add docstring note about legacy n_fft default (#12587)

Author: drammock

mne/time_frequency/spectrum.py

@@ -279,7 +279,7 @@ def _set_legacy_nfft_default(self, tmin, tmax, method, method_kw):
         This method returns ``None`` and has a side effect of (maybe) updating
         the ``method_kw`` dict.
         """
-        if method == "welch" and method_kw.get("n_fft", None) is None:
+        if method == "welch" and method_kw.get("n_fft") is None:
             tm = _time_mask(self.times, tmin, tmax, sfreq=self.info["sfreq"])
             method_kw["n_fft"] = min(np.sum(tm), 2048)
 

mne/utils/docs.py

@@ -1704,7 +1704,8 @@ def _reflow_param_docstring(docstring, has_first_line=True, width=75):
 
 _fmin_fmax = """\
 fmin, fmax : float
-    The lower- and upper-bound on frequencies of interest. Default is {}"""
+    The lower- and upper-bound on frequencies of interest. Default is
+    {}"""
 
 docdict["fmin_fmax_psd"] = _fmin_fmax.format(
     "``fmin=0, fmax=np.inf`` (spans all frequencies present in the data)."
@@ -2578,10 +2579,13 @@ def _reflow_param_docstring(docstring, has_first_line=True, width=75):
 **method_kw
     Additional keyword arguments passed to the spectral estimation
     function (e.g., ``n_fft, n_overlap, n_per_seg, average, window``
-    for Welch method, or
-    ``bandwidth, adaptive, low_bias, normalization`` for multitaper
-    method). See :func:`~mne.time_frequency.psd_array_welch` and
-    :func:`~mne.time_frequency.psd_array_multitaper` for details.
+    for Welch method, or ``bandwidth, adaptive, low_bias, normalization``
+    for multitaper method). See :func:`~mne.time_frequency.psd_array_welch`
+    and :func:`~mne.time_frequency.psd_array_multitaper` for details. Note
+    that for Welch method if ``n_fft`` is unspecified its default will be
+    the smaller of ``2048`` or the number of available time samples (taking into
+    account ``tmin`` and ``tmax``), not ``256`` as in
+    :func:`~mne.time_frequency.psd_array_welch`.
 """
 
 docdict["method_kw_tfr"] = _method_kw_tfr_template.format(