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.. highlight:: c
.. _string-conversion:
String conversion and formatting
Functions for number conversion and formatted string output.
.. c:function:: int PyOS_snprintf(char *str, size_t size, const char *format, ...)
Output not more than *size* bytes to *str* according to the format string
*format* and the extra arguments. See the Unix man page :manpage:`snprintf(3)`.
.. c:function:: int PyOS_vsnprintf(char *str, size_t size, const char *format, va_list va)
Output not more than *size* bytes to *str* according to the format string
*format* and the variable argument list *va*. Unix man page
:c:func:`PyOS_snprintf` and :c:func:`PyOS_vsnprintf` wrap the Standard C library
functions :c:func:`snprintf` and :c:func:`vsnprintf`. Their purpose is to
guarantee consistent behavior in corner cases, which the Standard C functions do
The wrappers ensure that ``str[size-1]`` is always ``'\0'`` upon return. They
never write more than *size* bytes (including the trailing ``'\0'``) into str.
Both functions require that ``str != NULL``, ``size > 0``, ``format != NULL``
and ``size < INT_MAX``. Note that this means there is no equivalent to the C99
``n = snprintf(NULL, 0, ...)`` which would determine the necessary buffer size.
The return value (*rv*) for these functions should be interpreted as follows:
* When ``0 <= rv < size``, the output conversion was successful and *rv*
characters were written to *str* (excluding the trailing ``'\0'`` byte at
* When ``rv >= size``, the output conversion was truncated and a buffer with
``rv + 1`` bytes would have been needed to succeed. ``str[size-1]`` is ``'\0'``
in this case.
* When ``rv < 0``, "something bad happened." ``str[size-1]`` is ``'\0'`` in
this case too, but the rest of *str* is undefined. The exact cause of the error
depends on the underlying platform.
The following functions provide locale-independent string to number conversions.
.. c:function:: double PyOS_string_to_double(const char *s, char **endptr, PyObject *overflow_exception)
Convert a string ``s`` to a :c:expr:`double`, raising a Python
exception on failure. The set of accepted strings corresponds to
the set of strings accepted by Python's :func:`float` constructor,
except that ``s`` must not have leading or trailing whitespace.
The conversion is independent of the current locale.
If ``endptr`` is ``NULL``, convert the whole string. Raise
:exc:`ValueError` and return ``-1.0`` if the string is not a valid
representation of a floating-point number.
If endptr is not ``NULL``, convert as much of the string as
possible and set ``*endptr`` to point to the first unconverted
character. If no initial segment of the string is the valid
representation of a floating-point number, set ``*endptr`` to point
to the beginning of the string, raise ValueError, and return
If ``s`` represents a value that is too large to store in a float
(for example, ``"1e500"`` is such a string on many platforms) then
if ``overflow_exception`` is ``NULL`` return ``Py_HUGE_VAL`` (with
an appropriate sign) and don't set any exception. Otherwise,
``overflow_exception`` must point to a Python exception object;
raise that exception and return ``-1.0``. In both cases, set
``*endptr`` to point to the first character after the converted value.
If any other error occurs during the conversion (for example an
out-of-memory error), set the appropriate Python exception and
return ``-1.0``.
.. versionadded:: 3.1
.. c:function:: char* PyOS_double_to_string(double val, char format_code, int precision, int flags, int *ptype)
Convert a :c:expr:`double` *val* to a string using supplied
*format_code*, *precision*, and *flags*.
*format_code* must be one of ``'e'``, ``'E'``, ``'f'``, ``'F'``,
``'g'``, ``'G'`` or ``'r'``. For ``'r'``, the supplied *precision*
must be 0 and is ignored. The ``'r'`` format code specifies the
standard :func:`repr` format.
*flags* can be zero or more of the values ``Py_DTSF_SIGN``,
``Py_DTSF_ADD_DOT_0``, or ``Py_DTSF_ALT``, or-ed together:
* ``Py_DTSF_SIGN`` means to always precede the returned string with a sign
character, even if *val* is non-negative.
* ``Py_DTSF_ADD_DOT_0`` means to ensure that the returned string will not look
like an integer.
* ``Py_DTSF_ALT`` means to apply "alternate" formatting rules. See the
documentation for the :c:func:`PyOS_snprintf` ``'#'`` specifier for
If *ptype* is non-``NULL``, then the value it points to will be set to one of
``Py_DTST_FINITE``, ``Py_DTST_INFINITE``, or ``Py_DTST_NAN``, signifying that
*val* is a finite number, an infinite number, or not a number, respectively.
The return value is a pointer to *buffer* with the converted string or
``NULL`` if the conversion failed. The caller is responsible for freeing the
returned string by calling :c:func:`PyMem_Free`.
.. versionadded:: 3.1
.. c:function:: int PyOS_stricmp(const char *s1, const char *s2)
Case insensitive comparison of strings. The function works almost
identically to :c:func:`strcmp` except that it ignores the case.
.. c:function:: int PyOS_strnicmp(const char *s1, const char *s2, Py_ssize_t size)
Case insensitive comparison of strings. The function works almost
identically to :c:func:`strncmp` except that it ignores the case.