Add preload keyward argument to Inst.load_data() method call (#13858)

Author: Aniketsy

doc/changes/dev/13858.bugfix.rst

@@ -0,0 +1 @@
+Add a ``memmap`` keyword argument to :meth:`mne.io.BaseRaw.load_data`, allowing loading deferred raw data into a memory-mapped file after object creation, by `Aniket Singh Yadav`_.

mne/io/base.py

@@ -566,11 +566,16 @@ def _check_bad_segment(
         return self._getitem((picks, slice(start, stop)), return_times=False)
 
     @verbose
-    def load_data(self, verbose=None):
+    def load_data(self, *, memmap=None, verbose=None):
         """Load raw data.
 
         Parameters
         ----------
+        memmap : path-like | None
+            If not ``None``, preload data into a memory-mapped file at this
+            path. If ``None`` (default), preload data into RAM.
+
+            .. versionadded:: 1.13
         %(verbose)s
 
         Returns
@@ -586,7 +591,9 @@ def load_data(self, verbose=None):
         .. versionadded:: 0.10.0
         """
         if not self.preload:
-            self._preload_data(True)
+            if memmap is not None:
+                _validate_type(memmap, "path-like", "memmap")
+            self._preload_data(memmap if memmap is not None else True)
         return self
 
     def _preload_data(self, preload):

mne/io/tests/test_raw.py

@@ -843,6 +843,18 @@ def _read_raw_arange(preload=False, verbose=None):
     return _RawArange(preload, verbose)
 
 
+def test_load_data_memmap(tmp_path):
+    """Test loading raw data into a memmap via load_data."""
+    raw = _read_raw_arange(preload=False)
+    memmap_fname = tmp_path / "raw-load-data-memmap.dat"
+    raw.load_data(memmap=memmap_fname)
+
+    assert raw.preload
+    assert isinstance(raw._data, np.memmap)
+    assert Path(raw._data.filename) == memmap_fname
+    assert_array_equal(raw._data[:, 0], np.arange(1, 9))
+
+
 def test_test_raw_reader():
     """Test _test_raw_reader."""
     _test_raw_reader(_read_raw_arange, test_scaling=False, test_rank="less")