diff --git a/lark/common.py b/lark/common.py
index 698a3ec..ec7012b 100644
--- a/lark/common.py
+++ b/lark/common.py
@@ -1,7 +1,7 @@
 import re
 import sys
 
-from .utils import get_regexp_width, STRING_TYPE
+from .utils import get_regexp_width
 
 Py36 = (sys.version_info[:2] >= (3, 6))
 
@@ -40,6 +40,9 @@ class Pattern(object):
     def __eq__(self, other):
         return type(self) == type(other) and self.value == other.value and self.flags == other.flags
 
+    def to_regexp(self):
+        raise NotImplementedError()
+
     if Py36:
         # Python 3.6 changed syntax for flags in regular expression
         def _get_flags(self, value):
diff --git a/lark/exceptions.py b/lark/exceptions.py
index 73b206a..ceca493 100644
--- a/lark/exceptions.py
+++ b/lark/exceptions.py
@@ -13,6 +13,8 @@ class LexError(LarkError):
     pass
 
 class UnexpectedInput(LarkError):
+    pos_in_stream = None
+
     def get_context(self, text, span=40):
         pos = self.pos_in_stream
         start = max(pos - span, 0)
diff --git a/lark/lexer.py b/lark/lexer.py
index 3ab91b4..0950a00 100644
--- a/lark/lexer.py
+++ b/lark/lexer.py
@@ -113,7 +113,6 @@ class UnlessCallback:
         for mre, type_from_index in self.mres:
             m = mre.match(t.value)
             if m:
-                value = m.group(0)
                 t.type = type_from_index[m.lastindex]
                 break
         return t
diff --git a/lark/parser_frontends.py b/lark/parser_frontends.py
index 7bd9378..7dd972d 100644
--- a/lark/parser_frontends.py
+++ b/lark/parser_frontends.py
@@ -5,16 +5,19 @@ from .utils import get_regexp_width
 from .parsers.grammar_analysis import GrammarAnalyzer
 from .lexer import TraditionalLexer, ContextualLexer, Lexer, Token
 
-from .exceptions import GrammarError
 from .parsers import lalr_parser, earley, xearley, resolve_ambig, cyk
 from .tree import Tree
 
 class WithLexer:
+    lexer = None
+    parser = None
+    lexer_conf = None
+
     def init_traditional_lexer(self, lexer_conf):
         self.lexer_conf = lexer_conf
         self.lexer = TraditionalLexer(lexer_conf.tokens, ignore=lexer_conf.ignore, user_callbacks=lexer_conf.callbacks)
 
-    def init_contextual_lexer(self, lexer_conf, parser_conf):
+    def init_contextual_lexer(self, lexer_conf):
         self.lexer_conf = lexer_conf
         states = {idx:list(t.keys()) for idx, t in self.parser._parse_table.states.items()}
         always_accept = lexer_conf.postlex.always_accept if lexer_conf.postlex else ()
@@ -27,8 +30,7 @@ class WithLexer:
         stream = self.lexer.lex(text)
         if self.lexer_conf.postlex:
             return self.lexer_conf.postlex.process(stream)
-        else:
-            return stream
+        return stream
 
     def parse(self, text):
         token_stream = self.lex(text)
@@ -43,7 +45,7 @@ class LALR_TraditionalLexer(WithLexer):
 class LALR_ContextualLexer(WithLexer):
     def __init__(self, lexer_conf, parser_conf, options=None):
         self.parser = lalr_parser.Parser(parser_conf)
-        self.init_contextual_lexer(lexer_conf, parser_conf)
+        self.init_contextual_lexer(lexer_conf)
 
 class LALR_CustomLexer(WithLexer):
     def __init__(self, lexer_cls, lexer_conf, parser_conf, options=None):
diff --git a/lark/utils.py b/lark/utils.py
index 6f603bb..c86a66d 100644
--- a/lark/utils.py
+++ b/lark/utils.py
@@ -105,8 +105,7 @@ except NameError:
             return 0
         elif a > b:
             return 1
-        else:
-            return -1
+        return -1
 
 
 import sre_parse
diff --git a/lark/visitors.py b/lark/visitors.py
index cb7e32b..ec9bde6 100644
--- a/lark/visitors.py
+++ b/lark/visitors.py
@@ -1,4 +1,4 @@
-from inspect import isclass, getmembers, getmro
+from inspect import getmembers, getmro
 from functools import wraps
 
 from .utils import smart_decorator