Escape the comment char only in regular expressions (#864)

* Raise on the first unexpected warning

* Ignore the DeprecationWarning about Validator.iter_errors

* Escape the comment char only in regular expressions

* Full coverage for the no_warning context manager

* Update CHANGELOG.md

docs/CHANGELOG.md

@@ -1,6 +1,13 @@
 Jupytext ChangeLog
 ==================
 
+1.13.1 (2021-10-07)
+-------------------
+
+**Fixed**
+- The magic commands in `py:percent` scripts with no explicit format information remain commented over a round trip ([#848](https://github.com/mwouts/jupytext/issues/848))
+
+
 1.13.0 (2021-09-25)
 -------------------
 

jupytext/formats.py

@@ -313,7 +313,8 @@ def guess_format(text, ext):
     # Is this a Hydrogen-like script?
     # Or a Sphinx-gallery script?
     if ext in _SCRIPT_EXTENSIONS:
-        comment = re.escape(_SCRIPT_EXTENSIONS[ext]["comment"])
+        unescaped_comment = _SCRIPT_EXTENSIONS[ext]["comment"]
+        comment = re.escape(unescaped_comment)
         language = _SCRIPT_EXTENSIONS[ext]["language"]
         twenty_hash_re = re.compile(r"^#( |)#{19,}\s*$")
         double_percent_re = re.compile(r"^{}( %%|%%)$".format(comment))
@@ -345,7 +346,7 @@ def guess_format(text, ext):
             ):
                 double_percent_count += 1
 
-            if not line.startswith(comment) and is_magic(line, language):
+            if not line.startswith(unescaped_comment) and is_magic(line, language):
                 magic_command_count += 1
 
             if twenty_hash_re.match(line) and ext == ".py":

jupytext/version.py

@@ -1,3 +1,3 @@
 """Jupytext's version number"""
 
-__version__ = "1.13.0"
+__version__ = "1.13.1"

tests/test_cli.py

@@ -4,6 +4,7 @@
 import sys
 import time
 import unittest.mock as mock
+import warnings
 from argparse import ArgumentTypeError
 from contextlib import contextmanager
 from io import StringIO
@@ -1133,11 +1134,11 @@ def test_sync_script_dotdot_folder_564(tmpdir):
 
 @contextmanager
 def no_warning():
-    with pytest.warns(None) as warnings:
+    with pytest.warns(None) as records:
         yield
 
     # There should be no warning
-    for record in warnings:
+    for record in records:
         # Temporary exception for for this one, see #865
         if (
             "Passing a schema to Validator.iter_errors is deprecated "
@@ -1147,6 +1148,12 @@ def no_warning():
         raise RuntimeError(record)
 
 
+def test_no_warning():
+    with pytest.raises(RuntimeError, match="a sample warning"):
+        with no_warning():
+            warnings.warn("a sample warning")
+
+
 def test_jupytext_to_file_emits_a_warning(tmpdir):
     """The user may type
         jupytext notebook.ipynb --to script.py

tests/test_formats.py

@@ -31,6 +31,32 @@ def test_guess_format_percent(nb_file):
         assert guess_format(stream.read(), ext=".py")[0] == "percent"
 
 
+def test_guess_format_simple_percent(
+    nb="""# %%
+print("hello world!")
+""",
+):
+    assert guess_format(nb, ext=".py")[0] == "percent"
+
+
+def test_guess_format_simple_percent_with_magic(
+    nb="""# %%
+# %time
+print("hello world!")
+""",
+):
+    assert guess_format(nb, ext=".py")[0] == "percent"
+
+
+def test_guess_format_simple_hydrogen_with_magic(
+    nb="""# %%
+%time
+print("hello world!")
+""",
+):
+    assert guess_format(nb, ext=".py")[0] == "hydrogen"
+
+
 @pytest.mark.parametrize("nb_file", list_notebooks("sphinx"))
 def test_guess_format_sphinx(nb_file):
     with open(nb_file) as stream: