Fix transition bandwidth reported in 'filter too short' error (#11406) (#14005)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: CedricConday

doc/changes/dev/14005.bugfix.rst

@@ -0,0 +1 @@
+Fix the error message raised when a requested FIR ``filter_length`` is too short, which reported half the actual transition bandwidth, by `Cedric Conday`_.

mne/filter.py

@@ -440,7 +440,7 @@ def _firwin_design(N, freq, gain, window, sfreq):
             if this_N > N:
                 raise ValueError(
                     f"The requested filter length {N} is too short for the requested "
-                    f"{transition * sfreq / 2.0:0.2f} Hz transition band, which "
+                    f"{transition * sfreq:0.2f} Hz transition band, which "
                     f"requires {this_N} samples"
                 )
             # Construct a lowpass

mne/tests/test_filter.py

@@ -1134,3 +1134,11 @@ def test_smart_pad(dc, sfreq):
     x_want = np.r_[np.zeros_like(x), x_want, np.zeros_like(x)]
     x_pad = _smart_pad(x, (len(x) * 2,) * 2, "reflect_limited")
     assert_allclose(x_pad, x_want, atol=0.1, err_msg="reflect_limited with zeros")
+
+
+def test_filter_too_short_error_reports_correct_transition(raw):
+    """The 'too short' filter error must report the true transition band (gh-11406)."""
+    # filter_length far too short for a 1 Hz transition band; the error must
+    # report 1.00 Hz (the requested l_trans_bandwidth), not half that.
+    with pytest.raises(ValueError, match="1.00 Hz transition band"):
+        raw.filter(l_freq=10, h_freq=None, l_trans_bandwidth=1.0, filter_length="11ms")