Avoid flagging nested f-strings (#1516)

resources/test/fixtures/pyflakes/F541.py

@@ -1,6 +1,8 @@
+# OK
 a = "abc"
 b = f"ghi{'jkl'}"
 
+# Errors
 c = f"def"
 d = f"def" + "ghi"
 e = (
@@ -8,5 +10,17 @@ e = (
     "ghi"
 )
 
+# OK
 g = f"ghi{123:{45}}"
+
+# Error
 h = "x" "y" f"z"
+
+v = 23.234234
+
+# OK
+f"{v:0.2f}"
+
+# OK (false negatives)
+f"{v:{f'0.2f'}}"
+f"{f''}"

src/checkers/ast.rs

@@ -91,6 +91,7 @@ pub struct Checker<'a> {
     in_type_definition: bool,
     in_deferred_string_type_definition: bool,
     in_deferred_type_definition: bool,
+    in_f_string: bool,
     in_literal: bool,
     in_subscript: bool,
     seen_import_boundary: bool,
@@ -147,6 +148,7 @@ impl<'a> Checker<'a> {
             in_type_definition: false,
             in_deferred_string_type_definition: false,
             in_deferred_type_definition: false,
+            in_f_string: false,
             in_literal: false,
             in_subscript: false,
             seen_import_boundary: false,
@@ -1466,6 +1468,7 @@ where
 
         self.push_expr(expr);
 
+        let prev_in_f_string = self.in_f_string;
         let prev_in_literal = self.in_literal;
         let prev_in_type_definition = self.in_type_definition;
 
@@ -2151,7 +2154,9 @@ where
                 }
             }
             ExprKind::JoinedStr { values } => {
-                if self.settings.enabled.contains(&CheckCode::F541) {
+                // Conversion flags are parsed as f-strings without placeholders, so skip
+                // nested f-strings, which would lead to false positives.
+                if !self.in_f_string && self.settings.enabled.contains(&CheckCode::F541) {
                     if !values
                         .iter()
                         .any(|value| matches!(value.node, ExprKind::FormattedValue { .. }))
@@ -2654,6 +2659,11 @@ where
                 }
                 self.in_subscript = prev_in_subscript;
             }
+            ExprKind::JoinedStr { .. } => {
+                self.in_f_string = true;
+                visitor::walk_expr(self, expr);
+                self.in_f_string = prev_in_f_string;
+            }
             _ => visitor::walk_expr(self, expr),
         }
 
@@ -2671,6 +2681,7 @@ where
 
         self.in_type_definition = prev_in_type_definition;
         self.in_literal = prev_in_literal;
+        self.in_f_string = prev_in_f_string;
 
         self.pop_expr();
     }

src/pyflakes/snapshots/ruff__pyflakes__tests__F541_F541.py.snap

@@ -4,19 +4,10 @@ expression: checks
 ---
 - kind: FStringMissingPlaceholders
   location:
-    row: 4
+    row: 6
     column: 4
   end_location:
-    row: 4
-    column: 10
-  fix: ~
-  parent: ~
-- kind: FStringMissingPlaceholders
-  location:
-    row: 5
-    column: 4
-  end_location:
-    row: 5
+    row: 6
     column: 10
   fix: ~
   parent: ~
@@ -31,10 +22,19 @@ expression: checks
   parent: ~
 - kind: FStringMissingPlaceholders
   location:
-    row: 12
+    row: 9
     column: 4
   end_location:
-    row: 12
+    row: 9
+    column: 10
+  fix: ~
+  parent: ~
+- kind: FStringMissingPlaceholders
+  location:
+    row: 17
+    column: 4
+  end_location:
+    row: 17
     column: 16
   fix: ~
   parent: ~