sandbox/morph / Doc / c-api / conversion.rst

String conversion and formatting

Functions for number conversion and formatted string output.

: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.