cpython/Doc/c-api/conversion.rst

.. highlightlang:: 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(2)`.


.. 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
   :manpage:`vsnprintf(2)`.

: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
not.

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`` and ``format !=
NULL``.

If the platform doesn't have :c:func:`vsnprintf` and the buffer size needed to
avoid truncation exceeds *size* by more than 512 bytes, Python aborts with a
*Py_FatalError*.

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
  *str*[*rv*]).

* 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:type:`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
   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
   ``-1.0``.

   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:: 2.7


.. c:function:: double PyOS_ascii_strtod(const char *nptr, char **endptr)

   Convert a string to a :c:type:`double`. This function behaves like the Standard C
   function :c:func:`strtod` does in the C locale. It does this without changing the
   current locale, since that would not be thread-safe.

   :c:func:`PyOS_ascii_strtod` should typically be used for reading configuration
   files or other non-user input that should be locale independent.

   See the Unix man page :manpage:`strtod(2)` for details.

   .. versionadded:: 2.4

   .. deprecated:: 2.7
      Use :c:func:`PyOS_string_to_double` instead.



.. c:function:: char* PyOS_ascii_formatd(char *buffer, size_t buf_len, const char *format, double d)

   Convert a :c:type:`double` to a string using the ``'.'`` as the decimal
   separator. *format* is a :c:func:`printf`\ -style format string specifying the
   number format. Allowed conversion characters are ``'e'``, ``'E'``, ``'f'``,
   ``'F'``, ``'g'`` and ``'G'``.

   The return value is a pointer to *buffer* with the converted string or NULL if
   the conversion failed.

   .. versionadded:: 2.4
   .. deprecated:: 2.7
      This function is removed in Python 2.7 and 3.1.  Use :func:`PyOS_double_to_string`
      instead.


.. c:function:: char* PyOS_double_to_string(double val, char format_code, int precision, int flags, int *ptype)

   Convert a :c:type:`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
     details.

   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:: 2.7


.. c:function:: double PyOS_ascii_atof(const char *nptr)

   Convert a string to a :c:type:`double` in a locale-independent way.

   See the Unix man page :manpage:`atof(2)` for details.

   .. versionadded:: 2.4

   .. deprecated:: 3.1
      Use :c:func:`PyOS_string_to_double` instead.


.. c:function:: char* PyOS_stricmp(char *s1, char *s2)

   Case insensitive comparison of strings. The function works almost
   identically to :c:func:`strcmp` except that it ignores the case.

   .. versionadded:: 2.6


.. c:function:: char* PyOS_strnicmp(char *s1, 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.

   .. versionadded:: 2.6