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\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. 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{y} (bytes object)
{[const char *]}]
This variant on \samp{s} convert a Python bytes object to a C pointer to a
character string. The bytes object must not contain embedded NUL bytes;
if it does, a \exception{TypeError} exception is raised.
\item[\samp{y\#} (bytes 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. This only
accepts bytes objects.
\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{U} (string) {[char *]}]
Convert a null-terminated C string to a Python unicode object.
If the C string pointer is \NULL, \code{None} is used.
\item[\samp{U\#} (string) {[char *, int]}]
Convert a C string and its length to a Python unicode object.
If the C string 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}
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