Implement inference for JoinedStr and FormattedValue (#2459)

Author: ericvergnaud

ChangeLog

@@ -17,6 +17,9 @@ Release date: TBA
 
   Closes pylint-dev/pylint#7126
 
+* Implement inference for JoinedStr and FormattedValue
+
+
 
 What's New in astroid 3.2.5?
 ============================

astroid/nodes/node_classes.py

@@ -4673,6 +4673,37 @@ def get_children(self):
         if self.format_spec is not None:
             yield self.format_spec
 
+    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
+        uninferable_already_generated = False
+        for format_spec in self.format_spec.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):
+                if not isinstance(value, Const):
+                    if not uninferable_already_generated:
+                        yield util.Uninferable
+                        uninferable_already_generated = True
+                    continue
+                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,
+                )
+
+
+MISSING_VALUE = "{MISSING_VALUE}"
+
 
 class JoinedStr(NodeNG):
     """Represents a list of string expressions to be joined.
@@ -4734,6 +4765,34 @@ def postinit(self, values: list[NodeNG] | None = None) -> None:
     def get_children(self):
         yield from self.values
 
+    def _infer(
+        self, context: InferenceContext | None = None, **kwargs: Any
+    ) -> Generator[InferenceResult, None, InferenceErrorInfo | None]:
+        yield from self._infer_from_values(self.values, context)
+
+    @classmethod
+    def _infer_from_values(
+        cls, nodes: list[NodeNG], context: InferenceContext | None = None, **kwargs: Any
+    ) -> Generator[InferenceResult, None, InferenceErrorInfo | None]:
+        if len(nodes) == 1:
+            yield from nodes[0]._infer(context, **kwargs)
+            return
+        uninferable_already_generated = False
+        for prefix in nodes[0]._infer(context, **kwargs):
+            for suffix in cls._infer_from_values(nodes[1:], context, **kwargs):
+                result = ""
+                for node in (prefix, suffix):
+                    if isinstance(node, Const):
+                        result += str(node.value)
+                        continue
+                    result += MISSING_VALUE
+                if MISSING_VALUE in result:
+                    if not uninferable_already_generated:
+                        uninferable_already_generated = True
+                        yield util.Uninferable
+                else:
+                    yield Const(result)
+
 
 class NamedExpr(_base_nodes.AssignTypeNode):
     """Represents the assignment from the assignment expression

tests/test_inference.py

@@ -19,6 +19,7 @@
 import pytest
 
 from astroid import (
+    Const,
     Slice,
     Uninferable,
     arguments,
@@ -652,6 +653,34 @@ def process_line(word_pos):
             )
         )
 
+    def test_fstring_inference(self) -> None:
+        code = """
+            name = "John"
+            result = f"Hello {name}!"
+            """
+        ast = parse(code, __name__)
+        node = ast["result"]
+        inferred = node.inferred()
+        self.assertEqual(len(inferred), 1)
+        value_node = inferred[0]
+        self.assertIsInstance(value_node, Const)
+        self.assertEqual(value_node.value, "Hello John!")
+
+    def test_formatted_fstring_inference(self) -> None:
+        code = """
+            width = 10
+            precision = 4
+            value = 12.34567
+            result = f"result: {value:{width}.{precision}}!"
+            """
+        ast = parse(code, __name__)
+        node = ast["result"]
+        inferred = node.inferred()
+        self.assertEqual(len(inferred), 1)
+        value_node = inferred[0]
+        self.assertIsInstance(value_node, Const)
+        self.assertEqual(value_node.value, "result:      12.35!")
+
     def test_float_complex_ambiguity(self) -> None:
         code = '''
             def no_conjugate_member(magic_flag):  #@