Fix zero padding for string format specs (#8407)

* Fix zero padding for string format specs

Assisted-by: Codex:gpt-5

* Add string alignment format regression test

Assisted-by: Codex:gpt-5

Lib/test/test_str.py

@@ -1070,7 +1070,7 @@ class StrTest(string_tests.StringLikeTest,
         '\U00100000'.ljust(3, '\U00010000')
         '\U00100000'.rjust(3, '\U00010000')
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; '{0:08s}'.format('result') misalign — '0' fill treated as numeric zero-pad for str type
+    @unittest.expectedFailure  # TODO: RUSTPYTHON; '{0.}'.format() raises ValueError instead of IndexError
     def test_format(self):
         self.assertEqual(''.format(), '')
         self.assertEqual('a'.format(), 'a')

crates/common/src/format.rs

@@ -227,6 +227,7 @@ pub struct FormatSpec {
     conversion: Option<FormatConversion>,
     fill: Option<CodePoint>,
     align: Option<FormatAlign>,
+    align_specified: bool,
     sign: Option<FormatSign>,
     alternate_form: bool,
     width: Option<usize>,
@@ -346,6 +347,7 @@ impl FormatSpec {
         // get_integer in CPython
         let (conversion, text) = FormatConversion::parse(text);
         let (mut fill, mut align, text) = parse_fill_and_align(text);
+        let align_specified = align.is_some();
         let (sign, text) = FormatSign::parse(text);
         let (alternate_form, text) = parse_alternate_form(text);
         let (zero, text) = parse_zero(text);
@@ -374,6 +376,7 @@ impl FormatSpec {
             conversion,
             fill,
             align,
+            align_specified,
             sign,
             alternate_form,
             width,
@@ -1020,13 +1023,24 @@ impl FormatSpec {
         self.validate_format(FormatType::String)?;
         match self.format_type {
             Some(FormatType::String) | None => {
+                if self.align == Some(FormatAlign::AfterSign) && self.align_specified {
+                    return Err(FormatSpecError::StringAlignmentFlag);
+                }
                 // CPython parity: precision truncates BEFORE width pads.
                 // `'{:3.2s}'.format('abc')` -> 'ab ' (truncate to 'ab', pad to 3).
                 let truncated: String = match self.precision {
                     Some(p) => s.deref().chars().take(p).collect(),
                     None => s.deref().to_owned(),
                 };
-                Ok(self.format_sign_and_align(&truncated, "", FormatAlign::Left))
+                let spec = Self {
+                    align: if self.align == Some(FormatAlign::AfterSign) {
+                        Some(FormatAlign::Left)
+                    } else {
+                        self.align
+                    },
+                    ..*self
+                };
+                Ok(spec.format_sign_and_align(&truncated, "", FormatAlign::Left))
             }
             _ => {
                 let ch = char::from(self.format_type.as_ref().unwrap());
@@ -1260,6 +1274,7 @@ pub enum FormatSpecError {
     CodeNotInRange,
     ZeroPadding,
     AlignmentFlag,
+    StringAlignmentFlag,
     NotImplemented(char, &'static str),
 }
 
@@ -1609,6 +1624,7 @@ mod tests {
             conversion: None,
             fill: None,
             align: None,
+            align_specified: false,
             sign: None,
             alternate_form: false,
             width: Some(33),
@@ -1626,6 +1642,7 @@ mod tests {
             conversion: None,
             fill: Some('<'.into()),
             align: Some(FormatAlign::Right),
+            align_specified: true,
             sign: None,
             alternate_form: false,
             width: Some(33),
@@ -1643,6 +1660,7 @@ mod tests {
             conversion: None,
             fill: Some('<'.into()),
             align: Some(FormatAlign::Right),
+            align_specified: true,
             sign: Some(FormatSign::Minus),
             alternate_form: true,
             width: Some(23),
@@ -1690,6 +1708,35 @@ mod tests {
         assert_eq!(format_bool("%", false), Ok("0.000000%".to_owned()));
     }
 
+    #[test]
+    fn format_string_zero_padding_uses_left_alignment() {
+        let spec = FormatSpec::parse("08s").unwrap();
+        let value = "result".to_owned();
+
+        assert_eq!(spec.format_string(&value), Ok("result00".to_owned()));
+    }
+
+    #[test]
+    fn format_string_explicit_after_sign_alignment_is_invalid() {
+        let spec = FormatSpec::parse("=8s").unwrap();
+        let value = "result".to_owned();
+
+        assert_eq!(
+            spec.format_string(&value),
+            Err(FormatSpecError::StringAlignmentFlag)
+        );
+    }
+
+    #[test]
+    fn format_int_zero_padding_stays_after_sign() {
+        let spec = FormatSpec::parse("08").unwrap();
+
+        assert_eq!(
+            spec.format_int(&BigInt::from(-42)),
+            Ok("-0000042".to_owned())
+        );
+    }
+
     #[test]
     fn format_int() {
         assert_eq!(

crates/vm/src/format.rs

@@ -77,6 +77,9 @@ impl IntoPyException for FormatSpecError {
             Self::AlignmentFlag => {
                 vm.new_value_error("'=' alignment flag is not allowed in complex format specifier")
             }
+            Self::StringAlignmentFlag => {
+                vm.new_value_error("'=' alignment not allowed in string format specifier")
+            }
             Self::NotImplemented(c, s) => {
                 let msg = format!("Format code '{c}' for object of type '{s}' not implemented yet");
                 vm.new_value_error(msg)

extra_tests/snippets/builtin_format.py

@@ -24,6 +24,14 @@ def test_zero_padding():
 
 test_zero_padding()
 
+try:
+    format("result", "=8s")
+except ValueError as error:
+    if str(error) != "'=' alignment not allowed in string format specifier":
+        raise AssertionError(f"unexpected error message: {error}") from error
+else:
+    raise AssertionError("expected ValueError for '=8s' string format specifier")
+
 assert "{:,}".format(100) == "100"
 assert "{:,}".format(1024) == "1,024"
 assert "{:_}".format(65536) == "65_536"