Reject the string format flags CPython refuses (#8571)

A sign, a space or an alternate form parsed fine on a string spec and
then had no effect:

    >>> format('ab', '+')
    'ab'
    >>> f"{'ab':#5}"
    'ab   '

CPython raises ValueError for all three. format_string already refused z
and an explicit '=' alignment with the same family of messages, so the
three checks go next to those, in the order CPython reports them: sign,
then z, then the alternate form, then the alignment.

Assisted-by: Claude Code:claude-opus-5

crates/common/src/format.rs

@@ -1112,9 +1112,19 @@ impl FormatSpec {
         self.validate_format(FormatType::String)?;
         match self.format_type {
             Some(FormatType::String) | None => {
+                // CPython rejects these four in this order: sign, z, #, then '='.
+                if let Some(sign) = self.sign {
+                    return Err(FormatSpecError::StringSpecNotAllowed(match sign {
+                        FormatSign::MinusOrSpace => "Space",
+                        FormatSign::Plus | FormatSign::Minus => "Sign",
+                    }));
+                }
                 if self.no_neg_0 {
                     return Err(FormatSpecError::NegativeZeroCoercionNotAllowed("string"));
                 }
+                if self.alternate_form {
+                    return Err(FormatSpecError::StringSpecNotAllowed("Alternate form (#)"));
+                }
                 if self.align == Some(FormatAlign::AfterSign) && self.align_specified {
                     return Err(FormatSpecError::StringAlignmentFlag);
                 }
@@ -1363,6 +1373,7 @@ pub enum FormatSpecError {
     AlignmentFlag,
     NegativeZeroCoercionNotAllowed(&'static str),
     StringAlignmentFlag,
+    StringSpecNotAllowed(&'static str),
     NotImplemented(char, &'static str),
 }
 
@@ -1845,6 +1856,52 @@ mod tests {
         );
     }
 
+    #[test]
+    fn format_string_rejects_sign_space_and_alternate_form() {
+        let value = "result".to_owned();
+        let cases = [
+            ("+", "Sign"),
+            ("-", "Sign"),
+            ("+8s", "Sign"),
+            (" ", "Space"),
+            (" 8s", "Space"),
+            ("#", "Alternate form (#)"),
+            ("#8s", "Alternate form (#)"),
+        ];
+
+        for (text, flag) in cases {
+            let spec = FormatSpec::parse(text).unwrap();
+            assert_eq!(
+                spec.format_string(&value),
+                Err(FormatSpecError::StringSpecNotAllowed(flag)),
+                "{text}"
+            );
+        }
+    }
+
+    #[test]
+    fn format_string_reports_the_flag_cpython_reports_first() {
+        let value = "result".to_owned();
+        // Sign beats z, z beats the alternate form, and the alternate form
+        // beats an explicit '=' alignment.
+        let cases = [
+            ("+z#5", FormatSpecError::StringSpecNotAllowed("Sign")),
+            (
+                "x=z#5",
+                FormatSpecError::NegativeZeroCoercionNotAllowed("string"),
+            ),
+            (
+                "x=#5",
+                FormatSpecError::StringSpecNotAllowed("Alternate form (#)"),
+            ),
+        ];
+
+        for (text, expected) in cases {
+            let spec = FormatSpec::parse(text).unwrap();
+            assert_eq!(spec.format_string(&value), Err(expected), "{text}");
+        }
+    }
+
     #[test]
     fn format_complex_rejects_zero_padding_before_after_sign_alignment() {
         for text in [

crates/vm/src/format.rs

@@ -86,6 +86,10 @@ impl IntoPyException for FormatSpecError {
             Self::StringAlignmentFlag => {
                 vm.new_value_error("'=' alignment not allowed in string format specifier")
             }
+            Self::StringSpecNotAllowed(s) => {
+                let msg = format!("{s} not allowed in string format specifier");
+                vm.new_value_error(msg)
+            }
             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

@@ -32,6 +32,30 @@ except ValueError as error:
 else:
     raise AssertionError("expected ValueError for '=8s' string format specifier")
 
+# regression: a sign, a space or an alternate form used to be accepted and dropped.
+for spec, flag in [
+    ("+", "Sign"),
+    ("-", "Sign"),
+    ("+5", "Sign"),
+    (" ", "Space"),
+    (" 5", "Space"),
+    ("#", "Alternate form (#)"),
+    ("#5", "Alternate form (#)"),
+    ("+.2", "Sign"),
+]:
+    try:
+        format("result", spec)
+    except ValueError as error:
+        expected = f"{flag} not allowed in string format specifier"
+        if str(error) != expected:
+            raise AssertionError(
+                f"{spec!r}: unexpected error message: {error}"
+            ) from error
+    else:
+        raise AssertionError(
+            f"expected ValueError for {spec!r} string format specifier"
+        )
+
 # regression: unknown conversion specifiers used to be silently ignored instead of raising.
 # The ValueError case itself is covered by test_str, but here we're testing the error message.
 try: