code: def _check_get_psf_ctf_params(mode, n_comp, return_pca_vars):
      """Check input parameters of _get_psf_ctf() for consistency."""
      if mode in [None, "sum", "mean"] and n_comp > 1:
          msg = "n_comp must be 1 for mode=%s." % mode
          raise ValueError(msg)
      if mode != "pca" and return_pca_vars:
          msg = "SVD variances can only be returned if mode=" "pca" "."
          raise ValueError(msg)
  
imports: from copy import deepcopy
  import numpy
  from mne.minimum_norm.inverse import InverseOperator
  from .._fiff.constants import FIFF
  from .._fiff.pick import pick_channels_forward
  from ..evoked import EvokedArray
  from ..forward.forward import Forward, convert_forward_solution
  from ..label import Label
  from ..source_estimate import _get_src_type, _make_stc, _prepare_label_extraction
  from ..source_space._source_space import SourceSpaces, _get_vertno
  from ..utils import _validate_type, logger, verbose
  from .inverse import apply_inverse
label_desc: Use image node_id 3 for calling a global function with example usage: _check_get_psf_ctf_params(mode, n_comp, return_pca_vars) without return types