ref: dab539cd14726de15932b0310532212f8da29bf7
dir: /sys/src/cmd/python/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, const 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). \constant{USE_STACKCHECK} 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}{const 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 finalization 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}{const 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. Before Python 2.4, the module may still be created in the failure case --- examine \code{sys.modules} to find out. Starting with Python 2.4, a failing import of a module no longer leaves the module in \code{sys.modules}. \versionchanged[failing imports remove incomplete module objects]{2.4} \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 (before Python 2.4, 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. \versionchanged[failing imports remove incomplete module objects]{2.4} \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}{const 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 it in the modules dictionary. Return \NULL{} with an exception set on failure. \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. Package structures implied by a dotted name for \var{name} are not created if not already present.} \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. Before Python 2.4, the module could still be created in error cases. Starting with Python 2.4, \var{name} is removed from \code{sys.modules} in error cases, and even if \var{name} was already in \code{sys.modules} on entry to \cfunction{PyImport_ExecCodeModule()}. Leaving incompletely initialized modules in \code{sys.modules} is dangerous, as imports of such modules have no way to know that the module object is an unknown (and probably damaged with respect to the module author's intents) state. This function will reload the module if it was already imported. See \cfunction{PyImport_ReloadModule()} for the intended way to reload a module. If \var{name} points to a dotted name of the form \code{package.module}, any package structures not already created will still not be created. \versionchanged[\var{name} is removed from \code{sys.modules} in error cases]{2.4} \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. The module supports two versions of the data format: version 0 is the historical version, version 1 (new in Python 2.4) shares interned strings in the file, and upon unmarshalling. \var{Py_MARSHAL_VERSION} indicates the current file format (currently 1). \begin{cfuncdesc}{void}{PyMarshal_WriteLongToFile}{long value, FILE *file, int version} 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. \versionchanged[\var{version} indicates the file format]{2.4} \end{cfuncdesc} \begin{cfuncdesc}{void}{PyMarshal_WriteObjectToFile}{PyObject *value, FILE *file, int version} Marshal a Python object, \var{value}, to \var{file}. \versionchanged[\var{version} indicates the file format]{2.4} \end{cfuncdesc} \begin{cfuncdesc}{PyObject*}{PyMarshal_WriteObjectToString}{PyObject *value, int version} Return a string object containing the marshalled representation of \var{value}. \versionchanged[\var{version} indicates the file format]{2.4} \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{short}. \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, Py_ssize_t 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}. The first three of these functions described, \cfunction{PyArg_ParseTuple()}, \cfunction{PyArg_ParseTupleAndKeywords()}, and \cfunction{PyArg_Parse()}, all use \emph{format strings} which are used to tell the function about the expected arguments. The format strings use the same syntax for each of these functions. A format string consists of zero or more ``format units.'' A format unit describes one Python object; it is usually a single character or a parenthesized sequence of format units. With a few exceptions, a format unit that is not a parenthesized sequence normally corresponds to a single address argument to these functions. In the following description, the quoted form is the format unit; the entry in (round) parentheses is the Python object type that matches the format unit; and the entry in [square] brackets is the type of the C variable(s) whose address should be passed. \begin{description} \item[\samp{s} (string or Unicode object) {[const char *]}] Convert a Python string or Unicode object to a C pointer to a character string. You must not provide storage for the string itself; a pointer to an existing string is stored into the character pointer variable whose address you pass. The C string is NUL-terminated. The Python string must not contain embedded NUL bytes; if it does, a \exception{TypeError} exception is raised. Unicode objects are converted to C strings using the default encoding. If this conversion fails, a \exception{UnicodeError} is raised. \item[\samp{s\#} (string, Unicode or any read buffer compatible object) {[const char *, int]}] This variant on \samp{s} stores into two C variables, the first one a pointer to a character string, the second one its length. In this case the Python string may contain embedded null bytes. Unicode objects pass back a pointer to the default encoded string version of the object if such a conversion is possible. All other read-buffer compatible objects pass back a reference to the raw internal data representation. \item[\samp{z} (string or \code{None}) {[const char *]}] Like \samp{s}, but the Python object may also be \code{None}, in which case the C pointer is set to \NULL. \item[\samp{z\#} (string or \code{None} or any read buffer compatible object) {[const char *, int]}] This is to \samp{s\#} as \samp{z} is to \samp{s}. \item[\samp{u} (Unicode object) {[Py_UNICODE *]}] Convert a Python Unicode object to a C pointer to a NUL-terminated buffer of 16-bit Unicode (UTF-16) data. As with \samp{s}, there is no need to provide storage for the Unicode data buffer; a pointer to the existing Unicode data is stored into the \ctype{Py_UNICODE} pointer variable whose address you pass. \item[\samp{u\#} (Unicode object) {[Py_UNICODE *, int]}] This variant on \samp{u} stores into two C variables, the first one a pointer to a Unicode data buffer, the second one its length. Non-Unicode objects are handled by interpreting their read-buffer pointer as pointer to a \ctype{Py_UNICODE} array. \item[\samp{es} (string, Unicode object or character buffer compatible object) {[const char *encoding, char **buffer]}] This variant on \samp{s} is used for encoding Unicode and objects convertible to Unicode into a character buffer. It only works for encoded data without embedded NUL bytes. This format requires two arguments. The first is only used as input, and must be a \ctype{const char*} which points to the name of an encoding as a NUL-terminated string, or \NULL, in which case the default encoding is used. An exception is raised if the named encoding is not known to Python. The second argument must be a \ctype{char**}; the value of the pointer it references will be set to a buffer with the contents of the argument text. The text will be encoded in the encoding specified by the first argument. \cfunction{PyArg_ParseTuple()} will allocate a buffer of the needed size, copy the encoded data into this buffer and adjust \var{*buffer} to reference the newly allocated storage. The caller is responsible for calling \cfunction{PyMem_Free()} to free the allocated buffer after use. \item[\samp{et} (string, Unicode object or character buffer compatible object) {[const char *encoding, char **buffer]}] Same as \samp{es} except that 8-bit string objects are passed through without recoding them. Instead, the implementation assumes that the string object uses the encoding passed in as parameter. \item[\samp{es\#} (string, Unicode object or character buffer compatible object) {[const char *encoding, char **buffer, int *buffer_length]}] This variant on \samp{s\#} is used for encoding Unicode and objects convertible to Unicode into a character buffer. Unlike the \samp{es} format, this variant allows input data which contains NUL characters. It requires three arguments. The first is only used as input, and must be a \ctype{const char*} which points to the name of an encoding as a NUL-terminated string, or \NULL, in which case the default encoding is used. An exception is raised if the named encoding is not known to Python. The second argument must be a \ctype{char**}; the value of the pointer it references will be set to a buffer with the contents of the argument text. The text will be encoded in the encoding specified by the first argument. The third argument must be a pointer to an integer; the referenced integer will be set to the number of bytes in the output buffer. There are two modes of operation: If \var{*buffer} points a \NULL{} pointer, the function will allocate a buffer of the needed size, copy the encoded data into this buffer and set \var{*buffer} to reference the newly allocated storage. The caller is responsible for calling \cfunction{PyMem_Free()} to free the allocated buffer after usage. If \var{*buffer} points to a non-\NULL{} pointer (an already allocated buffer), \cfunction{PyArg_ParseTuple()} will use this location as the buffer and interpret the initial value of \var{*buffer_length} as the buffer size. It will then copy the encoded data into the buffer and NUL-terminate it. If the buffer is not large enough, a \exception{ValueError} will be set. In both cases, \var{*buffer_length} is set to the length of the encoded data without the trailing NUL byte. \item[\samp{et\#} (string, Unicode object or character buffer compatible object) {[const char *encoding, char **buffer]}] Same as \samp{es\#} except that string objects are passed through without recoding them. Instead, the implementation assumes that the string object uses the encoding passed in as parameter. \item[\samp{b} (integer) {[char]}] Convert a Python integer to a tiny int, stored in a C \ctype{char}. \item[\samp{B} (integer) {[unsigned char]}] Convert a Python integer to a tiny int without overflow checking, stored in a C \ctype{unsigned char}. \versionadded{2.3} \item[\samp{h} (integer) {[short int]}] Convert a Python integer to a C \ctype{short int}. \item[\samp{H} (integer) {[unsigned short int]}] Convert a Python integer to a C \ctype{unsigned short int}, without overflow checking. \versionadded{2.3} \item[\samp{i} (integer) {[int]}] Convert a Python integer to a plain C \ctype{int}. \item[\samp{I} (integer) {[unsigned int]}] Convert a Python integer to a C \ctype{unsigned int}, without overflow checking. \versionadded{2.3} \item[\samp{l} (integer) {[long int]}] Convert a Python integer to a C \ctype{long int}. \item[\samp{k} (integer) {[unsigned long]}] Convert a Python integer or long integer to a C \ctype{unsigned long} without overflow checking. \versionadded{2.3} \item[\samp{L} (integer) {[PY_LONG_LONG]}] Convert a Python integer to a C \ctype{long long}. This format is only available on platforms that support \ctype{long long} (or \ctype{_int64} on Windows). \item[\samp{K} (integer) {[unsigned PY_LONG_LONG]}] Convert a Python integer or long integer to a C \ctype{unsigned long long} without overflow checking. This format is only available on platforms that support \ctype{unsigned long long} (or \ctype{unsigned _int64} on Windows). \versionadded{2.3} \item[\samp{n} (integer) {[Py_ssize_t]}] Convert a Python integer or long integer to a C \ctype{Py_ssize_t}. \versionadded{2.5} \item[\samp{c} (string of length 1) {[char]}] Convert a Python character, represented as a string of length 1, to a C \ctype{char}. \item[\samp{f} (float) {[float]}] Convert a Python floating point number to a C \ctype{float}. \item[\samp{d} (float) {[double]}] Convert a Python floating point number to a C \ctype{double}. \item[\samp{D} (complex) {[Py_complex]}] Convert a Python complex number to a C \ctype{Py_complex} structure. \item[\samp{O} (object) {[PyObject *]}] Store a Python object (without any conversion) in a C object pointer. The C program thus receives the actual object that was passed. The object's reference count is not increased. The pointer stored is not \NULL. \item[\samp{O!} (object) {[\var{typeobject}, PyObject *]}] Store a Python object in a C object pointer. This is similar to \samp{O}, but takes two C arguments: the first is the address of a Python type object, the second is the address of the C variable (of type \ctype{PyObject*}) into which the object pointer is stored. If the Python object does not have the required type, \exception{TypeError} is raised. \item[\samp{O\&} (object) {[\var{converter}, \var{anything}]}] Convert a Python object to a C variable through a \var{converter} function. This takes two arguments: the first is a function, the second is the address of a C variable (of arbitrary type), converted to \ctype{void *}. The \var{converter} function in turn is called as follows: \var{status}\code{ = }\var{converter}\code{(}\var{object}, \var{address}\code{);} where \var{object} is the Python object to be converted and \var{address} is the \ctype{void*} argument that was passed to the \cfunction{PyArg_Parse*()} function. The returned \var{status} should be \code{1} for a successful conversion and \code{0} if the conversion has failed. When the conversion fails, the \var{converter} function should raise an exception. \item[\samp{S} (string) {[PyStringObject *]}] Like \samp{O} but requires that the Python object is a string object. Raises \exception{TypeError} if the object is not a string object. The C variable may also be declared as \ctype{PyObject*}. \item[\samp{U} (Unicode string) {[PyUnicodeObject *]}] Like \samp{O} but requires that the Python object is a Unicode object. Raises \exception{TypeError} if the object is not a Unicode object. The C variable may also be declared as \ctype{PyObject*}. \item[\samp{t\#} (read-only character buffer) {[char *, int]}] Like \samp{s\#}, but accepts any object which implements the read-only buffer interface. The \ctype{char*} variable is set to point to the first byte of the buffer, and the \ctype{int} is set to the length of the buffer. Only single-segment buffer objects are accepted; \exception{TypeError} is raised for all others. \item[\samp{w} (read-write character buffer) {[char *]}] Similar to \samp{s}, but accepts any object which implements the read-write buffer interface. The caller must determine the length of the buffer by other means, or use \samp{w\#} instead. Only single-segment buffer objects are accepted; \exception{TypeError} is raised for all others. \item[\samp{w\#} (read-write character buffer) {[char *, int]}] Like \samp{s\#}, but accepts any object which implements the read-write buffer interface. The \ctype{char *} variable is set to point to the first byte of the buffer, and the \ctype{int} is set to the length of the buffer. Only single-segment buffer objects are accepted; \exception{TypeError} is raised for all others. \item[\samp{(\var{items})} (tuple) {[\var{matching-items}]}] The object must be a Python sequence whose length is the number of format units in \var{items}. The C arguments must correspond to the individual format units in \var{items}. Format units for sequences may be nested. \note{Prior to Python version 1.5.2, this format specifier only accepted a tuple containing the individual parameters, not an arbitrary sequence. Code which previously caused \exception{TypeError} to be raised here may now proceed without an exception. This is not expected to be a problem for existing code.} \end{description} It is possible to pass Python long integers where integers are requested; however no proper range checking is done --- the most significant bits are silently truncated when the receiving field is too small to receive the value (actually, the semantics are inherited from downcasts in C --- your mileage may vary). A few other characters have a meaning in a format string. These may not occur inside nested parentheses. They are: \begin{description} \item[\samp{|}] Indicates that the remaining arguments in the Python argument list are optional. The C variables corresponding to optional arguments should be initialized to their default value --- when an optional argument is not specified, \cfunction{PyArg_ParseTuple()} does not touch the contents of the corresponding C variable(s). \item[\samp{:}] The list of format units ends here; the string after the colon is used as the function name in error messages (the ``associated value'' of the exception that \cfunction{PyArg_ParseTuple()} raises). \item[\samp{;}] The list of format units ends here; the string after the semicolon is used as the error message \emph{instead} of the default error message. Clearly, \samp{:} and \samp{;} mutually exclude each other. \end{description} Note that any Python object references which are provided to the caller are \emph{borrowed} references; do not decrement their reference count! Additional arguments passed to these functions must be addresses of variables whose type is determined by the format string; these are used to store values from the input tuple. There are a few cases, as described in the list of format units above, where these parameters are used as input values; they should match what is specified for the corresponding format unit in that case. For the conversion to succeed, the \var{arg} object must match the format and the format must be exhausted. On success, the \cfunction{PyArg_Parse*()} functions return true, otherwise they return false and raise an appropriate exception. \begin{cfuncdesc}{int}{PyArg_ParseTuple}{PyObject *args, const 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. \end{cfuncdesc} \begin{cfuncdesc}{int}{PyArg_VaParse}{PyObject *args, const char *format, va_list vargs} Identical to \cfunction{PyArg_ParseTuple()}, except that it accepts a va_list rather than a variable number of arguments. \end{cfuncdesc} \begin{cfuncdesc}{int}{PyArg_ParseTupleAndKeywords}{PyObject *args, PyObject *kw, const 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. \end{cfuncdesc} \begin{cfuncdesc}{int}{PyArg_VaParseTupleAndKeywords}{PyObject *args, PyObject *kw, const char *format, char *keywords[], va_list vargs} Identical to \cfunction{PyArg_ParseTupleAndKeywords()}, except that it accepts a va_list rather than a variable number of arguments. \end{cfuncdesc} \begin{cfuncdesc}{int}{PyArg_Parse}{PyObject *args, const 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, const char *name, Py_ssize_t min, Py_ssize_t 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}{const 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. \cfunction{Py_BuildValue()} does not always build a tuple. It builds a tuple only if its format string contains two or more format units. If the format string is empty, it returns \code{None}; if it contains exactly one format unit, it returns whatever object is described by that format unit. To force it to return a tuple of size 0 or one, parenthesize the format string. When memory buffers are passed as parameters to supply data to build objects, as for the \samp{s} and \samp{s\#} formats, the required data is copied. Buffers provided by the caller are never referenced by the objects created by \cfunction{Py_BuildValue()}. In other words, if your code invokes \cfunction{malloc()} and passes the allocated memory to \cfunction{Py_BuildValue()}, your code is responsible for calling \cfunction{free()} for that memory once \cfunction{Py_BuildValue()} returns. In the following description, the quoted form is the format unit; the entry in (round) parentheses is the Python object type that the format unit will return; and the entry in [square] brackets is the type of the C value(s) to be passed. The characters space, tab, colon and comma are ignored in format strings (but not within format units such as \samp{s\#}). This can be used to make long format strings a tad more readable. \begin{description} \item[\samp{s} (string) {[char *]}] Convert a null-terminated C string to a Python object. If the C string pointer is \NULL, \code{None} is used. \item[\samp{s\#} (string) {[char *, int]}] Convert a C string and its length to a Python object. If the C string pointer is \NULL, the length is ignored and \code{None} is returned. \item[\samp{z} (string or \code{None}) {[char *]}] Same as \samp{s}. \item[\samp{z\#} (string or \code{None}) {[char *, int]}] Same as \samp{s\#}. \item[\samp{u} (Unicode string) {[Py_UNICODE *]}] Convert a null-terminated buffer of Unicode (UCS-2 or UCS-4) data to a Python Unicode object. If the Unicode buffer pointer is \NULL, \code{None} is returned. \item[\samp{u\#} (Unicode string) {[Py_UNICODE *, int]}] Convert a Unicode (UCS-2 or UCS-4) data buffer and its length to a Python Unicode object. If the Unicode buffer pointer is \NULL, the length is ignored and \code{None} is returned. \item[\samp{i} (integer) {[int]}] Convert a plain C \ctype{int} to a Python integer object. \item[\samp{b} (integer) {[char]}] Convert a plain C \ctype{char} to a Python integer object. \item[\samp{h} (integer) {[short int]}] Convert a plain C \ctype{short int} to a Python integer object. \item[\samp{l} (integer) {[long int]}] Convert a C \ctype{long int} to a Python integer object. \item[\samp{B} (integer) {[unsigned char]}] Convert a C \ctype{unsigned char} to a Python integer object. \item[\samp{H} (integer) {[unsigned short int]}] Convert a C \ctype{unsigned short int} to a Python integer object. \item[\samp{I} (integer/long) {[unsigned int]}] Convert a C \ctype{unsigned int} to a Python integer object or a Python long integer object, if it is larger than \code{sys.maxint}. \item[\samp{k} (integer/long) {[unsigned long]}] Convert a C \ctype{unsigned long} to a Python integer object or a Python long integer object, if it is larger than \code{sys.maxint}. \item[\samp{L} (long) {[PY_LONG_LONG]}] Convert a C \ctype{long long} to a Python long integer object. Only available on platforms that support \ctype{long long}. \item[\samp{K} (long) {[unsigned PY_LONG_LONG]}] Convert a C \ctype{unsigned long long} to a Python long integer object. Only available on platforms that support \ctype{unsigned long long}. \item[\samp{n} (int) {[Py_ssize_t]}] Convert a C \ctype{Py_ssize_t} to a Python integer or long integer. \versionadded{2.5} \item[\samp{c} (string of length 1) {[char]}] Convert a C \ctype{int} representing a character to a Python string of length 1. \item[\samp{d} (float) {[double]}] Convert a C \ctype{double} to a Python floating point number. \item[\samp{f} (float) {[float]}] Same as \samp{d}. \item[\samp{D} (complex) {[Py_complex *]}] Convert a C \ctype{Py_complex} structure to a Python complex number. \item[\samp{O} (object) {[PyObject *]}] Pass a Python object untouched (except for its reference count, which is incremented by one). If the object passed in is a \NULL{} pointer, it is assumed that this was caused because the call producing the argument found an error and set an exception. Therefore, \cfunction{Py_BuildValue()} will return \NULL{} but won't raise an exception. If no exception has been raised yet, \exception{SystemError} is set. \item[\samp{S} (object) {[PyObject *]}] Same as \samp{O}. \item[\samp{N} (object) {[PyObject *]}] Same as \samp{O}, except it doesn't increment the reference count on the object. Useful when the object is created by a call to an object constructor in the argument list. \item[\samp{O\&} (object) {[\var{converter}, \var{anything}]}] Convert \var{anything} to a Python object through a \var{converter} function. The function is called with \var{anything} (which should be compatible with \ctype{void *}) as its argument and should return a ``new'' Python object, or \NULL{} if an error occurred. \item[\samp{(\var{items})} (tuple) {[\var{matching-items}]}] Convert a sequence of C values to a Python tuple with the same number of items. \item[\samp{[\var{items}]} (list) {[\var{matching-items}]}] Convert a sequence of C values to a Python list with the same number of items. \item[\samp{\{\var{items}\}} (dictionary) {[\var{matching-items}]}] Convert a sequence of C values to a Python dictionary. Each pair of consecutive C values adds one item to the dictionary, serving as key and value, respectively. \end{description} If there is an error in the format string, the \exception{SystemError} exception is set and \NULL{} returned. \end{cfuncdesc} \section{String conversion and formatting \label{string-formatting}} Functions for number conversion and formatted string output. \begin{cfuncdesc}{int}{PyOS_snprintf}{char *str, size_t size, const char *format, \moreargs} Output not more than \var{size} bytes to \var{str} according to the format string \var{format} and the extra arguments. See the \UNIX{} man page \manpage{snprintf}{2}. \end{cfuncdesc} \begin{cfuncdesc}{int}{PyOS_vsnprintf}{char *str, size_t size, const char *format, va_list va} Output not more than \var{size} bytes to \var{str} according to the format string \var{format} and the variable argument list \var{va}. \UNIX{} man page \manpage{vsnprintf}{2}. \end{cfuncdesc} \cfunction{PyOS_snprintf} and \cfunction{PyOS_vsnprintf} wrap the Standard C library functions \cfunction{snprintf()} and \cfunction{vsnprintf()}. Their purpose is to guarantee consistent behavior in corner cases, which the Standard C functions do not. The wrappers ensure that \var{str}[\var{size}-1] is always \character{\textbackslash0} upon return. They never write more than \var{size} bytes (including the trailing \character{\textbackslash0} into str. Both functions require that \code{\var{str} != NULL}, \code{\var{size} > 0} and \code{\var{format} != NULL}. If the platform doesn't have \cfunction{vsnprintf()} and the buffer size needed to avoid truncation exceeds \var{size} by more than 512 bytes, Python aborts with a \var{Py_FatalError}. The return value (\var{rv}) for these functions should be interpreted as follows: \begin{itemize} \item When \code{0 <= \var{rv} < \var{size}}, the output conversion was successful and \var{rv} characters were written to \var{str} (excluding the trailing \character{\textbackslash0} byte at \var{str}[\var{rv}]). \item When \code{\var{rv} >= \var{size}}, the output conversion was truncated and a buffer with \code{\var{rv} + 1} bytes would have been needed to succeed. \var{str}[\var{size}-1] is \character{\textbackslash0} in this case. \item When \code{\var{rv} < 0}, ``something bad happened.'' \var{str}[\var{size}-1] is \character{\textbackslash0} in this case too, but the rest of \var{str} is undefined. The exact cause of the error depends on the underlying platform. \end{itemize} The following functions provide locale-independent string to number conversions. \begin{cfuncdesc}{double}{PyOS_ascii_strtod}{const char *nptr, char **endptr} Convert a string to a \ctype{double}. This function behaves like the Standard C function \cfunction{strtod()} does in the C locale. It does this without changing the current locale, since that would not be thread-safe. \cfunction{PyOS_ascii_strtod} should typically be used for reading configuration files or other non-user input that should be locale independent. \versionadded{2.4} See the \UNIX{} man page \manpage{strtod}{2} for details. \end{cfuncdesc} \begin{cfuncdesc}{char *}{PyOS_ascii_formatd}{char *buffer, size_t buf_len, const char *format, double d} Convert a \ctype{double} to a string using the \character{.} as the decimal separator. \var{format} is a \cfunction{printf()}-style format string specifying the number format. Allowed conversion characters are \character{e}, \character{E}, \character{f}, \character{F}, \character{g} and \character{G}. The return value is a pointer to \var{buffer} with the converted string or NULL if the conversion failed. \versionadded{2.4} \end{cfuncdesc} \begin{cfuncdesc}{double}{PyOS_ascii_atof}{const char *nptr} Convert a string to a \ctype{double} in a locale-independent way. \versionadded{2.4} See the \UNIX{} man page \manpage{atof}{2} for details. \end{cfuncdesc}