Fixes behavior of stc.save() as mentioned in issue #13158

mne/source_estimate.py

@@ -1884,7 +1884,7 @@ class SourceEstimate(_BaseSurfaceSourceEstimate):
     """
 
     @verbose
-    def save(self, fname, ftype="stc", *, overwrite=False, verbose=None):
+    def save(self, fname, ftype="auto", overwrite=False, verbose=None):
         """Save the source estimates to a file.
 
         Parameters
@@ -1894,18 +1894,29 @@ def save(self, fname, ftype="stc", *, overwrite=False, verbose=None):
             spaces are obtained by adding ``"-lh.stc"`` and ``"-rh.stc"`` (or
             ``"-lh.w"`` and ``"-rh.w"``) to the stem provided, for the left and
             the right hemisphere, respectively.
-        ftype : str
-            File format to use. Allowed values are ``"stc"`` (default),
-            ``"w"``, and ``"h5"``. The ``"w"`` format only supports a single
-            time point.
+        ftype : "auto" | "stc" | "w" | "h5"
+            File format to use. If "auto", the file format will be inferred from the
+            file extension if possible. Other allowed values are ``"stc"``, ``"w"``, and
+            ``"h5"``. The ``"w"`` format only supports a single time point.
         %(overwrite)s
 
             .. versionadded:: 1.0
         %(verbose)s
         """
         fname = str(_check_fname(fname=fname, overwrite=True))  # checked below
+        if ftype == "auto":
+            if fname.endswith((".stc", "-lh.stc", "-rh.stc")):
+                ftype = "stc"
+            elif fname.endswith((".w", "-lh.w", "-rh.w")):
+                ftype = "w"
+            elif fname.endswith(".h5"):
+                ftype = "h5"
+            else:
+                logger.info(
+                    "Cannot infer file type from `fname`; falling back to `.stc` format"
+                )
+                ftype = "stc"
         _check_option("ftype", ftype, ["stc", "w", "h5"])
-
         lh_data = self.data[: len(self.lh_vertno)]
         rh_data = self.data[-len(self.rh_vertno) :]
 
@@ -1918,6 +1929,8 @@ def save(self, fname, ftype="stc", *, overwrite=False, verbose=None):
                     "real numbers before saving."
                 )
             logger.info("Writing STC to disk...")
+            if fname.endswith(".stc"):
+                fname = fname[:-4]
             fname_l = str(_check_fname(fname + "-lh.stc", overwrite=overwrite))
             fname_r = str(_check_fname(fname + "-rh.stc", overwrite=overwrite))
             _write_stc(

mne/tests/test_source_estimate.py

@@ -482,7 +482,6 @@ def test_io_stc(tmp_path):
     stc = _fake_stc()
     stc.save(tmp_path / "tmp.stc")
     stc2 = read_source_estimate(tmp_path / "tmp.stc")
-
     assert_array_almost_equal(stc.data, stc2.data)
     assert_array_almost_equal(stc.tmin, stc2.tmin)
     assert_equal(len(stc.vertices), len(stc2.vertices))