Add support for custom python cell magics (#2744)

Fixes #2742.

This PR adds the ability to configure additional python cell magics. This
will allow formatting cells in Jupyter Notebooks that are using custom (python)
magics.

CHANGES.md

@@ -30,6 +30,8 @@
 - Fix handling of standalone `match()` or `case()` when there is a trailing newline or a
   comment inside of the parentheses. (#2760)
 - Black now normalizes string prefix order (#2297)
+- Add configuration option (`python-cell-magics`) to format cells with custom magics in
+  Jupyter Notebooks (#2744)
 - Deprecate `--experimental-string-processing` and move the functionality under
   `--preview` (#2789)
 

src/black/__init__.py

@@ -24,6 +24,7 @@
     MutableMapping,
     Optional,
     Pattern,
+    Sequence,
     Set,
     Sized,
     Tuple,
@@ -225,6 +226,16 @@ def validate_regex(
         "(useful when piping source on standard input)."
     ),
 )
+@click.option(
+    "--python-cell-magics",
+    multiple=True,
+    help=(
+        "When processing Jupyter Notebooks, add the given magic to the list"
+        f" of known python-magics ({', '.join(PYTHON_CELL_MAGICS)})."
+        " Useful for formatting cells with custom python magics."
+    ),
+    default=[],
+)
 @click.option(
     "-S",
     "--skip-string-normalization",
@@ -401,6 +412,7 @@ def main(
     fast: bool,
     pyi: bool,
     ipynb: bool,
+    python_cell_magics: Sequence[str],
     skip_string_normalization: bool,
     skip_magic_trailing_comma: bool,
     experimental_string_processing: bool,
@@ -476,6 +488,7 @@ def main(
         magic_trailing_comma=not skip_magic_trailing_comma,
         experimental_string_processing=experimental_string_processing,
         preview=preview,
+        python_cell_magics=set(python_cell_magics),
     )
 
     if code is not None:
@@ -981,7 +994,7 @@ def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileCo
     return dst_contents
 
 
-def validate_cell(src: str) -> None:
+def validate_cell(src: str, mode: Mode) -> None:
     """Check that cell does not already contain TransformerManager transformations,
     or non-Python cell magics, which might cause tokenizer_rt to break because of
     indentations.
@@ -1000,7 +1013,10 @@ def validate_cell(src: str) -> None:
     """
     if any(transformed_magic in src for transformed_magic in TRANSFORMED_MAGICS):
         raise NothingChanged
-    if src[:2] == "%%" and src.split()[0][2:] not in PYTHON_CELL_MAGICS:
+    if (
+        src[:2] == "%%"
+        and src.split()[0][2:] not in PYTHON_CELL_MAGICS | mode.python_cell_magics
+    ):
         raise NothingChanged
 
 
@@ -1020,7 +1036,7 @@ def format_cell(src: str, *, fast: bool, mode: Mode) -> str:
     could potentially be automagics or multi-line magics, which
     are currently not supported.
     """
-    validate_cell(src)
+    validate_cell(src, mode)
     src_without_trailing_semicolon, has_trailing_semicolon = remove_trailing_semicolon(
         src
     )

src/black/mode.py

@@ -4,6 +4,7 @@
 chosen by the user.
 """
 
+from hashlib import md5
 import sys
 
 from dataclasses import dataclass, field
@@ -142,6 +143,7 @@ class Mode:
     is_ipynb: bool = False
     magic_trailing_comma: bool = True
     experimental_string_processing: bool = False
+    python_cell_magics: Set[str] = field(default_factory=set)
     preview: bool = False
 
     def __post_init__(self) -> None:
@@ -180,5 +182,6 @@ def get_cache_key(self) -> str:
             str(int(self.magic_trailing_comma)),
             str(int(self.experimental_string_processing)),
             str(int(self.preview)),
+            md5((",".join(sorted(self.python_cell_magics))).encode()).hexdigest(),
         ]
         return ".".join(parts)

tests/test.toml

@@ -7,6 +7,7 @@ line-length = 79
 target-version = ["py36", "py37", "py38"]
 exclude='\.pyi?$'
 include='\.py?$'
+python-cell-magics = ["custom1", "custom2"]
 
 [v1.0.0-syntax]
 # This shouldn't break Black.

tests/test_black.py

@@ -1322,6 +1322,7 @@ def test_parse_pyproject_toml(self) -> None:
         self.assertEqual(config["color"], True)
         self.assertEqual(config["line_length"], 79)
         self.assertEqual(config["target_version"], ["py36", "py37", "py38"])
+        self.assertEqual(config["python_cell_magics"], ["custom1", "custom2"])
         self.assertEqual(config["exclude"], r"\.pyi?$")
         self.assertEqual(config["include"], r"\.py?$")
 

tests/test_ipynb.py

@@ -1,5 +1,8 @@
+from dataclasses import replace
 import pathlib
 import re
+from contextlib import ExitStack as does_not_raise
+from typing import ContextManager
 
 from click.testing import CliRunner
 from black.handle_ipynb_magics import jupyter_dependencies_are_installed
@@ -63,9 +66,19 @@ def test_trailing_semicolon_noop() -> None:
         format_cell(src, fast=True, mode=JUPYTER_MODE)
 
 
-def test_cell_magic() -> None:
+@pytest.mark.parametrize(
+    "mode",
+    [
+        pytest.param(JUPYTER_MODE, id="default mode"),
+        pytest.param(
+            replace(JUPYTER_MODE, python_cell_magics={"cust1", "cust1"}),
+            id="custom cell magics mode",
+        ),
+    ],
+)
+def test_cell_magic(mode: Mode) -> None:
     src = "%%time\nfoo =bar"
-    result = format_cell(src, fast=True, mode=JUPYTER_MODE)
+    result = format_cell(src, fast=True, mode=mode)
     expected = "%%time\nfoo = bar"
     assert result == expected
 
@@ -76,6 +89,16 @@ def test_cell_magic_noop() -> None:
         format_cell(src, fast=True, mode=JUPYTER_MODE)
 
 
+@pytest.mark.parametrize(
+    "mode",
+    [
+        pytest.param(JUPYTER_MODE, id="default mode"),
+        pytest.param(
+            replace(JUPYTER_MODE, python_cell_magics={"cust1", "cust1"}),
+            id="custom cell magics mode",
+        ),
+    ],
+)
 @pytest.mark.parametrize(
     "src, expected",
     (
@@ -96,8 +119,8 @@ def test_cell_magic_noop() -> None:
         pytest.param("env =  %env", "env = %env", id="Assignment to magic"),
     ),
 )
-def test_magic(src: str, expected: str) -> None:
-    result = format_cell(src, fast=True, mode=JUPYTER_MODE)
+def test_magic(src: str, expected: str, mode: Mode) -> None:
+    result = format_cell(src, fast=True, mode=mode)
     assert result == expected
 
 
@@ -139,6 +162,41 @@ def test_cell_magic_with_magic() -> None:
     assert result == expected
 
 
+@pytest.mark.parametrize(
+    "mode, expected_output, expectation",
+    [
+        pytest.param(
+            JUPYTER_MODE,
+            "%%custom_python_magic -n1 -n2\nx=2",
+            pytest.raises(NothingChanged),
+            id="No change when cell magic not registered",
+        ),
+        pytest.param(
+            replace(JUPYTER_MODE, python_cell_magics={"cust1", "cust1"}),
+            "%%custom_python_magic -n1 -n2\nx=2",
+            pytest.raises(NothingChanged),
+            id="No change when other cell magics registered",
+        ),
+        pytest.param(
+            replace(JUPYTER_MODE, python_cell_magics={"custom_python_magic", "cust1"}),
+            "%%custom_python_magic -n1 -n2\nx = 2",
+            does_not_raise(),
+            id="Correctly change when cell magic registered",
+        ),
+    ],
+)
+def test_cell_magic_with_custom_python_magic(
+    mode: Mode, expected_output: str, expectation: ContextManager[object]
+) -> None:
+    with expectation:
+        result = format_cell(
+            "%%custom_python_magic -n1 -n2\nx=2",
+            fast=True,
+            mode=mode,
+        )
+        assert result == expected_output
+
+
 def test_cell_magic_nested() -> None:
     src = "%%time\n%%time\n2+2"
     result = format_cell(src, fast=True, mode=JUPYTER_MODE)