fix: check_code misses lines_before_imports-only changes (#2242)

isort/core.py

@@ -373,12 +373,15 @@ def process(
                     not_imports = True
 
         if not_imports:
+            above_import_section: str = ""
             if not was_in_quote and config.lines_before_imports > -1:
                 if line.strip() == "" and not end_of_file:
                     lines_before += line
                     continue
                 if not import_section:
                     output_stream.write("".join(lines_before))
+                else:
+                    above_import_section = "".join(lines_before)
                 lines_before = []
 
             raw_import_section: str = import_section
@@ -451,7 +454,7 @@ def process(
                             )
 
                         made_changes = made_changes or _has_changed(
-                            before=raw_import_section,
+                            before=above_import_section + raw_import_section,
                             after=sorted_import_section,
                             line_separator=line_separator,
                             ignore_whitespace=config.ignore_whitespace,
@@ -522,4 +525,4 @@ def _has_changed(before: str, after: str, line_separator: str, ignore_whitespace
             remove_whitespace(before, line_separator=line_separator).strip()
             != remove_whitespace(after, line_separator=line_separator).strip()
         )
-    return before.strip() != after.strip()
+    return before.rstrip() != after.rstrip()

tests/unit/test_isort.py

@@ -1703,6 +1703,31 @@ def test_custom_lines_before_import_section(has_body: bool) -> None:
     )
 
 
+def test_check_code_detects_lines_before_import_changes() -> None:
+    """check_code must return False when lines_before_imports requires adding or removing blank
+    lines before the import section, even if the imports themselves are already sorted.
+
+    Regression test for https://github.com/PyCQA/isort/issues/2242
+    """
+    ln = "\n"
+    sorted_imports = "from a import b, x\n"
+    body = "\nfoo = 'bar'\n"
+
+    # Adding a blank line before imports (0 present, 1 required)
+    code_without_blank = sorted_imports + body
+    assert isort.code(code_without_blank, lines_before_imports=1) == ln + code_without_blank
+    assert not isort.check_code(code_without_blank, lines_before_imports=1)
+
+    # Removing a blank line before imports (1 present, 0 required)
+    code_with_blank = ln + sorted_imports + body
+    assert isort.code(code_with_blank, lines_before_imports=0) == sorted_imports + body
+    assert not isort.check_code(code_with_blank, lines_before_imports=0)
+
+    # Already correct: no change needed
+    assert isort.check_code(code_without_blank, lines_before_imports=0)
+    assert isort.check_code(code_with_blank, lines_before_imports=1)
+
+
 def test_custom_lines_after_import_section() -> None:
     """Test the case where the number of lines to output after imports has been explicitly set."""
     test_input = "from a import b\nfoo = 'bar'\n"