python_doc_md/Advanced/c-api/conversion.md

String conversion and formatting {#string-conversion}

Functions for number conversion and formatted string output.

Output not more than size bytes to str according to the format string format and the extra arguments. See the Unix man page snprintf(3){.interpreted-text role="manpage"}.

Output not more than size bytes to str according to the format string format and the variable argument list va. Unix man page vsnprintf(3){.interpreted-text role="manpage"}.

PyOS_snprintf{.interpreted-text role="c:func"} and PyOS_vsnprintf{.interpreted-text role="c:func"} wrap the Standard C library functions snprintf{.interpreted-text role="c:func"} and vsnprintf{.interpreted-text role="c:func"}. 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, format != NULL and size < INT_MAX. Note that this means there is no equivalent to the C99 n = snprintf(NULL, 0, ...) which would determine the necessary buffer size.

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, the output conversion failed and 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.

Convert the initial part of the string in str to an unsigned long{.interpreted-text role="c:expr"} value according to the given base, which must be between 2 and 36 inclusive, or be the special value 0.

Leading white space and case of characters are ignored. If base is zero it looks for a leading 0b, 0o or 0x to tell which base. If these are absent it defaults to 10. Base must be 0 or between 2 and 36 (inclusive). If ptr is non-NULL it will contain a pointer to the end of the scan.

If the converted value falls out of range of corresponding return type, range error occurs (errno{.interpreted-text role="c:data"} is set to !ERANGE{.interpreted-text role="c:macro"}) and !ULONG_MAX{.interpreted-text role="c:macro"} is returned. If no conversion can be performed, 0 is returned.

See also the Unix man page strtoul(3){.interpreted-text role="manpage"}.

::: versionadded 3.2 :::

Convert the initial part of the string in str to an long{.interpreted-text role="c:expr"} value according to the given base, which must be between 2 and 36 inclusive, or be the special value 0.

Same as PyOS_strtoul{.interpreted-text role="c:func"}, but return a long{.interpreted-text role="c:expr"} value instead and LONG_MAX{.interpreted-text role="c:macro"} on overflows.

See also the Unix man page strtol(3){.interpreted-text role="manpage"}.

::: versionadded 3.2 :::

Convert a string s to a double{.interpreted-text role="c:expr"}, raising a Python exception on failure. The set of accepted strings corresponds to the set of strings accepted by Python's float{.interpreted-text role="func"} 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{.interpreted-text role="exc"} 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 !INFINITY{.interpreted-text role="c:macro"} (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 :::

Convert a double{.interpreted-text role="c:expr"} 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{.interpreted-text role="func"} format.

flags can be zero or more of the following values or-ed together:

Always precede the returned string with a sign character, even if val is non-negative.

Ensure that the returned string will not look like an integer.

Apply "alternate" formatting rules. See the documentation for the PyOS_snprintf{.interpreted-text role="c:func"} '#' specifier for details.

Negative zero is converted to positive zero.

::: versionadded 3.11 :::

If ptype is non-NULL, then the value it points to will be set to one of the following constants depending on the type of val:

*ptype type of val

---

          finite number
          infinite number
          not a number

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{.interpreted-text role="c:func"}.

::: versionadded 3.1 :::

Character classification and conversion

The following macros provide locale-independent (unlike the C standard library ctype.h) character classification and conversion. The argument must be a signed or unsigned char{.interpreted-text role="c:expr"}.

Return true if the character c is an alphanumeric character.

Return true if the character c is an alphabetic character (a-z and A-Z).

Return true if the character c is a decimal digit (0-9).

Return true if the character c is a lowercase ASCII letter (a-z).

Return true if the character c is an uppercase ASCII letter (A-Z).

Return true if the character c is a whitespace character (space, tab, carriage return, newline, vertical tab, or form feed).

Return true if the character c is a hexadecimal digit (0-9, a-f, and A-F).

Return the lowercase equivalent of the character c.

Return the uppercase equivalent of the character c.