From c7495b9aa098ef7a358fc74556359d21c6a4ba11 Mon Sep 17 00:00:00 2001
From: Joe Antonakakis <jma353@cornell.edu>
Date: Sun, 4 Aug 2019 02:03:19 -0700
Subject: [PATCH 1/1] Fix unstable format involving backslash + whitespace at
 beginning of file (#948)

---
 black.py                          |  8 +++++++-
 tests/data/beginning_backslash.py | 12 ++++++++++++
 tests/test_black.py               |  8 ++++++++
 3 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 tests/data/beginning_backslash.py

diff --git a/black.py b/black.py
index f7022d8..05edf1a 100644
--- 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
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 7b3a8b6..015243f 100644
--- 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"
-- 
2.39.5