Examples: Added example of extending the Python grammar

examples/advanced/extend_python.py

@@ -0,0 +1,46 @@
+"""
+Extend the Python Grammar
+==============================
+
+This example demonstrates how to use the `%extend` statement,
+to add new syntax to the example Python grammar.
+
+"""
+
+from lark.lark import Lark
+from python_parser import PythonIndenter
+
+GRAMMAR = r"""
+%import .python3 (compound_stmt, single_input, file_input, eval_input, test, suite, _NEWLINE, _INDENT, _DEDENT, COMMENT)
+
+%extend compound_stmt: match_stmt
+
+match_stmt: "match" test ":" cases
+cases: _NEWLINE _INDENT case+ _DEDENT
+
+case: "case" test ":" suite // test is not quite correct.
+
+%ignore /[\t \f]+/          // WS
+%ignore /\\[\t \f]*\r?\n/   // LINE_CONT
+%ignore COMMENT
+"""
+
+parser = Lark(GRAMMAR, parser='lalr', start=['single_input', 'file_input', 'eval_input'], postlex=PythonIndenter())
+
+tree = parser.parse(r"""
+def name(n):
+    match n:
+        case 1:
+            print("one")
+        case 2:
+            print("two")
+        case _:
+            print("number is too big")
+
+""", start='file_input')
+
+# Remove the 'python3__' prefix that was add to the implicitely imported rules.
+for t in tree.iter_subtrees():
+    t.data = t.data.rsplit('__', 1)[-1]
+
+print(tree.pretty())

examples/advanced/grammar_building.py

@@ -1,59 +0,0 @@
-from pathlib import Path
-
-from lark.indenter import Indenter
-from lark.lark import Lark
-from lark.load_grammar import GrammarBuilder
-
-MATCH_GRAMMAR = ('match', """
-
-%extend compound_stmt: match_stmt
-
-match_stmt: "match" test ":" cases
-
-cases: _NEWLINE _INDENT case+ _DEDENT
-
-case: "case" test ":" suite // test is not quite correct. 
-
-""", ('compound_stmt', 'test', 'suite', '_DEDENT', '_INDENT', '_NEWLINE'))
-
-EXTENSIONS = (MATCH_GRAMMAR,)
-
-builder = GrammarBuilder()
-
-builder.load_grammar((Path(__file__).with_name('python3.lark')).read_text(), 'python3')
-
-for name, ext_grammar, needed_names in EXTENSIONS:
-    mangle = builder.get_mangle(name, dict(zip(needed_names, needed_names)))
-    builder.load_grammar(ext_grammar, name, mangle)
-
-grammar = builder.build()
-
-
-class PythonIndenter(Indenter):
-    NL_type = '_NEWLINE'
-    OPEN_PAREN_types = ['LPAR', 'LSQB', 'LBRACE']
-    CLOSE_PAREN_types = ['RPAR', 'RSQB', 'RBRACE']
-    INDENT_type = '_INDENT'
-    DEDENT_type = '_DEDENT'
-    tab_len = 8
-
-
-parser = Lark(grammar, parser='lalr', start=['single_input', 'file_input', 'eval_input'], postlex=PythonIndenter())
-
-tree = parser.parse(r"""
-
-a = 5
-
-def name(n):
-    match n:
-        case 1:
-            print("one")
-        case 2:
-            print("two")
-        case _:
-            print("number is to big") 
-
-name(a)
-""", start='file_input')
-
-print(tree.pretty())

lark/load_grammar.py

@@ -977,6 +977,9 @@ class GrammarBuilder:
             aliases = dict(zip(names, names))  # Can't have aliased multi import, so all aliases will be the same as names
         else:  # Single import
             dotted_path = tuple(path_node.children[:-1])
+            if not dotted_path:
+                name ,= path_node.children
+                raise GrammarError("Nothing was imported from grammar `%s`" % name)
             name = path_node.children[-1]  # Get name from dotted path
             aliases = {name.value: (arg1 or name).value}  # Aliases if exist