class_docstr: Structure to discover and store `Dataset` dependencies.
  
  Used primarily by the `Scene` object to organize dependency finding.
  Dependencies are stored used a series of `Node` objects which this
  class is a subclass of.
class_docstr_tok: ["Structure","to","discover","and","store","`","Dataset","`","dependencies",".","Used","primarily","by","the","`","Scene","`","object","to","organize","dependency","finding",".","Dependencies","are","stored","used","a","series","of","`","Node","`","objects","which","this","class","is","a","subclass","of","."]
code: def _get_subtree_for_existing_name(self, dsq):
      try:
          # assume that there is no such thing as a "better" composite
          # version so if we find any DataIDs already loaded then
          # we want to use them
          node = self[dsq]
          LOG.trace(
              "Composite already loaded:\n\tRequested: {}\n\tFound: {}".format(
                  dsq, node.name
              )
          )
          return node
      except KeyError:
          # composite hasn't been loaded yet, let's load it below
          LOG.trace(
              "Composite hasn't been loaded yet, will load: {}".format(
                  dsq
              )
          )
          raise MissingDependencies({dsq})
  
imports: from __future__ import annotations
  from typing import Container, Iterable, Optional
  import numpy
  from satpy import DataID, DatasetDict
  from satpy.dataset import ModifierTuple, create_filtered_query
  from satpy.dataset.data_dict import TooManyResults, get_key
  from satpy.node import EMPTY_LEAF_NAME, LOG, CompositorNode, MissingDependencies, Node, ReaderNode
label_desc: Use image node_id 14 for calling the DependencyTree obj's underlying member method code with example usage: obj._get_subtree_for_existing_name(dsq) and returns: node