Return NothingChanged if non-Python cell magic is detected, to avoid tokenize error

CHANGES.md

@@ -1,5 +1,11 @@
 # Change Log
 
+## Unreleased
+
+### _Black_
+
+- Fixed non-Python cell magics sometimes failing due to indentation (#2630)
+
 ## 21.11b1
 
 ### _Black_

docs/faq.md

@@ -47,6 +47,7 @@ _Black_ is timid about formatting Jupyter Notebooks. Cells containing any of the
 following will not be formatted:
 
 - automagics (e.g. `pip install black`)
+- non-Python cell magics (e.g. `%%writeline`)
 - multiline magics, e.g.:
 
   ```python

src/black/__init__.py

@@ -57,6 +57,7 @@
     remove_trailing_semicolon,
     put_trailing_semicolon_back,
     TRANSFORMED_MAGICS,
+    NON_PYTHON_CELL_MAGICS,
     jupyter_dependencies_are_installed,
 )
 
@@ -943,7 +944,9 @@ def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileCo
 
 
 def validate_cell(src: str) -> None:
-    """Check that cell does not already contain TransformerManager transformations.
+    """Check that cell does not already contain TransformerManager transformations,
+    or non-Python cell magics, which might cause tokenizer_rt to break because of
+    indentations.
 
     If a cell contains ``!ls``, then it'll be transformed to
     ``get_ipython().system('ls')``. However, if the cell originally contained
@@ -959,6 +962,8 @@ def validate_cell(src: str) -> None:
     """
     if any(transformed_magic in src for transformed_magic in TRANSFORMED_MAGICS):
         raise NothingChanged
+    if any("%%" + cell_magic in src for cell_magic in NON_PYTHON_CELL_MAGICS):
+        raise NothingChanged
 
 
 def format_cell(src: str, *, fast: bool, mode: Mode) -> str:

tests/test_ipynb.py

@@ -102,6 +102,7 @@ def test_magic(src: str, expected: str) -> None:
     (
         "%%bash\n2+2",
         "%%html --isolated\n2+2",
+        "%%writefile e.txt\n  meh\n meh",
     ),
 )
 def test_non_python_magics(src: str) -> None: