bpo-45995: add "z" format specifer to coerce negative 0 to zero (GH-3…

…0049) Add "z" format specifier to coerce negative 0 to zero. See #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 <[email protected]>
python · Apr 11, 2022 · b0b836b · b0b836b
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diff --git a/Doc/library/string.rst b/Doc/library/string.rst
@@ -309,7 +309,7 @@ non-empty format specification typically modifies the result.
 The general form of a *standard format specifier* is:
 
 .. productionlist:: format-spec
-   format_spec: [[`fill`]`align`][`sign`][#][0][`width`][`grouping_option`][.`precision`][`type`]
+   format_spec: [[`fill`]`align`][`sign`][z][#][0][`width`][`grouping_option`][.`precision`][`type`]
    fill: <any character>
    align: "<" | ">" | "=" | "^"
    sign: "+" | "-" | " "
@@ -380,6 +380,15 @@ following:
    +---------+----------------------------------------------------------+
 
 
+.. index:: single: z; in string formatting
+
+The ``'z'`` option coerces negative zero floating-point values to positive
+zero after rounding to the format precision.  This option is only valid for
+floating-point presentation types.
+
+.. versionchanged:: 3.11
+   Added the ``'z'`` option (see also :pep:`682`).
+
 .. index:: single: # (hash); in string formatting
 
 The ``'#'`` option causes the "alternate form" to be used for the

diff --git a/Include/internal/pycore_format.h b/Include/internal/pycore_format.h
@@ -14,12 +14,14 @@ extern "C" {
  * F_BLANK      ' '
  * F_ALT        '#'
  * F_ZERO       '0'
+ * F_NO_NEG_0   'z'
  */
 #define F_LJUST (1<<0)
 #define F_SIGN  (1<<1)
 #define F_BLANK (1<<2)
 #define F_ALT   (1<<3)
 #define F_ZERO  (1<<4)
+#define F_NO_NEG_0 (1<<5)
 
 #ifdef __cplusplus
 }

diff --git a/Include/pystrtod.h b/Include/pystrtod.h
@@ -32,6 +32,7 @@ PyAPI_FUNC(double) _Py_parse_inf_or_nan(const char *p, char **endptr);
 #define Py_DTSF_ADD_DOT_0 0x02 /* if the result is an integer add ".0" */
 #define Py_DTSF_ALT       0x04 /* "alternate" formatting. it's format_code
                                   specific */
+#define Py_DTSF_NO_NEG_0  0x08 /* negative zero result is coerced to 0 */
 
 /* PyOS_double_to_string's "type", if non-NULL, will be set to one of: */
 #define Py_DTST_FINITE 0

diff --git a/Lib/_pydecimal.py b/Lib/_pydecimal.py
@@ -3795,6 +3795,10 @@ def __format__(self, specifier, context=None, _localeconv=None):
         # represented in fixed point; rescale them to 0e0.
         if not self and self._exp > 0 and spec['type'] in 'fF%':
             self = self._rescale(0, rounding)
+        if not self and spec['no_neg_0'] and self._sign:
+            adjusted_sign = 0
+        else:
+            adjusted_sign = self._sign
 
         # figure out placement of the decimal point
         leftdigits = self._exp + len(self._int)
@@ -3825,7 +3829,7 @@ def __format__(self, specifier, context=None, _localeconv=None):
 
         # done with the decimal-specific stuff;  hand over the rest
         # of the formatting to the _format_number function
-        return _format_number(self._sign, intpart, fracpart, exp, spec)
+        return _format_number(adjusted_sign, intpart, fracpart, exp, spec)
 
 def _dec_from_triple(sign, coefficient, exponent, special=False):
     """Create a decimal instance directly, without any validation,
@@ -6143,14 +6147,15 @@ def _convert_for_comparison(self, other, equality_op=False):
 #
 # A format specifier for Decimal looks like:
 #
-#   [[fill]align][sign][#][0][minimumwidth][,][.precision][type]
+#   [[fill]align][sign][z][#][0][minimumwidth][,][.precision][type]
 
 _parse_format_specifier_regex = re.compile(r"""\A
 (?:
    (?P<fill>.)?
    (?P<align>[<>=^])
 )?
 (?P<sign>[-+ ])?
+(?P<no_neg_0>z)?
 (?P<alt>\#)?
 (?P<zeropad>0)?
 (?P<minimumwidth>(?!0)\d+)?

diff --git a/Lib/pydoc_data/topics.py b/Lib/pydoc_data/topics.py
@@ -6119,7 +6119,7 @@
                   'The general form of a *standard format specifier* is:\n'
                   '\n'
                   '   format_spec     ::= '
-                  '[[fill]align][sign][#][0][width][grouping_option][.precision][type]\n'
+                  '[[fill]align][sign][z][#][0][width][grouping_option][.precision][type]\n'
                   '   fill            ::= <any character>\n'
                   '   align           ::= "<" | ">" | "=" | "^"\n'
                   '   sign            ::= "+" | "-" | " "\n'
@@ -6221,6 +6221,15 @@
                   '   '
                   '+-----------+------------------------------------------------------------+\n'
                   '\n'
+                  'The "\'z\'" option coerces negative zero floating-point '
+                  'values to positive\n'
+                  'zero after rounding to the format precision.  This option '
+                  'is only valid for\n'
+                  'floating-point presentation types.\n'
+                  '\n'
+                  'Changed in version 3.11: Added the "\'z\'" option (see also '
+                  '**PEP 682**).\n'
+                  '\n'
                   'The "\'#\'" option causes the “alternate form” to be used '
                   'for the\n'
                   'conversion.  The alternate form is defined differently for '

diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
@@ -1072,6 +1072,57 @@ def test_formatting(self):
             (',e', '123456', '1.23456e+5'),
             (',E', '123456', '1.23456E+5'),
 
+            # negative zero: default behavior
+            ('.1f', '-0', '-0.0'),
+            ('.1f', '-.0', '-0.0'),
+            ('.1f', '-.01', '-0.0'),
+
+            # negative zero: z option
+            ('z.1f', '0.', '0.0'),
+            ('z6.1f', '0.', '   0.0'),
+            ('z6.1f', '-1.', '  -1.0'),
+            ('z.1f', '-0.', '0.0'),
+            ('z.1f', '.01', '0.0'),
+            ('z.1f', '-.01', '0.0'),
+            ('z.2f', '0.', '0.00'),
+            ('z.2f', '-0.', '0.00'),
+            ('z.2f', '.001', '0.00'),
+            ('z.2f', '-.001', '0.00'),
+
+            ('z.1e', '0.', '0.0e+1'),
+            ('z.1e', '-0.', '0.0e+1'),
+            ('z.1E', '0.', '0.0E+1'),
+            ('z.1E', '-0.', '0.0E+1'),
+
+            ('z.2e', '-0.001', '-1.00e-3'),  # tests for mishandled rounding
+            ('z.2g', '-0.001', '-0.001'),
+            ('z.2%', '-0.001', '-0.10%'),
+
+            ('zf', '-0.0000', '0.0000'),  # non-normalized form is preserved
+
+            ('z.1f', '-00000.000001', '0.0'),
+            ('z.1f', '-00000.', '0.0'),
+            ('z.1f', '-.0000000000', '0.0'),
+
+            ('z.2f', '-00000.000001', '0.00'),
+            ('z.2f', '-00000.', '0.00'),
+            ('z.2f', '-.0000000000', '0.00'),
+
+            ('z.1f', '.09', '0.1'),
+            ('z.1f', '-.09', '-0.1'),
+
+            (' z.0f', '-0.', ' 0'),
+            ('+z.0f', '-0.', '+0'),
+            ('-z.0f', '-0.', '0'),
+            (' z.0f', '-1.', '-1'),
+            ('+z.0f', '-1.', '-1'),
+            ('-z.0f', '-1.', '-1'),
+
+            ('z>6.1f', '-0.', 'zz-0.0'),
+            ('z>z6.1f', '-0.', 'zzz0.0'),
+            ('x>z6.1f', '-0.', 'xxx0.0'),
+            ('🖤>z6.1f', '-0.', '🖤🖤🖤0.0'),  # multi-byte fill char
+
             # issue 6850
             ('a=-7.0', '0.12345', 'aaaa0.1'),
 
@@ -1086,6 +1137,15 @@ def test_formatting(self):
         # bytes format argument
         self.assertRaises(TypeError, Decimal(1).__format__, b'-020')
 
+    def test_negative_zero_format_directed_rounding(self):
+        with self.decimal.localcontext() as ctx:
+            ctx.rounding = ROUND_CEILING
+            self.assertEqual(format(self.decimal.Decimal('-0.001'), 'z.2f'),
+                            '0.00')
+
+    def test_negative_zero_bad_format(self):
+        self.assertRaises(ValueError, format, self.decimal.Decimal('1.23'), 'fz')
+
     def test_n_format(self):
         Decimal = self.decimal.Decimal
 

diff --git a/Lib/test/test_float.py b/Lib/test/test_float.py
@@ -701,18 +701,16 @@ def test_format(self):
         # conversion to string should fail
         self.assertRaises(ValueError, format, 3.0, "s")
 
-        # other format specifiers shouldn't work on floats,
-        #  in particular int specifiers
-        for format_spec in ([chr(x) for x in range(ord('a'), ord('z')+1)] +
-                            [chr(x) for x in range(ord('A'), ord('Z')+1)]):
-            if not format_spec in 'eEfFgGn%':
-                self.assertRaises(ValueError, format, 0.0, format_spec)
-                self.assertRaises(ValueError, format, 1.0, format_spec)
-                self.assertRaises(ValueError, format, -1.0, format_spec)
-                self.assertRaises(ValueError, format, 1e100, format_spec)
-                self.assertRaises(ValueError, format, -1e100, format_spec)
-                self.assertRaises(ValueError, format, 1e-100, format_spec)
-                self.assertRaises(ValueError, format, -1e-100, format_spec)
+        # confirm format options expected to fail on floats, such as integer
+        # presentation types
+        for format_spec in 'sbcdoxX':
+            self.assertRaises(ValueError, format, 0.0, format_spec)
+            self.assertRaises(ValueError, format, 1.0, format_spec)
+            self.assertRaises(ValueError, format, -1.0, format_spec)
+            self.assertRaises(ValueError, format, 1e100, format_spec)
+            self.assertRaises(ValueError, format, -1e100, format_spec)
+            self.assertRaises(ValueError, format, 1e-100, format_spec)
+            self.assertRaises(ValueError, format, -1e-100, format_spec)
 
         # issue 3382
         self.assertEqual(format(NAN, 'f'), 'nan')

diff --git a/Lib/test/test_format.py b/Lib/test/test_format.py
@@ -546,6 +546,80 @@ def test_unicode_in_error_message(self):
         with self.assertRaisesRegex(ValueError, str_err):
             "{a:%ЫйЯЧ}".format(a='a')
 
+    def test_negative_zero(self):
+        ## default behavior
+        self.assertEqual(f"{-0.:.1f}", "-0.0")
+        self.assertEqual(f"{-.01:.1f}", "-0.0")
+        self.assertEqual(f"{-0:.1f}", "0.0")  # integers do not distinguish -0
+
+        ## z sign option
+        self.assertEqual(f"{0.:z.1f}", "0.0")
+        self.assertEqual(f"{0.:z6.1f}", "   0.0")
+        self.assertEqual(f"{-1.:z6.1f}", "  -1.0")
+        self.assertEqual(f"{-0.:z.1f}", "0.0")
+        self.assertEqual(f"{.01:z.1f}", "0.0")
+        self.assertEqual(f"{-0:z.1f}", "0.0")  # z is allowed for integer input
+        self.assertEqual(f"{-.01:z.1f}", "0.0")
+        self.assertEqual(f"{0.:z.2f}", "0.00")
+        self.assertEqual(f"{-0.:z.2f}", "0.00")
+        self.assertEqual(f"{.001:z.2f}", "0.00")
+        self.assertEqual(f"{-.001:z.2f}", "0.00")
+
+        self.assertEqual(f"{0.:z.1e}", "0.0e+00")
+        self.assertEqual(f"{-0.:z.1e}", "0.0e+00")
+        self.assertEqual(f"{0.:z.1E}", "0.0E+00")
+        self.assertEqual(f"{-0.:z.1E}", "0.0E+00")
+
+        self.assertEqual(f"{-0.001:z.2e}", "-1.00e-03")  # tests for mishandled
+                                                         # rounding
+        self.assertEqual(f"{-0.001:z.2g}", "-0.001")
+        self.assertEqual(f"{-0.001:z.2%}", "-0.10%")
+
+        self.assertEqual(f"{-00000.000001:z.1f}", "0.0")
+        self.assertEqual(f"{-00000.:z.1f}", "0.0")
+        self.assertEqual(f"{-.0000000000:z.1f}", "0.0")
+
+        self.assertEqual(f"{-00000.000001:z.2f}", "0.00")
+        self.assertEqual(f"{-00000.:z.2f}", "0.00")
+        self.assertEqual(f"{-.0000000000:z.2f}", "0.00")
+
+        self.assertEqual(f"{.09:z.1f}", "0.1")
+        self.assertEqual(f"{-.09:z.1f}", "-0.1")
+
+        self.assertEqual(f"{-0.: z.0f}", " 0")
+        self.assertEqual(f"{-0.:+z.0f}", "+0")
+        self.assertEqual(f"{-0.:-z.0f}", "0")
+        self.assertEqual(f"{-1.: z.0f}", "-1")
+        self.assertEqual(f"{-1.:+z.0f}", "-1")
+        self.assertEqual(f"{-1.:-z.0f}", "-1")
+
+        self.assertEqual(f"{0.j:z.1f}", "0.0+0.0j")
+        self.assertEqual(f"{-0.j:z.1f}", "0.0+0.0j")
+        self.assertEqual(f"{.01j:z.1f}", "0.0+0.0j")
+        self.assertEqual(f"{-.01j:z.1f}", "0.0+0.0j")
+
+        self.assertEqual(f"{-0.:z>6.1f}", "zz-0.0")  # test fill, esp. 'z' fill
+        self.assertEqual(f"{-0.:z>z6.1f}", "zzz0.0")
+        self.assertEqual(f"{-0.:x>z6.1f}", "xxx0.0")
+        self.assertEqual(f"{-0.:🖤>z6.1f}", "🖤🖤🖤0.0")  # multi-byte fill char
+
+    def test_specifier_z_error(self):
+        error_msg = re.compile("Invalid format specifier '.*z.*'")
+        with self.assertRaisesRegex(ValueError, error_msg):
+            f"{0:z+f}"  # wrong position
+        with self.assertRaisesRegex(ValueError, error_msg):
+            f"{0:fz}"  # wrong position
+
+        error_msg = re.escape("Negative zero coercion (z) not allowed")
+        with self.assertRaisesRegex(ValueError, error_msg):
+            f"{0:zd}"  # can't apply to int presentation type
+        with self.assertRaisesRegex(ValueError, error_msg):
+            f"{'x':zs}"  # can't apply to string
+
+        error_msg = re.escape("unsupported format character 'z'")
+        with self.assertRaisesRegex(ValueError, error_msg):
+            "%z.1f" % 0  # not allowed in old style string interpolation
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
@@ -524,18 +524,16 @@ def test(f, format_spec, result):
         self.assertRaises(TypeError, 3.0.__format__, None)
         self.assertRaises(TypeError, 3.0.__format__, 0)
 
-        # other format specifiers shouldn't work on floats,
-        #  in particular int specifiers
-        for format_spec in ([chr(x) for x in range(ord('a'), ord('z')+1)] +
-                            [chr(x) for x in range(ord('A'), ord('Z')+1)]):
-            if not format_spec in 'eEfFgGn%':
-                self.assertRaises(ValueError, format, 0.0, format_spec)
-                self.assertRaises(ValueError, format, 1.0, format_spec)
-                self.assertRaises(ValueError, format, -1.0, format_spec)
-                self.assertRaises(ValueError, format, 1e100, format_spec)
-                self.assertRaises(ValueError, format, -1e100, format_spec)
-                self.assertRaises(ValueError, format, 1e-100, format_spec)
-                self.assertRaises(ValueError, format, -1e-100, format_spec)
+        # confirm format options expected to fail on floats, such as integer
+        # presentation types
+        for format_spec in 'sbcdoxX':
+            self.assertRaises(ValueError, format, 0.0, format_spec)
+            self.assertRaises(ValueError, format, 1.0, format_spec)
+            self.assertRaises(ValueError, format, -1.0, format_spec)
+            self.assertRaises(ValueError, format, 1e100, format_spec)
+            self.assertRaises(ValueError, format, -1e100, format_spec)
+            self.assertRaises(ValueError, format, 1e-100, format_spec)
+            self.assertRaises(ValueError, format, -1e-100, format_spec)
 
         # Alternate float formatting
         test(1.0, '.0e', '1e+00')

diff --git a/Misc/NEWS.d/next/Library/2021-12-14-13-15-41.bpo-45995.Am9pNL.rst b/Misc/NEWS.d/next/Library/2021-12-14-13-15-41.bpo-45995.Am9pNL.rst
@@ -0,0 +1,3 @@
+Add a "z" option to the string formatting specification that coerces negative
+zero floating-point values to positive zero after rounding to the format
+precision.  Contributed by John Belmonte.