Fix issue #1322: Occasionally two extra newlines before comment with & .

CHANGELOG.md

@@ -3,6 +3,9 @@ Changelog
 
 NOTE: isort follows the [semver](https://semver.org/) versioning standard.
 
+### 5.1.1 July 15, 2020
+  - Fix issue #1322: Occasionally two extra newlines before comment with `-n` & `--fss`.
+
 ### 5.1.0 July 14, 2020
   - isort now throws an exception if an invalid settings path is given (issue #1174).
   - Implemented support for automatic redundant alias removal (issue #1281).

isort/output.py

@@ -108,6 +108,8 @@ def sorted_imports(
             comments_above = []
             new_section_output: List[str] = []
             for line in section_output:
+                if not line:
+                    continue
                 if line.startswith("#"):
                     comments_above.append(line)
                 elif comments_above:
@@ -132,7 +134,11 @@ def sorted_imports(
             for line in new_section_output:
                 comments = getattr(line, "comments", ())
                 if comments:
-                    if section_output and config.ensure_newline_before_comments:
+                    if (
+                        config.ensure_newline_before_comments
+                        and section_output
+                        and section_output[-1]
+                    ):
                         section_output.append("")
                     section_output.extend(comments)
                 section_output.append(str(line))

tests/test_regressions.py

@@ -360,3 +360,29 @@ def test_isort_doesnt_rewrite_import_with_dot_to_from_import_issue_1280():
     """,
         show_diff=True,
     )
+
+
+def test_isort_shouldnt_introduce_extra_lines_with_fass_issue_1322():
+    """Tests to ensure isort doesn't introduce extra lines when used with fass option.
+    See: https://github.com/timothycrosley/isort/issues/1322
+    """
+    assert (
+        isort.code(
+            """
+        import logging
+
+# Comment canary
+from foo import bar
+import quux
+""",
+            force_sort_within_sections=True,
+            ensure_newline_before_comments=True,
+        )
+        == """
+        import logging
+
+# Comment canary
+from foo import bar
+import quux
+"""
+    )