Fix issue when inferring single-node or non-const JoinedStr (#2578)

ChangeLog

@@ -13,6 +13,10 @@ What's New in astroid 3.3.4?
 ============================
 Release date: TBA
 
+* Fix regression with f-string inference.
+
+  Closes pylint-dev/pylint#9947
+
 * Fix bug with ``manager.clear_cache()`` not fully clearing cache
 
   Refs https://github.com/pylint-dev/pylint/pull/9932#issuecomment-2364985551

astroid/nodes/node_classes.py

@@ -4685,32 +4685,30 @@ def get_children(self):
     def _infer(
         self, context: InferenceContext | None = None, **kwargs: Any
     ) -> Generator[InferenceResult, None, InferenceErrorInfo | None]:
-        if self.format_spec is None:
-            yield from self.value.infer(context, **kwargs)
-            return
+        format_specs = Const("") if self.format_spec is None else self.format_spec
         uninferable_already_generated = False
-        for format_spec in self.format_spec.infer(context, **kwargs):
+        for format_spec in format_specs.infer(context, **kwargs):
             if not isinstance(format_spec, Const):
                 if not uninferable_already_generated:
                     yield util.Uninferable
                     uninferable_already_generated = True
                 continue
             for value in self.value.infer(context, **kwargs):
+                value_to_format = value
                 if isinstance(value, Const):
-                    try:
-                        formatted = format(value.value, format_spec.value)
-                        yield Const(
-                            formatted,
-                            lineno=self.lineno,
-                            col_offset=self.col_offset,
-                            end_lineno=self.end_lineno,
-                            end_col_offset=self.end_col_offset,
-                        )
-                        continue
-                    except (ValueError, TypeError):
-                        # happens when format_spec.value is invalid
-                        pass  # fall through
-                if not uninferable_already_generated:
+                    value_to_format = value.value
+                try:
+                    formatted = format(value_to_format, format_spec.value)
+                    yield Const(
+                        formatted,
+                        lineno=self.lineno,
+                        col_offset=self.col_offset,
+                        end_lineno=self.end_lineno,
+                        end_col_offset=self.end_col_offset,
+                    )
+                    continue
+                except (ValueError, TypeError):
+                    # happens when format_spec.value is invalid
                     yield util.Uninferable
                     uninferable_already_generated = True
                 continue

tests/test_inference.py

@@ -19,6 +19,7 @@
 import pytest
 
 from astroid import (
+    Assign,
     Const,
     Slice,
     Uninferable,
@@ -7388,3 +7389,33 @@ def test_empty_format_spec() -> None:
     assert isinstance(node, nodes.JoinedStr)
 
     assert list(node.infer()) == [util.Uninferable]
+
+
+@pytest.mark.parametrize(
+    "source, expected",
+    [
+        (
+            """
+class Cls:
+    # pylint: disable=too-few-public-methods
+    pass
+
+c_obj = Cls()
+
+s1 = f'{c_obj!r}' #@
+""",
+            "<__main__.Cls",
+        ),
+        ("s1 = f'{5}' #@", "5"),
+    ],
+)
+def test_joined_str_returns_string(source, expected) -> None:
+    """Regression test for https://github.com/pylint-dev/pylint/issues/9947."""
+    node = extract_node(source)
+    assert isinstance(node, Assign)
+    target = node.targets[0]
+    assert target
+    inferred = list(target.inferred())
+    assert len(inferred) == 1
+    assert isinstance(inferred[0], Const)
+    inferred[0].value.startswith(expected)