Fix cache load failures by including open() in the try/except block

lark/lark.py

@@ -311,26 +311,28 @@ def __init__(self, grammar: 'Union[Grammar, str, IO[str]]', **options) -> None:
 
                     cache_fn = tempfile.gettempdir() + "/.lark_cache_%s_%s_%s_%s.tmp" % (getpass.getuser(), cache_md5, *sys.version_info[:2])
 
-                if FS.exists(cache_fn):
-                    logger.debug('Loading grammar from cache: %s', cache_fn)
-                    # Remove options that aren't relevant for loading from cache
-                    for name in (set(options) - _LOAD_ALLOWED_OPTIONS):
-                        del options[name]
+                old_options = self.options
+                try:
                     with FS.open(cache_fn, 'rb') as f:
-                        old_options = self.options
-                        try:
-                            file_md5 = f.readline().rstrip(b'\n')
-                            cached_used_files = pickle.load(f)
-                            if file_md5 == cache_md5.encode('utf8') and verify_used_files(cached_used_files):
-                                cached_parser_data = pickle.load(f)
-                                self._load(cached_parser_data, **options)
-                                return
-                        except Exception: # We should probably narrow done which errors we catch here.
-                            logger.exception("Failed to load Lark from cache: %r. We will try to carry on." % cache_fn)
-
-                            # In theory, the Lark instance might have been messed up by the call to `_load`.
-                            # In practice the only relevant thing that might have been overriden should be `options`
-                            self.options = old_options
+                        logger.debug('Loading grammar from cache: %s', cache_fn)
+                        # Remove options that aren't relevant for loading from cache
+                        for name in (set(options) - _LOAD_ALLOWED_OPTIONS):
+                            del options[name]
+                        file_md5 = f.readline().rstrip(b'\n')
+                        cached_used_files = pickle.load(f)
+                        if file_md5 == cache_md5.encode('utf8') and verify_used_files(cached_used_files):
+                            cached_parser_data = pickle.load(f)
+                            self._load(cached_parser_data, **options)
+                            return
+                except FileNotFoundError:
+                    # The cache file doesn't exist; parse and compose the grammar as normal
+                    pass
+                except Exception: # We should probably narrow done which errors we catch here.
+                    logger.exception("Failed to load Lark from cache: %r. We will try to carry on." % cache_fn)
+
+                    # In theory, the Lark instance might have been messed up by the call to `_load`.
+                    # In practice the only relevant thing that might have been overriden should be `options`
+                    self.options = old_options
 
 
             # Parse the grammar file and compose the grammars