Fix formatting cells with magic methods and starting or trailing empty lines

CHANGES.md

@@ -10,6 +10,9 @@
 
 <!-- Changes that affect Black's stable style -->
 
+- Fix formatting cells in IPython notebooks with magic methods and starting or trailing
+  empty lines (#4484)
+
 ### Preview style
 
 <!-- Changes that affect Black's preview style -->

src/black/handle_ipynb_magics.py

@@ -178,12 +178,14 @@ def mask_cell(src: str) -> tuple[str, list[Replacement]]:
     from IPython.core.inputtransformer2 import TransformerManager
 
     transformer_manager = TransformerManager()
+    # A side effect of the following transformation is that it also removes any
+    # empty lines at the beginning of the cell.
     transformed = transformer_manager.transform_cell(src)
     transformed, cell_magic_replacements = replace_cell_magics(transformed)
     replacements += cell_magic_replacements
     transformed = transformer_manager.transform_cell(transformed)
     transformed, magic_replacements = replace_magics(transformed)
-    if len(transformed.splitlines()) != len(src.splitlines()):
+    if len(transformed.strip().splitlines()) != len(src.strip().splitlines()):
         # Multi-line magic, not supported.
         raise NothingChanged
     replacements += magic_replacements
@@ -269,7 +271,7 @@ def replace_magics(src: str) -> tuple[str, list[Replacement]]:
     magic_finder = MagicFinder()
     magic_finder.visit(ast.parse(src))
     new_srcs = []
-    for i, line in enumerate(src.splitlines(), start=1):
+    for i, line in enumerate(src.split("\n"), start=1):
         if i in magic_finder.magics:
             offsets_and_magics = magic_finder.magics[i]
             if len(offsets_and_magics) != 1:  # pragma: nocover

tests/test_ipynb.py

@@ -173,6 +173,22 @@ def test_cell_magic_with_magic() -> None:
     assert result == expected
 
 
+@pytest.mark.parametrize(
+    "src, expected",
+    (
+        ("\n\n\n%time \n\n", "%time"),
+        ("  \n\t\n%%timeit -n4 \t \nx=2  \n\r\n", "%%timeit -n4\nx = 2"),
+        (
+            "  \t\n\n%%capture \nx=2 \n%config \n\n%env\n\t  \n \n\n",
+            "%%capture\nx = 2\n%config\n\n%env",
+        ),
+    ),
+)
+def test_cell_magic_with_empty_lines(src: str, expected: str) -> None:
+    result = format_cell(src, fast=True, mode=JUPYTER_MODE)
+    assert result == expected
+
+
 @pytest.mark.parametrize(
     "mode, expected_output, expectation",
     [