RustPython · shAn-kor · Aug 9, 2026
diff --git a/Lib/test/test_format.py b/Lib/test/test_format.py
@@ -529,7 +529,6 @@ def test_with_an_underscore_and_a_comma_in_format_specifier(self):
         with self.assertRaisesRegex(ValueError, error_msg):
             '{:._,f}'.format(1.1)
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON
     def test_better_error_message_format(self):
         # https://bugs.python.org/issue20524
         for value in [12j, 12, 12.0, "12"]:
@@ -551,7 +550,6 @@ def test_better_error_message_format(self):
                 with self.assertRaisesRegex(ValueError, err):
                     eval("f'xx{value:{bad_format_spec}}yy'")
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON
     def test_unicode_in_error_message(self):
         str_err = re.escape(
             "Invalid format specifier '%ЫйЯЧ' for object of type 'str'")
@@ -615,7 +613,6 @@ def test_negative_zero(self):
         self.assertEqual(f"{-0.:x>z6.1f}", "xxx0.0")
         self.assertEqual(f"{-0.:🖤>z6.1f}", "🖤🖤🖤0.0")  # multi-byte fill char
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON
     def test_specifier_z_error(self):
         error_msg = re.compile("Invalid format specifier '.*z.*'")
         with self.assertRaisesRegex(ValueError, error_msg):

diff --git a/crates/vm/src/builtins/bool.rs b/crates/vm/src/builtins/bool.rs
@@ -1,5 +1,4 @@
 use super::{PyInt, PyStrRef, PyType, PyTypeRef, PyUtf8StrRef};
-use crate::common::format::FormatSpec;
 use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyResult, TryFromBorrowedObject, VirtualMachine,
     class::PyClassImpl,
@@ -100,9 +99,10 @@ impl Constructor for PyBool {
 impl PyBool {
     #[pymethod]
     fn __format__(obj: PyObjectRef, spec: PyUtf8StrRef, vm: &VirtualMachine) -> PyResult<String> {
+        let format_spec = crate::format::parse_format_spec(obj.as_object(), spec.as_str(), vm)?;
         let new_bool = obj.try_to_bool(vm)?;
-        FormatSpec::parse(spec.as_str())
-            .and_then(|format_spec| format_spec.format_bool(new_bool))
+        format_spec
+            .format_bool(new_bool)
             .map_err(|err| err.into_pyexception(vm))
     }
 }

diff --git a/crates/vm/src/builtins/complex.rs b/crates/vm/src/builtins/complex.rs
@@ -3,7 +3,7 @@ use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
     builtins::PyUtf8StrRef,
     class::PyClassImpl,
-    common::{format::FormatSpec, wtf8::Wtf8Buf},
+    common::wtf8::Wtf8Buf,
     convert::{IntoPyException, ToPyObject, ToPyResult},
     function::{FuncArgs, OptionalArg, PyComparisonValue},
     protocol::PyNumberMethods,
@@ -367,8 +367,7 @@ impl PyComplex {
         if spec.is_empty() {
             return Ok(zelf.as_object().str(vm)?.as_wtf8().to_owned());
         }
-        let format_spec =
-            FormatSpec::parse(spec.as_str()).map_err(|err| err.into_pyexception(vm))?;
+        let format_spec = crate::format::parse_format_spec(zelf.as_object(), spec.as_str(), vm)?;
         let result = if format_spec.has_locale_format() {
             let locale = crate::format::get_locale_info();
             format_spec.format_complex_locale(&zelf.value, &locale)

diff --git a/crates/vm/src/builtins/float.rs b/crates/vm/src/builtins/float.rs
@@ -6,7 +6,7 @@ use crate::{
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult,
     TryFromBorrowedObject, TryFromObject, VirtualMachine,
     class::PyClassImpl,
-    common::{float_ops, format::FormatSpec, hash, wtf8::Wtf8Buf},
+    common::{float_ops, hash, wtf8::Wtf8Buf},
     convert::{IntoPyException, ToPyObject, ToPyResult},
     function::{
         ArgBytesLike, FuncArgs, OptionalArg, OptionalOption, PyArithmeticValue, PyComparisonValue,
@@ -269,8 +269,7 @@ impl PyFloat {
         if spec.is_empty() {
             return Ok(zelf.as_object().str(vm)?.as_wtf8().to_owned());
         }
-        let format_spec =
-            FormatSpec::parse(spec.as_str()).map_err(|err| err.into_pyexception(vm))?;
+        let format_spec = crate::format::parse_format_spec(zelf.as_object(), spec.as_str(), vm)?;
         let result = if format_spec.has_locale_format() {
             let locale = crate::format::get_locale_info();
             format_spec.format_float_locale(zelf.value, &locale)

diff --git a/crates/vm/src/builtins/int.rs b/crates/vm/src/builtins/int.rs
@@ -6,7 +6,6 @@ use crate::{
     bytes_inner::PyBytesInner,
     class::PyClassImpl,
     common::{
-        format::FormatSpec,
         hash,
         int::{bigint_to_finite_float, bytes_to_int, true_div},
         wtf8::Wtf8Buf,
@@ -511,8 +510,7 @@ impl PyInt {
         if spec.is_empty() && !zelf.class().is(vm.ctx.types.int_type) {
             return Ok(zelf.as_object().str(vm)?.as_wtf8().to_owned());
         }
-        let format_spec =
-            FormatSpec::parse(spec.as_str()).map_err(|err| err.into_pyexception(vm))?;
+        let format_spec = crate::format::parse_format_spec(zelf.as_object(), spec.as_str(), vm)?;
         if format_spec.is_decimal_int_format() {
             check_int_to_str_digits(&zelf.value, vm)?;
         }

diff --git a/crates/vm/src/builtins/str.rs b/crates/vm/src/builtins/str.rs
@@ -42,7 +42,7 @@ use num_traits::ToPrimitive;
 use rustpython_common::{
     ascii,
     atomic::{self, PyAtomic, Radium},
-    format::{FormatSpec, FormatString, FromTemplate},
+    format::{FormatString, FromTemplate},
     hash,
     lock::PyMutex,
     str::DeduceStrKind,
@@ -1017,10 +1017,9 @@ impl PyStr {
             };
         }
         let zelf = zelf.try_into_utf8(vm)?;
-        let s = FormatSpec::parse(spec.as_str())
-            .and_then(|format_spec| {
-                format_spec.format_string(&CharLenStr(zelf.as_str(), zelf.char_len()))
-            })
+        let format_spec = crate::format::parse_format_spec(zelf.as_object(), spec.as_str(), vm)?;
+        let s = format_spec
+            .format_string(&CharLenStr(zelf.as_str(), zelf.char_len()))
             .map_err(|err| err.into_pyexception(vm))?;
         Ok(vm.ctx.new_str(s))
     }

diff --git a/crates/vm/src/format.rs b/crates/vm/src/format.rs
@@ -41,6 +41,20 @@ pub(crate) fn get_locale_info() -> LocaleInfo {
     }
 }
 
+pub(crate) fn parse_format_spec(
+    object: &PyObject,
+    spec: &str,
+    vm: &VirtualMachine,
+) -> PyResult<FormatSpec> {
+    FormatSpec::parse(spec).map_err(|err| match err {
+        FormatSpecError::InvalidFormatSpecifier => vm.new_value_error(format!(
+            "Invalid format specifier '{spec}' for object of type '{}'",
+            object.class().name()
+        )),
+        _ => err.into_pyexception(vm),
+    })
+}
+
 impl IntoPyException for FormatSpecError {
     fn into_pyexception(self, vm: &VirtualMachine) -> PyBaseExceptionRef {
         match self {