class_docstr: CertStore represents a persistent file-based record of certificates, each associated with a
  particular master url.
code: def _load_store_file(self) -> Dict[str, str]:
      if not self.path.exists():
          return {}
  
      try:
          try:
              with self.path.open() as f:
                  store = json.load(f)
          except json.JSONDecodeError:
              raise api.errors.CorruptCertificateCacheException()
  
          # Store must be a dictionary.
          if not isinstance(store, dict):
              raise api.errors.CorruptCertificateCacheException()
  
          # All keys are url's, all values are pem-encoded certs.
          for k, v in store.items():
              if not isinstance(k, str):
                  raise api.errors.CorruptCertificateCacheException()
              if not isinstance(v, str):
                  raise api.errors.CorruptCertificateCacheException()
  
          return cast(Dict[str, str], store)
  
      except api.errors.CorruptCertificateCacheException:
          self.path.unlink()
          raise
  
imports: import atexit
  import contextlib
  import json
  import logging
  import os
  import pathlib
  import tempfile
  from typing import Dict, Iterator, Optional, Union, cast
  import certifi
  import filelock
  from determined.common import api, util
label_desc: Use image node_id 2 for calling the CertStore obj's underlying member method code with example usage: obj._load_store_file() and returns: dict, cast