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Copy pathstring_format.h
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1274 lines (1095 loc) · 36.7 KB
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/*
string_format.h -- implementation of string.format().
It uses the Objects/stringlib conventions, so that it can be
compiled for both unicode and string objects.
*/
/* Defines for Python 2.6 compatability */
#if PY_VERSION_HEX < 0x03000000
#define PyLong_FromSsize_t _PyLong_FromSsize_t
#endif
/* Defines for more efficiently reallocating the string buffer */
#define INITIAL_SIZE_INCREMENT 100
#define SIZE_MULTIPLIER 2
#define MAX_SIZE_INCREMENT 3200
/************************************************************************/
/*********** Global data structures and forward declarations *********/
/************************************************************************/
/*
A SubString consists of the characters between two string or
unicode pointers.
*/
typedef struct {
STRINGLIB_CHAR *ptr;
STRINGLIB_CHAR *end;
} SubString;
/* forward declaration for recursion */
static PyObject *
build_string(SubString *input, PyObject *args, PyObject *kwargs,
int recursion_depth);
/************************************************************************/
/************************** Utility functions ************************/
/************************************************************************/
/* fill in a SubString from a pointer and length */
Py_LOCAL_INLINE(void)
SubString_init(SubString *str, STRINGLIB_CHAR *p, Py_ssize_t len)
{
str->ptr = p;
if (p == NULL)
str->end = NULL;
else
str->end = str->ptr + len;
}
/* return a new string. if str->ptr is NULL, return None */
Py_LOCAL_INLINE(PyObject *)
SubString_new_object(SubString *str)
{
if (str->ptr == NULL) {
Py_INCREF(Py_None);
return Py_None;
}
return STRINGLIB_NEW(str->ptr, str->end - str->ptr);
}
/* return a new string. if str->ptr is NULL, return None */
Py_LOCAL_INLINE(PyObject *)
SubString_new_object_or_empty(SubString *str)
{
if (str->ptr == NULL) {
return STRINGLIB_NEW(NULL, 0);
}
return STRINGLIB_NEW(str->ptr, str->end - str->ptr);
}
/************************************************************************/
/*********** Output string management functions ****************/
/************************************************************************/
typedef struct {
STRINGLIB_CHAR *ptr;
STRINGLIB_CHAR *end;
PyObject *obj;
Py_ssize_t size_increment;
} OutputString;
/* initialize an OutputString object, reserving size characters */
static int
output_initialize(OutputString *output, Py_ssize_t size)
{
output->obj = STRINGLIB_NEW(NULL, size);
if (output->obj == NULL)
return 0;
output->ptr = STRINGLIB_STR(output->obj);
output->end = STRINGLIB_LEN(output->obj) + output->ptr;
output->size_increment = INITIAL_SIZE_INCREMENT;
return 1;
}
/*
output_extend reallocates the output string buffer.
It returns a status: 0 for a failed reallocation,
1 for success.
*/
static int
output_extend(OutputString *output, Py_ssize_t count)
{
STRINGLIB_CHAR *startptr = STRINGLIB_STR(output->obj);
Py_ssize_t curlen = output->ptr - startptr;
Py_ssize_t maxlen = curlen + count + output->size_increment;
if (STRINGLIB_RESIZE(&output->obj, maxlen) < 0)
return 0;
startptr = STRINGLIB_STR(output->obj);
output->ptr = startptr + curlen;
output->end = startptr + maxlen;
if (output->size_increment < MAX_SIZE_INCREMENT)
output->size_increment *= SIZE_MULTIPLIER;
return 1;
}
/*
output_data dumps characters into our output string
buffer.
In some cases, it has to reallocate the string.
It returns a status: 0 for a failed reallocation,
1 for success.
*/
static int
output_data(OutputString *output, const STRINGLIB_CHAR *s, Py_ssize_t count)
{
if ((count > output->end - output->ptr) && !output_extend(output, count))
return 0;
memcpy(output->ptr, s, count * sizeof(STRINGLIB_CHAR));
output->ptr += count;
return 1;
}
/************************************************************************/
/*********** Format string parsing -- integers and identifiers *********/
/************************************************************************/
static Py_ssize_t
get_integer(const SubString *str)
{
Py_ssize_t accumulator = 0;
Py_ssize_t digitval;
Py_ssize_t oldaccumulator;
STRINGLIB_CHAR *p;
/* empty string is an error */
if (str->ptr >= str->end)
return -1;
for (p = str->ptr; p < str->end; p++) {
digitval = STRINGLIB_TODECIMAL(*p);
if (digitval < 0)
return -1;
/*
This trick was copied from old Unicode format code. It's cute,
but would really suck on an old machine with a slow divide
implementation. Fortunately, in the normal case we do not
expect too many digits.
*/
oldaccumulator = accumulator;
accumulator *= 10;
if ((accumulator+10)/10 != oldaccumulator+1) {
PyErr_Format(PyExc_ValueError,
"Too many decimal digits in format string");
return -1;
}
accumulator += digitval;
}
return accumulator;
}
/************************************************************************/
/******** Functions to get field objects and specification strings ******/
/************************************************************************/
/* do the equivalent of obj.name */
static PyObject *
getattr(PyObject *obj, SubString *name)
{
PyObject *newobj;
PyObject *str = SubString_new_object(name);
if (str == NULL)
return NULL;
newobj = PyObject_GetAttr(obj, str);
Py_DECREF(str);
return newobj;
}
/* do the equivalent of obj[idx], where obj is a sequence */
static PyObject *
getitem_sequence(PyObject *obj, Py_ssize_t idx)
{
return PySequence_GetItem(obj, idx);
}
/* do the equivalent of obj[idx], where obj is not a sequence */
static PyObject *
getitem_idx(PyObject *obj, Py_ssize_t idx)
{
PyObject *newobj;
PyObject *idx_obj = PyLong_FromSsize_t(idx);
if (idx_obj == NULL)
return NULL;
newobj = PyObject_GetItem(obj, idx_obj);
Py_DECREF(idx_obj);
return newobj;
}
/* do the equivalent of obj[name] */
static PyObject *
getitem_str(PyObject *obj, SubString *name)
{
PyObject *newobj;
PyObject *str = SubString_new_object(name);
if (str == NULL)
return NULL;
newobj = PyObject_GetItem(obj, str);
Py_DECREF(str);
return newobj;
}
typedef struct {
/* the entire string we're parsing. we assume that someone else
is managing its lifetime, and that it will exist for the
lifetime of the iterator. can be empty */
SubString str;
/* pointer to where we are inside field_name */
STRINGLIB_CHAR *ptr;
} FieldNameIterator;
static int
FieldNameIterator_init(FieldNameIterator *self, STRINGLIB_CHAR *ptr,
Py_ssize_t len)
{
SubString_init(&self->str, ptr, len);
self->ptr = self->str.ptr;
return 1;
}
static int
_FieldNameIterator_attr(FieldNameIterator *self, SubString *name)
{
STRINGLIB_CHAR c;
name->ptr = self->ptr;
/* return everything until '.' or '[' */
while (self->ptr < self->str.end) {
switch (c = *self->ptr++) {
case '[':
case '.':
/* backup so that we this character will be seen next time */
self->ptr--;
break;
default:
continue;
}
break;
}
/* end of string is okay */
name->end = self->ptr;
return 1;
}
static int
_FieldNameIterator_item(FieldNameIterator *self, SubString *name)
{
int bracket_seen = 0;
STRINGLIB_CHAR c;
name->ptr = self->ptr;
/* return everything until ']' */
while (self->ptr < self->str.end) {
switch (c = *self->ptr++) {
case ']':
bracket_seen = 1;
break;
default:
continue;
}
break;
}
/* make sure we ended with a ']' */
if (!bracket_seen) {
PyErr_SetString(PyExc_ValueError, "Missing ']' in format string");
return 0;
}
/* end of string is okay */
/* don't include the ']' */
name->end = self->ptr-1;
return 1;
}
/* returns 0 on error, 1 on non-error termination, and 2 if it returns a value */
static int
FieldNameIterator_next(FieldNameIterator *self, int *is_attribute,
Py_ssize_t *name_idx, SubString *name)
{
/* check at end of input */
if (self->ptr >= self->str.end)
return 1;
switch (*self->ptr++) {
case '.':
*is_attribute = 1;
if (_FieldNameIterator_attr(self, name) == 0)
return 0;
*name_idx = -1;
break;
case '[':
*is_attribute = 0;
if (_FieldNameIterator_item(self, name) == 0)
return 0;
*name_idx = get_integer(name);
break;
default:
/* interal error, can't get here */
assert(0);
return 0;
}
/* empty string is an error */
if (name->ptr == name->end) {
PyErr_SetString(PyExc_ValueError, "Empty attribute in format string");
return 0;
}
return 2;
}
/* input: field_name
output: 'first' points to the part before the first '[' or '.'
'first_idx' is -1 if 'first' is not an integer, otherwise
it's the value of first converted to an integer
'rest' is an iterator to return the rest
*/
static int
field_name_split(STRINGLIB_CHAR *ptr, Py_ssize_t len, SubString *first,
Py_ssize_t *first_idx, FieldNameIterator *rest)
{
STRINGLIB_CHAR c;
STRINGLIB_CHAR *p = ptr;
STRINGLIB_CHAR *end = ptr + len;
/* find the part up until the first '.' or '[' */
while (p < end) {
switch (c = *p++) {
case '[':
case '.':
/* backup so that we this character is available to the
"rest" iterator */
p--;
break;
default:
continue;
}
break;
}
/* set up the return values */
SubString_init(first, ptr, p - ptr);
FieldNameIterator_init(rest, p, end - p);
/* see if "first" is an integer, in which case it's used as an index */
*first_idx = get_integer(first);
/* zero length string is an error */
if (first->ptr >= first->end) {
PyErr_SetString(PyExc_ValueError, "empty field name");
goto error;
}
return 1;
error:
return 0;
}
/*
get_field_object returns the object inside {}, before the
format_spec. It handles getindex and getattr lookups and consumes
the entire input string.
*/
static PyObject *
get_field_object(SubString *input, PyObject *args, PyObject *kwargs)
{
PyObject *obj = NULL;
int ok;
int is_attribute;
SubString name;
SubString first;
Py_ssize_t index;
FieldNameIterator rest;
if (!field_name_split(input->ptr, input->end - input->ptr, &first,
&index, &rest)) {
goto error;
}
if (index == -1) {
/* look up in kwargs */
PyObject *key = SubString_new_object(&first);
if (key == NULL)
goto error;
if ((kwargs == NULL) || (obj = PyDict_GetItem(kwargs, key)) == NULL) {
PyErr_SetObject(PyExc_KeyError, key);
Py_DECREF(key);
goto error;
}
Py_DECREF(key);
Py_INCREF(obj);
}
else {
/* look up in args */
obj = PySequence_GetItem(args, index);
if (obj == NULL)
goto error;
}
/* iterate over the rest of the field_name */
while ((ok = FieldNameIterator_next(&rest, &is_attribute, &index,
&name)) == 2) {
PyObject *tmp;
if (is_attribute)
/* getattr lookup "." */
tmp = getattr(obj, &name);
else
/* getitem lookup "[]" */
if (index == -1)
tmp = getitem_str(obj, &name);
else
if (PySequence_Check(obj))
tmp = getitem_sequence(obj, index);
else
/* not a sequence */
tmp = getitem_idx(obj, index);
if (tmp == NULL)
goto error;
/* assign to obj */
Py_DECREF(obj);
obj = tmp;
}
/* end of iterator, this is the non-error case */
if (ok == 1)
return obj;
error:
Py_XDECREF(obj);
return NULL;
}
/************************************************************************/
/***************** Field rendering functions **************************/
/************************************************************************/
/*
render_field() is the main function in this section. It takes the
field object and field specification string generated by
get_field_and_spec, and renders the field into the output string.
render_field calls fieldobj.__format__(format_spec) method, and
appends to the output.
*/
static int
render_field(PyObject *fieldobj, SubString *format_spec, OutputString *output)
{
int ok = 0;
PyObject *result = NULL;
PyObject *format_spec_object = NULL;
PyObject *(*formatter)(PyObject *, STRINGLIB_CHAR *, Py_ssize_t) = NULL;
STRINGLIB_CHAR* format_spec_start = format_spec->ptr ?
format_spec->ptr : NULL;
Py_ssize_t format_spec_len = format_spec->ptr ?
format_spec->end - format_spec->ptr : 0;
/* If we know the type exactly, skip the lookup of __format__ and just
call the formatter directly. */
#if STRINGLIB_IS_UNICODE
if (PyUnicode_CheckExact(fieldobj))
formatter = _PyUnicode_FormatAdvanced;
/* Unfortunately, there's a problem with checking for int, long,
and float here. If we're being included as unicode, their
formatters expect string format_spec args. For now, just skip
this optimization for unicode. This could be fixed, but it's a
hassle. */
#else
if (PyString_CheckExact(fieldobj))
formatter = _PyBytes_FormatAdvanced;
else if (PyInt_CheckExact(fieldobj))
formatter =_PyInt_FormatAdvanced;
else if (PyLong_CheckExact(fieldobj))
formatter =_PyLong_FormatAdvanced;
else if (PyFloat_CheckExact(fieldobj))
formatter = _PyFloat_FormatAdvanced;
#endif
if (formatter) {
/* we know exactly which formatter will be called when __format__ is
looked up, so call it directly, instead. */
result = formatter(fieldobj, format_spec_start, format_spec_len);
}
else {
/* We need to create an object out of the pointers we have, because
__format__ takes a string/unicode object for format_spec. */
format_spec_object = STRINGLIB_NEW(format_spec_start,
format_spec_len);
if (format_spec_object == NULL)
goto done;
result = PyObject_Format(fieldobj, format_spec_object);
}
if (result == NULL)
goto done;
#if PY_VERSION_HEX >= 0x03000000
assert(PyUnicode_Check(result));
#else
assert(PyString_Check(result) || PyUnicode_Check(result));
/* Convert result to our type. We could be str, and result could
be unicode */
{
PyObject *tmp = STRINGLIB_TOSTR(result);
if (tmp == NULL)
goto done;
Py_DECREF(result);
result = tmp;
}
#endif
ok = output_data(output,
STRINGLIB_STR(result), STRINGLIB_LEN(result));
done:
Py_XDECREF(format_spec_object);
Py_XDECREF(result);
return ok;
}
static int
parse_field(SubString *str, SubString *field_name, SubString *format_spec,
STRINGLIB_CHAR *conversion)
{
STRINGLIB_CHAR c = 0;
/* initialize these, as they may be empty */
*conversion = '\0';
SubString_init(format_spec, NULL, 0);
/* search for the field name. it's terminated by the end of the
string, or a ':' or '!' */
field_name->ptr = str->ptr;
while (str->ptr < str->end) {
switch (c = *(str->ptr++)) {
case ':':
case '!':
break;
default:
continue;
}
break;
}
if (c == '!' || c == ':') {
/* we have a format specifier and/or a conversion */
/* don't include the last character */
field_name->end = str->ptr-1;
/* the format specifier is the rest of the string */
format_spec->ptr = str->ptr;
format_spec->end = str->end;
/* see if there's a conversion specifier */
if (c == '!') {
/* there must be another character present */
if (format_spec->ptr >= format_spec->end) {
PyErr_SetString(PyExc_ValueError,
"end of format while looking for conversion "
"specifier");
return 0;
}
*conversion = *(format_spec->ptr++);
/* if there is another character, it must be a colon */
if (format_spec->ptr < format_spec->end) {
c = *(format_spec->ptr++);
if (c != ':') {
PyErr_SetString(PyExc_ValueError,
"expected ':' after format specifier");
return 0;
}
}
}
return 1;
}
else {
/* end of string, there's no format_spec or conversion */
field_name->end = str->ptr;
return 1;
}
}
/************************************************************************/
/******* Output string allocation and escape-to-markup processing ******/
/************************************************************************/
/* MarkupIterator breaks the string into pieces of either literal
text, or things inside {} that need to be marked up. it is
designed to make it easy to wrap a Python iterator around it, for
use with the Formatter class */
typedef struct {
SubString str;
} MarkupIterator;
static int
MarkupIterator_init(MarkupIterator *self, STRINGLIB_CHAR *ptr, Py_ssize_t len)
{
SubString_init(&self->str, ptr, len);
return 1;
}
/* returns 0 on error, 1 on non-error termination, and 2 if it got a
string (or something to be expanded) */
static int
MarkupIterator_next(MarkupIterator *self, SubString *literal,
SubString *field_name, SubString *format_spec,
STRINGLIB_CHAR *conversion,
int *format_spec_needs_expanding)
{
int at_end;
STRINGLIB_CHAR c = 0;
STRINGLIB_CHAR *start;
int count;
Py_ssize_t len;
int markup_follows = 0;
/* initialize all of the output variables */
SubString_init(literal, NULL, 0);
SubString_init(field_name, NULL, 0);
SubString_init(format_spec, NULL, 0);
*conversion = '\0';
*format_spec_needs_expanding = 0;
/* No more input, end of iterator. This is the normal exit
path. */
if (self->str.ptr >= self->str.end)
return 1;
start = self->str.ptr;
/* First read any literal text. Read until the end of string, an
escaped '{' or '}', or an unescaped '{'. In order to never
allocate memory and so I can just pass pointers around, if
there's an escaped '{' or '}' then we'll return the literal
including the brace, but no format object. The next time
through, we'll return the rest of the literal, skipping past
the second consecutive brace. */
while (self->str.ptr < self->str.end) {
switch (c = *(self->str.ptr++)) {
case '{':
case '}':
markup_follows = 1;
break;
default:
continue;
}
break;
}
at_end = self->str.ptr >= self->str.end;
len = self->str.ptr - start;
if ((c == '}') && (at_end || (c != *self->str.ptr))) {
PyErr_SetString(PyExc_ValueError, "Single '}' encountered "
"in format string");
return 0;
}
if (at_end && c == '{') {
PyErr_SetString(PyExc_ValueError, "Single '{' encountered "
"in format string");
return 0;
}
if (!at_end) {
if (c == *self->str.ptr) {
/* escaped } or {, skip it in the input. there is no
markup object following us, just this literal text */
self->str.ptr++;
markup_follows = 0;
}
else
len--;
}
/* record the literal text */
literal->ptr = start;
literal->end = start + len;
if (!markup_follows)
return 2;
/* this is markup, find the end of the string by counting nested
braces. note that this prohibits escaped braces, so that
format_specs cannot have braces in them. */
count = 1;
start = self->str.ptr;
/* we know we can't have a zero length string, so don't worry
about that case */
while (self->str.ptr < self->str.end) {
switch (c = *(self->str.ptr++)) {
case '{':
/* the format spec needs to be recursively expanded.
this is an optimization, and not strictly needed */
*format_spec_needs_expanding = 1;
count++;
break;
case '}':
count--;
if (count <= 0) {
/* we're done. parse and get out */
SubString s;
SubString_init(&s, start, self->str.ptr - 1 - start);
if (parse_field(&s, field_name, format_spec, conversion) == 0)
return 0;
/* a zero length field_name is an error */
if (field_name->ptr == field_name->end) {
PyErr_SetString(PyExc_ValueError, "zero length field name "
"in format");
return 0;
}
/* success */
return 2;
}
break;
}
}
/* end of string while searching for matching '}' */
PyErr_SetString(PyExc_ValueError, "unmatched '{' in format");
return 0;
}
/* do the !r or !s conversion on obj */
static PyObject *
do_conversion(PyObject *obj, STRINGLIB_CHAR conversion)
{
/* XXX in pre-3.0, do we need to convert this to unicode, since it
might have returned a string? */
switch (conversion) {
case 'r':
return PyObject_Repr(obj);
case 's':
return STRINGLIB_TOSTR(obj);
default:
if (conversion > 32 && conversion < 127) {
/* It's the ASCII subrange; casting to char is safe
(assuming the execution character set is an ASCII
superset). */
PyErr_Format(PyExc_ValueError,
"Unknown conversion specifier %c",
(char)conversion);
} else
PyErr_Format(PyExc_ValueError,
"Unknown conversion specifier \\x%x",
(unsigned int)conversion);
return NULL;
}
}
/* given:
{field_name!conversion:format_spec}
compute the result and write it to output.
format_spec_needs_expanding is an optimization. if it's false,
just output the string directly, otherwise recursively expand the
format_spec string. */
static int
output_markup(SubString *field_name, SubString *format_spec,
int format_spec_needs_expanding, STRINGLIB_CHAR conversion,
OutputString *output, PyObject *args, PyObject *kwargs,
int recursion_depth)
{
PyObject *tmp = NULL;
PyObject *fieldobj = NULL;
SubString expanded_format_spec;
SubString *actual_format_spec;
int result = 0;
/* convert field_name to an object */
fieldobj = get_field_object(field_name, args, kwargs);
if (fieldobj == NULL)
goto done;
if (conversion != '\0') {
tmp = do_conversion(fieldobj, conversion);
if (tmp == NULL)
goto done;
/* do the assignment, transferring ownership: fieldobj = tmp */
Py_DECREF(fieldobj);
fieldobj = tmp;
tmp = NULL;
}
/* if needed, recurively compute the format_spec */
if (format_spec_needs_expanding) {
tmp = build_string(format_spec, args, kwargs, recursion_depth-1);
if (tmp == NULL)
goto done;
/* note that in the case we're expanding the format string,
tmp must be kept around until after the call to
render_field. */
SubString_init(&expanded_format_spec,
STRINGLIB_STR(tmp), STRINGLIB_LEN(tmp));
actual_format_spec = &expanded_format_spec;
}
else
actual_format_spec = format_spec;
if (render_field(fieldobj, actual_format_spec, output) == 0)
goto done;
result = 1;
done:
Py_XDECREF(fieldobj);
Py_XDECREF(tmp);
return result;
}
/*
do_markup is the top-level loop for the format() method. It
searches through the format string for escapes to markup codes, and
calls other functions to move non-markup text to the output,
and to perform the markup to the output.
*/
static int
do_markup(SubString *input, PyObject *args, PyObject *kwargs,
OutputString *output, int recursion_depth)
{
MarkupIterator iter;
int format_spec_needs_expanding;
int result;
SubString literal;
SubString field_name;
SubString format_spec;
STRINGLIB_CHAR conversion;
MarkupIterator_init(&iter, input->ptr, input->end - input->ptr);
while ((result = MarkupIterator_next(&iter, &literal, &field_name,
&format_spec, &conversion,
&format_spec_needs_expanding)) == 2) {
if (!output_data(output, literal.ptr, literal.end - literal.ptr))
return 0;
if (field_name.ptr != field_name.end)
if (!output_markup(&field_name, &format_spec,
format_spec_needs_expanding, conversion, output,
args, kwargs, recursion_depth))
return 0;
}
return result;
}
/*
build_string allocates the output string and then
calls do_markup to do the heavy lifting.
*/
static PyObject *
build_string(SubString *input, PyObject *args, PyObject *kwargs,
int recursion_depth)
{
OutputString output;
PyObject *result = NULL;
Py_ssize_t count;
output.obj = NULL; /* needed so cleanup code always works */
/* check the recursion level */
if (recursion_depth <= 0) {
PyErr_SetString(PyExc_ValueError,
"Max string recursion exceeded");
goto done;
}
/* initial size is the length of the format string, plus the size
increment. seems like a reasonable default */
if (!output_initialize(&output,
input->end - input->ptr +
INITIAL_SIZE_INCREMENT))
goto done;
if (!do_markup(input, args, kwargs, &output, recursion_depth)) {
goto done;
}
count = output.ptr - STRINGLIB_STR(output.obj);
if (STRINGLIB_RESIZE(&output.obj, count) < 0) {
goto done;
}
/* transfer ownership to result */
result = output.obj;
output.obj = NULL;
done:
Py_XDECREF(output.obj);
return result;
}
/************************************************************************/
/*********** main routine ***********************************************/
/************************************************************************/
/* this is the main entry point */
static PyObject *
do_string_format(PyObject *self, PyObject *args, PyObject *kwargs)
{
SubString input;
/* PEP 3101 says only 2 levels, so that
"{0:{1}}".format('abc', 's') # works
"{0:{1:{2}}}".format('abc', 's', '') # fails
*/
int recursion_depth = 2;
SubString_init(&input, STRINGLIB_STR(self), STRINGLIB_LEN(self));
return build_string(&input, args, kwargs, recursion_depth);
}
/************************************************************************/
/*********** formatteriterator ******************************************/
/************************************************************************/
/* This is used to implement string.Formatter.vparse(). It exists so
Formatter can share code with the built in unicode.format() method.
It's really just a wrapper around MarkupIterator that is callable
from Python. */
typedef struct {
PyObject_HEAD
STRINGLIB_OBJECT *str;
MarkupIterator it_markup;
} formatteriterobject;
static void
formatteriter_dealloc(formatteriterobject *it)
{
Py_XDECREF(it->str);
PyObject_FREE(it);
}
/* returns a tuple:
(literal, field_name, format_spec, conversion)
literal is any literal text to output. might be zero length
field_name is the string before the ':'. might be None
format_spec is the string after the ':'. mibht be None
conversion is either None, or the string after the '!'
*/
static PyObject *
formatteriter_next(formatteriterobject *it)
{
SubString literal;
SubString field_name;
SubString format_spec;
STRINGLIB_CHAR conversion;
int format_spec_needs_expanding;
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