Fix isort no-lines-before preceded by an empty section (#3139)

Fix isort no-lines-before preceded by an empty section

Fix #3138.

crates/ruff/resources/test/fixtures/isort/no_lines_before_with_empty_sections.py

@@ -0,0 +1,3 @@
+from __future__ import annotations
+from typing import Any
+from . import my_local_folder_object

crates/ruff/src/rules/isort/categorize.rs

@@ -6,12 +6,13 @@ use log::debug;
 use ruff_python::sys::KNOWN_STANDARD_LIBRARY;
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
+use strum_macros::EnumIter;
 
 use super::types::{ImportBlock, Importable};
 use crate::settings::types::PythonVersion;
 
 #[derive(
-    Debug, PartialOrd, Ord, PartialEq, Eq, Clone, Serialize, Deserialize, JsonSchema, Hash,
+    Debug, PartialOrd, Ord, PartialEq, Eq, Clone, Serialize, Deserialize, JsonSchema, Hash, EnumIter,
 )]
 #[serde(deny_unknown_fields, rename_all = "kebab-case")]
 pub enum ImportType {

crates/ruff/src/rules/isort/mod.rs

@@ -12,6 +12,7 @@ use normalize::normalize_imports;
 use order::order_imports;
 use settings::RelativeImportsOrder;
 use sorting::cmp_either_import;
+use strum::IntoEnumIterator;
 use track::{Block, Trailer};
 use types::EitherImport::{Import, ImportFrom};
 use types::{AliasData, CommentSet, EitherImport, OrderedImportBlock, TrailingComma};
@@ -232,7 +233,7 @@ fn format_import_block(
     target_version: PythonVersion,
 ) -> String {
     // Categorize by type (e.g., first-party vs. third-party).
-    let block_by_type = categorize_imports(
+    let mut block_by_type = categorize_imports(
         block,
         src,
         package,
@@ -247,7 +248,17 @@ fn format_import_block(
 
     // Generate replacement source code.
     let mut is_first_block = true;
-    for (import_type, import_block) in block_by_type {
+    let mut pending_lines_before = false;
+    for import_type in ImportType::iter() {
+        let import_block = block_by_type.remove(&import_type);
+
+        if !no_lines_before.contains(&import_type) {
+            pending_lines_before = true;
+        }
+        let Some(import_block) = import_block else {
+            continue;
+        };
+
         let mut imports = order_imports(
             import_block,
             order_by_type,
@@ -280,8 +291,10 @@ fn format_import_block(
         // Add a blank line between every section.
         if is_first_block {
             is_first_block = false;
-        } else if !no_lines_before.contains(&import_type) {
+            pending_lines_before = false;
+        } else if pending_lines_before {
             output.push_str(stylist.line_ending());
+            pending_lines_before = false;
         }
 
         let mut lines_inserted = false;
@@ -791,6 +804,31 @@ mod tests {
         Ok(())
     }
 
+    #[test_case(Path::new("no_lines_before_with_empty_sections.py"))]
+    fn no_lines_before_with_empty_sections(path: &Path) -> Result<()> {
+        let snapshot = format!(
+            "no_lines_before_with_empty_sections.py_{}",
+            path.to_string_lossy()
+        );
+        let mut diagnostics = test_path(
+            Path::new("isort").join(path).as_path(),
+            &Settings {
+                isort: super::settings::Settings {
+                    no_lines_before: BTreeSet::from([
+                        ImportType::StandardLibrary,
+                        ImportType::LocalFolder,
+                    ]),
+                    ..super::settings::Settings::default()
+                },
+                src: vec![test_resource_path("fixtures/isort")],
+                ..Settings::for_rule(Rule::UnsortedImports)
+            },
+        )?;
+        diagnostics.sort_by_key(|diagnostic| diagnostic.location);
+        assert_yaml_snapshot!(snapshot, diagnostics);
+        Ok(())
+    }
+
     #[test_case(Path::new("lines_after_imports_nothing_after.py"))]
     #[test_case(Path::new("lines_after_imports_func_after.py"))]
     #[test_case(Path::new("lines_after_imports_class_after.py"))]

crates/ruff/src/rules/isort/snapshots/ruff__rules__isort__tests__no_lines_before_with_empty_sections.py_no_lines_before_with_empty_sections.py.snap

@@ -0,0 +1,22 @@
+---
+source: crates/ruff/src/rules/isort/mod.rs
+expression: diagnostics
+---
+- kind:
+    UnsortedImports: ~
+  location:
+    row: 1
+    column: 0
+  end_location:
+    row: 4
+    column: 0
+  fix:
+    content: "from __future__ import annotations\nfrom typing import Any\n\nfrom . import my_local_folder_object\n"
+    location:
+      row: 1
+      column: 0
+    end_location:
+      row: 4
+      column: 0
+  parent: ~
+