Fix unstable format involving backslash + whitespace at beginning of file (#948)

diff --git a/black.py b/black.py
index f7022d8014c4a9aef3474d28ca153a60984a56d2..05edf1a2ae320c3bd3b9a11c57b2edd3a314a4cf 100644 (file)
--- a/black.py
+++ b/black.py
@@ -1480,7 +1480,13 @@ class EmptyLineTracker:
         lines (two on module-level).
         """
         before, after = self._maybe_empty_lines(current_line)
-        before -= self.previous_after
+        before = (
+            # Black should not insert empty lines at the beginning
+            # of the file
+            0
+            if self.previous_line is None
+            else before - self.previous_after
+        )
         self.previous_after = after
         self.previous_line = current_line
         return before, after

diff --git a/tests/data/beginning_backslash.py b/tests/data/beginning_backslash.py
new file mode 100644 (file)
index 0000000..66c347d
--- /dev/null
+++ b/tests/data/beginning_backslash.py
@@ -0,0 +1,12 @@
+\
+
+
+
+
+
+print("hello, world")
+
+# output
+
+
+print("hello, world")

diff --git a/tests/test_black.py b/tests/test_black.py
index 7b3a8b66e2b3bd448676989056e39dc9aed83617..015243f7c1322fd28cb6ba33933acf6ef02f09f3 100644 (file)
--- a/tests/test_black.py
+++ b/tests/test_black.py
@@ -639,6 +639,14 @@ class BlackTestCase(unittest.TestCase):
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, black.FileMode())
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_beginning_backslash(self) -> None:
+        source, expected = read_data("beginning_backslash")
+        actual = fs(source)
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, black.FileMode())
+
     def test_tab_comment_indentation(self) -> None:
         contents_tab = "if 1:\n\tif 2:\n\t\tpass\n\t# comment\n\tpass\n"
         contents_spc = "if 1:\n    if 2:\n        pass\n    # comment\n    pass\n"