs}
  Parse the parameters of a function that takes only positional
  parameters into local variables.  Returns true on success; on
  failure, it returns false and raises the appropriate exception.  See
  \citetitle[../ext/parseTuple.html]{Extending and Embedding the
  Python Interpreter} for more information.
\end{cfuncdesc}

\begin{cfuncdesc}{int}{PyArg_ParseTupleAndKeywords}{PyObject *args,
                       PyObject *kw, char *format, char *keywords[],
                       \moreargs}
  Parse the parameters of a function that takes both positional and
  keyword parameters into local variables.  Returns true on success;
  on failure, it returns false and raises the appropriate exception.
  See \citetitle[../ext/parseTupleAndKeywords.html]{Extending and
  Embedding the Python Interpreter} for more information.
\end{cfuncdesc}

\begin{cfuncdesc}{int}{PyArg_Parse}{PyObject *args, char *format,
                                    \moreargs}
  Function used to deconstruct the argument lists of ``old-style''
  functions --- these are functions which use the
  \constant{METH_OLDARGS} parameter parsing method.  This is not
  recommended for use in parameter parsing in new code, and most code
  in the standard interpreter has been modified to no longer use this
  for that purpose.  It does remain a convenient way to decompose
  other tuples, however, and may continue to be used for that
  purpose.
\end{cfuncdesc}

\begin{cfuncdesc}{int}{PyArg_UnpackTuple}{PyObject *args, char *name,
                                          int min, int max, \moreargs}
  A simpler form of parameter retrieval which does not use a format
  string to specify the types of the arguments.  Functions which use
  this method to retrieve their parameters should be declared as
  \constant{METH_VARARGS} in function or method tables.  The tuple
  containing the actual parameters should be passed as \var{args}; it
  must actually be a tuple.  The length of the tuple must be at least
  \var{min} and no more than \var{max}; \var{min} and \var{max} may be
  equal.  Additional arguments must be passed to the function, each of
  which should be a pointer to a \ctype{PyObject*} variable; these
  will be filled in with the values from \var{args}; they will contain
  borrowed references.  The variables which correspond to optional
  parameters not given by \var{args} will not be filled in; these
  should be initialized by the caller.
  This function returns true on success and false if \var{args} is not
  a tuple or contains the wrong number of elements; an exception will
  be set if there was a failure.

  This is an example of the use of this function, taken from the
  sources for the \module{_weakref} helper module for weak references:

\begin{verbatim}
static PyObject *
weakref_ref(PyObject *self, PyObject *args)
{
    PyObject *object;
    PyObject *callback = NULL;
    PyObject *result = NULL;

    if (PyArg_UnpackTuple(args, "ref", 1, 2, &object, &callback)) {
        result = PyWeakref_NewRef(object, callback);
    }
    return result;
}
\end{verbatim}

  The call to \cfunction{PyArg_UnpackTuple()} in this example is
  entirely equivalent to this call to \cfunction{PyArg_ParseTuple()}:

\begin{verbatim}
PyArg_ParseTuple(args, "O|O:ref", &object, &callback)
\end{verbatim}

  \versionadded{2.2}
\end{cfuncdesc}

\begin{cfuncdesc}{PyObject*}{Py_BuildValue}{char *format,
                                            \moreargs}
  Create a new value based on a format string similar to those
  accepted by the \cfunction{PyArg_Parse*()} family of functions and a
  sequence of values.  Returns the value or \NULL{} in the case of an
  error; an exception will be raised if \NULL{} is returned.  For more
  information on the format string and additional parameters, see
  \citetitle[../ext/buildValue.html]{Extending and Embedding the
  Python Interpreter}.
\end{cfuncdesc}