bpo-45995: add "z" format specifer to coerce negative 0 to zero (GH-30049)

Add "z" format specifier to coerce negative 0 to zero.

See https://github.com/python/cpython/issues/90153 (originally https://bugs.python.org/issue45995) for discussion.
This covers `str.format()` and f-strings.  Old-style string interpolation is not supported.

Co-authored-by: Mark Dickinson <dickinsm@gmail.com>

1 files changed, 28 insertions, 0 deletions

diff --git a/Python/formatter_unicode.c b/Python/formatter_unicode.c
index a1e50e2..04d37c0 100644
--- a/Python/formatter_unicode.c
+++ b/Python/formatter_unicode.c
@@ -130,6 +130,7 @@ typedef struct {
     Py_UCS4 fill_char;
     Py_UCS4 align;
     int alternate;
+    int no_neg_0;
     Py_UCS4 sign;
     Py_ssize_t width;
     enum LocaleType thousands_separators;
@@ -166,6 +167,7 @@ parse_internal_render_format_spec(PyObject *obj,
     format->fill_char = ' ';
     format->align = default_align;
     format->alternate = 0;
+    format->no_neg_0 = 0;
     format->sign = '\0';
     format->width = -1;
     format->thousands_separators = LT_NO_LOCALE;
@@ -193,6 +195,13 @@ parse_internal_render_format_spec(PyObject *obj,
         ++pos;
     }
 
+    /* If the next character is z, request coercion of negative 0.
+       Applies only to floats. */
+    if (end-pos >= 1 && READ_spec(pos) == 'z') {
+        format->no_neg_0 = 1;
+        ++pos;
+    }
+
     /* If the next character is #, we're in alternate mode.  This only
        applies to integers. */
     if (end-pos >= 1 && READ_spec(pos) == '#') {
@@ -779,6 +788,14 @@ format_string_internal(PyObject *value, const InternalFormatSpec *format,
         goto done;
     }
 
+    /* negative 0 coercion is not allowed on strings */
+    if (format->no_neg_0) {
+        PyErr_SetString(PyExc_ValueError,
+                        "Negative zero coercion (z) not allowed in string format "
+                        "specifier");
+        goto done;
+    }
+
     /* alternate is not allowed on strings */
     if (format->alternate) {
         PyErr_SetString(PyExc_ValueError,
@@ -872,6 +889,13 @@ format_long_internal(PyObject *value, const InternalFormatSpec *format,
                         "Precision not allowed in integer format specifier");
         goto done;
     }
+    /* no negative zero coercion on integers */
+    if (format->no_neg_0) {
+        PyErr_SetString(PyExc_ValueError,
+                        "Negative zero coercion (z) not allowed in integer"
+                        " format specifier");
+        goto done;
+    }
 
     /* special case for character formatting */
     if (format->type == 'c') {
@@ -1049,6 +1073,8 @@ format_float_internal(PyObject *value,
 
     if (format->alternate)
         flags |= Py_DTSF_ALT;
+    if (format->no_neg_0)
+        flags |= Py_DTSF_NO_NEG_0;
 
     if (type == '\0') {
         /* Omitted type specifier.  Behaves in the same way as repr(x)
@@ -1238,6 +1264,8 @@ format_complex_internal(PyObject *value,
 
     if (format->alternate)
         flags |= Py_DTSF_ALT;
+    if (format->no_neg_0)
+        flags |= Py_DTSF_NO_NEG_0;
 
     if (type == '\0') {
         /* Omitted type specifier. Should be like str(self). */