Add documentation and error message to Reconstructor

lark/lark.py

@@ -100,6 +100,10 @@ class LarkOptions(Serialize):
             - When ``False``, does nothing (default)
             - When ``True``, caches to a temporary file in the local directory
             - When given a string, caches to the path pointed by the string
+    cache_grammar
+            For use with ``cache`` option. When ``True``, the unanalyzed grammar is also included in the cache.
+            Useful for classes that require the ``Lark.grammar`` to be present (e.g. Reconstructor).
+            (default= ``False``)
     regex
             When True, uses the ``regex`` module instead of the stdlib ``re``.
     g_regex_flags
@@ -336,7 +340,9 @@ def __init__(self, grammar: 'Union[Grammar, str, IO[str]]', **options) -> None:
                         # specific reason - we just want a username.
                         username = "unknown"
 
-                    cache_fn = tempfile.gettempdir() + "/.lark_cache_%s_%s_%s_%s.tmp" % (username, cache_sha256, *sys.version_info[:2])
+
+                    cache_fn = tempfile.gettempdir() + "/.lark_%s_%s_%s_%s_%s_%s.tmp" % (
+                        "cache_grammar" if self.options.cache_grammar else "cache", username, cache_sha256, *sys.version_info[:2])
 
                 old_options = self.options
                 try:

lark/reconstruct.py

@@ -3,6 +3,7 @@
 
 from typing import Dict, Callable, Iterable, Optional
 
+from .exceptions import ConfigurationError
 from .lark import Lark
 from .tree import Tree, ParseTree
 from .visitors import Transformer_InPlace
@@ -80,6 +81,9 @@ class Reconstructor(TreeMatcher):
     def __init__(self, parser: Lark, term_subs: Optional[Dict[str, Callable[[Symbol], str]]]=None) -> None:
         TreeMatcher.__init__(self, parser)
 
+        if not hasattr(parser, 'grammar') and parser.options.cache:
+            raise ConfigurationError('Unanalyzed grammar not available from cached parser, use cache_grammar=True')
+
         self.write_tokens = WriteTokensTransformer({t.name:t for t in self.tokens}, term_subs or {})
 
     def _reconstruct(self, tree):