Allow an empty prefix in format dict

CHANGELOG.md

@@ -6,9 +6,11 @@ Jupytext ChangeLog
 
 **Fixed**
 - We have fixed a typo when `build_jupytext_contents_manager_class` can't be imported ([#1162](https://github.com/mwouts/jupytext/issues/1162))
+- Some dependencies of the JupyterLab extensions were updated ([#1243](https://github.com/mwouts/jupytext/issues/1243), [#1245](https://github.com/mwouts/jupytext/issues/1245))
 
 **Added**
 - Added support for Lua notebooks ([#1252](https://github.com/mwouts/jupytext/pull/1252)) - thanks to [erentar](https://github.com/erentar) for this contribution
+- Empty prefixes are now allowed in Jupytext format when specified as a dictionary ([#1144](https://github.com/mwouts/jupytext/issues/1144))
 
 
 1.16.2 (2024-05-05)

src/jupytext/config.py

@@ -398,7 +398,9 @@ def load_jupytext_configuration_file(config_file, stream=None):
     # formats can be a dict prefix => format
     if isinstance(config.formats, dict):
         config.formats = [
-            (prefix[:-1] if prefix.endswith("/") else prefix) + "///" + fmt
+            fmt
+            if not prefix
+            else (prefix[:-1] if prefix.endswith("/") else prefix) + "///" + fmt
             for prefix, fmt in config.formats.items()
         ]
     config.formats = short_form_multiple_formats(config.formats)

tests/functional/config/test_config.py

@@ -158,6 +158,15 @@ def test_load_jupytext_configuration_file(tmpdir, config_file):
 """,
             "notebooks///ipynb,scripts///py:percent",
         ),
+        (
+            """# Pair local notebooks to scripts in 'notebooks_py' and md files in 'notebooks_md'
+[formats]
+"" = "ipynb"
+"notebooks_py" = "py:percent"
+"notebooks_md" = "md:myst"
+""",
+            "ipynb,notebooks_py///py:percent,notebooks_md///md:myst",
+        ),
     ],
 )
 def test_jupytext_formats(tmpdir, content_toml, formats_short_form):