fix: derive format from path in Plot.plot

src/pyobsplot/js_modules.py

@@ -1,4 +1,6 @@
+import io
 from functools import partial
+from pathlib import Path
 from typing import Callable, Literal
 
 from pyobsplot.obsplot import Obsplot
@@ -35,7 +37,16 @@ def plot(
             if provided, plot is saved to disk to an HTML file instead of displayed
             as a jupyter widget, by default None
         """
-        format_value = format or _plot_format
+        format_value = format
+        if path is not None and not isinstance(path, io.StringIO):
+            extension = Path(path).suffix.lower()[1:]
+            allowed_extensions = ["html", "svg", "pdf", "png"]
+            if extension not in allowed_extensions:
+                msg = f"Output file extension should be one of {allowed_extensions}"
+                raise ValueError(msg)
+            format_value = format_value or extension
+
+        format_value = format_value or _plot_format
         op = Obsplot(format=format_value)  # type: ignore
         return op(spec, path=path)