Implement F841 (for exception handlers) (#63)

src/linter.rs

@@ -354,6 +354,30 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn f841() -> Result<()> {
+        let actual = check_path(
+            &Path::new("./resources/test/src/F841.py"),
+            &settings::Settings {
+                line_length: 88,
+                exclude: vec![],
+                select: BTreeSet::from([CheckCode::F841]),
+            },
+            &cache::Mode::None,
+        )?;
+        let expected = vec![Message {
+            kind: CheckKind::UnusedVariable("e".to_string()),
+            location: Location::new(3, 1),
+            filename: "./resources/test/src/F841.py".to_string(),
+        }];
+        assert_eq!(actual.len(), expected.len());
+        for i in 0..actual.len() {
+            assert_eq!(actual[i], expected[i]);
+        }
+
+        Ok(())
+    }
+
     #[test]
     fn f901() -> Result<()> {
         let actual = check_path(