code: def distance_metrics():
      """Valid metrics for pairwise_distances.
  
      This function simply returns the valid pairwise distance metrics.
      It exists to allow for a description of the mapping for
      each of the valid strings.
  
      The valid distance metrics, and the function they map to, are:
  
      ============     ====================================
      metric           Function
      ============     ====================================
      'cityblock'      metrics.pairwise.manhattan_distances
      'cosine'         metrics.pairwise.cosine_distances
      'euclidean'      metrics.pairwise.euclidean_distances
      'l1'             metrics.pairwise.manhattan_distances
      'l2'             metrics.pairwise.euclidean_distances
      'manhattan'      metrics.pairwise.manhattan_distances
      ============     ====================================
  
      Read more in the :ref:`User Guide <metrics>`.
  
      """
      return PAIRWISE_DISTANCE_FUNCTIONS
  
imports: import itertools
  import numpy
  from scipy.spatial import distance
  from scipy.sparse import csr_matrix
  from scipy.sparse import issparse
  from ..utils import check_array
  from ..utils import gen_even_slices
  from ..utils import gen_batches
  from ..utils.fixes import partial
  from ..utils.extmath import row_norms, safe_sparse_dot
  from ..preprocessing import normalize
  from ..externals.joblib import Parallel
  from ..externals.joblib import delayed
  from ..externals.joblib import cpu_count
  from .pairwise_fast import _chi2_kernel_fast, _sparse_manhattan
label_desc: Use image node_id 21 for calling a global function with example usage: distance_metrics() and returns: PAIRWISE_DISTANCE_FUNCTIONS