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 2.7.

double PyOS_ascii_strtod(const char *nptr, char **endptr)

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

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 strtod(2) for details.

New in version 2.4.

Deprecated since version 2.7: Use PyOS_string_to_double() instead.

char* PyOS_ascii_formatd(char *buffer, size_t buf_len, const char *format, double d)

Convert a double to a string using the '.' as the decimal separator. format is a 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.

New in version 2.4.

Deprecated since version 2.7: This function is removed in Python 2.7 and 3.1. Use PyOS_double_to_string() instead.

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 2.7.

double PyOS_ascii_atof(const char *nptr)

Convert a string to a double in a locale-independent way.

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

New in version 2.4.

Deprecated since version 3.1: Use PyOS_string_to_double() instead.

char* PyOS_stricmp(char *s1, char *s2)

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

New in version 2.6.

char* PyOS_strnicmp(char *s1, char *s2, Py_ssize_t  size)

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

New in version 2.6.