Traverses directory collecting input and target files.

def _collect_data(directory, input_ext, target_ext):
  """Traverses directory collecting input and target files."""
  # Directory from string to tuple pair of strings
  # key: the filepath to a datafile including the datafile's basename. Example,
  #   if the datafile was "/path/to/datafile.wav" then the key would be
  #   "/path/to/datafile"
  # value: a pair of strings (input_filepath, target_filepath)
  data_files = {}
  for root, _, filenames in os.walk(directory):
    input_files = [filename for filename in filenames if input_ext in filename]
    for input_filename in input_files:
      basename = input_filename.strip(input_ext)
      input_file = os.path.join(root, input_filename)
      target_file = os.path.join(root, basename + target_ext)
      key = os.path.join(root, basename)
      assert os.path.exists(target_file)
      assert key not in data_files
      data_files[key] = (input_file, target_file)
  return data_files