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.. highlightlang:: c
.. _string-conversion:
String conversion and formatting
================================
Functions for number conversion and formatted string output.
.. cfunction:: 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)`.
.. cfunction:: 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)`.
:cfunc:`PyOS_snprintf` and :cfunc:`PyOS_vsnprintf` wrap the Standard C library
functions :cfunc:`snprintf` and :cfunc:`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 :cfunc:`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.
.. cfunction:: double PyOS_ascii_strtod(const char *nptr, char **endptr)
Convert a string to a :ctype:`double`. This function behaves like the Standard C
function :cfunc:`strtod` does in the C locale. It does this without changing the
current locale, since that would not be thread-safe.
:cfunc:`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.
.. deprecated:: 3.1
Use :cfunc:`PyOS_string_to_double` instead.
.. cfunction:: double PyOS_string_to_double(const char *s, char **endptr, PyObject *overflow_exception)
Convert a string ``s`` to a :ctype:`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:: 3.1
.. cfunction:: char* PyOS_ascii_formatd(char *buffer, size_t buf_len, const char *format, double d)
Convert a :ctype:`double` to a string using the ``'.'`` as the decimal
separator. *format* is a :cfunc:`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.
.. deprecated:: 3.1
Use :cfunc:`PyOS_double_to_string` instead.
.. cfunction:: char* PyOS_double_to_string(double val, char format_code, int precision, int flags, int *ptype)
Convert a :ctype:`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 :cfunc:`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 :cfunc:`PyMem_Free`.
.. versionadded:: 3.1
.. cfunction:: double PyOS_ascii_atof(const char *nptr)
Convert a string to a :ctype:`double` in a locale-independent way.
See the Unix man page :manpage:`atof(2)` for details.
.. deprecated:: 3.1
Use PyOS_string_to_double instead.
.. cfunction:: char* PyOS_stricmp(char *s1, char *s2)
Case insensitive comparison of strings. The function works almost
identically to :cfunc:`strcmp` except that it ignores the case.
.. cfunction:: char* PyOS_strnicmp(char *s1, char *s2, Py_ssize_t size)
Case insensitive comparison of strings. The function works almost
identically to :cfunc:`strncmp` except that it ignores the case.
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