String conversion and formatting — Python 3.8.4 documentation

# String conversion and formatting

Functions for number conversion and formatted string output.

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 snprintf(2).

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 vsnprintf(2).

`PyOS_snprintf()` and `PyOS_vsnprintf()` wrap the Standard C library functions `snprintf()` and `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 `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.

double `PyOS_string_to_double`(const char *s, char **endptr, PyObject *overflow_exception)

  Convert a string `s` to a `double`, raising a Python exception on failure. The set of accepted strings corresponds to the set of strings accepted by Python’s `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`.

  New in version 3.1.

char* `PyOS_double_to_string`(double val, char format_code, int precision, int flags, int *ptype)

  Convert a `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 `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 `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 `PyMem_Free()`.

  New in version 3.1.

int `PyOS_stricmp`(const char *s1, const char *s2)

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

int `PyOS_strnicmp`(const char *s1, const char *s2, Py_ssize_t size)

  Case insensitive comparison of strings. The function works almost identically to `strncmp()` except that it ignores the case.