source: pyyaml/trunk/lib/yaml/scanner.py @ 188

Revision 188, 51.6 KB checked in by xi, 9 years ago (diff)

Fix several problems caused by ill-formed documents.

The line number is not calculated correctly for DOS-style line breaks.

Fix error reporting in remove_possible_simple_key. The problem is caused by the document:

+foo: &A bar
+*A ]

Raise an error for a complex key which is not indented correctly, for instance:

? "foo"
 : "bar"
Line 
1
2# Scanner produces tokens of the following types:
3# STREAM-START
4# STREAM-END
5# DIRECTIVE(name, value)
6# DOCUMENT-START
7# DOCUMENT-END
8# BLOCK-SEQUENCE-START
9# BLOCK-MAPPING-START
10# BLOCK-END
11# FLOW-SEQUENCE-START
12# FLOW-MAPPING-START
13# FLOW-SEQUENCE-END
14# FLOW-MAPPING-END
15# BLOCK-ENTRY
16# FLOW-ENTRY
17# KEY
18# VALUE
19# ALIAS(value)
20# ANCHOR(value)
21# TAG(value)
22# SCALAR(value, plain)
23#
24# Read comments in the Scanner code for more details.
25#
26
27__all__ = ['Scanner', 'ScannerError']
28
29from error import MarkedYAMLError
30from tokens import *
31
32class ScannerError(MarkedYAMLError):
33    pass
34
35class SimpleKey:
36    # See below simple keys treatment.
37
38    def __init__(self, token_number, required, index, line, column, mark):
39        self.token_number = token_number
40        self.required = required
41        self.index = index
42        self.line = line
43        self.column = column
44        self.mark = mark
45
46class Scanner:
47
48    def __init__(self):
49        """Initialize the scanner."""
50        # It is assumed that Scanner and Reader will have a common descendant.
51        # Reader do the dirty work of checking for BOM and converting the
52        # input data to Unicode. It also adds NUL to the end.
53        #
54        # Reader supports the following methods
55        #   self.peek(i=0)       # peek the next i-th character
56        #   self.prefix(l=1)     # peek the next l characters
57        #   self.forward(l=1)    # read the next l characters and move the pointer.
58
59        # Had we reached the end of the stream?
60        self.done = False
61
62        # The number of unclosed '{' and '['. `flow_level == 0` means block
63        # context.
64        self.flow_level = 0
65
66        # List of processed tokens that are not yet emitted.
67        self.tokens = []
68
69        # Add the STREAM-START token.
70        self.fetch_stream_start()
71
72        # Number of tokens that were emitted through the `get_token` method.
73        self.tokens_taken = 0
74
75        # The current indentation level.
76        self.indent = -1
77
78        # Past indentation levels.
79        self.indents = []
80
81        # Variables related to simple keys treatment.
82
83        # A simple key is a key that is not denoted by the '?' indicator.
84        # Example of simple keys:
85        #   ---
86        #   block simple key: value
87        #   ? not a simple key:
88        #   : { flow simple key: value }
89        # We emit the KEY token before all keys, so when we find a potential
90        # simple key, we try to locate the corresponding ':' indicator.
91        # Simple keys should be limited to a single line and 1024 characters.
92
93        # Can a simple key start at the current position? A simple key may
94        # start:
95        # - at the beginning of the line, not counting indentation spaces
96        #       (in block context),
97        # - after '{', '[', ',' (in the flow context),
98        # - after '?', ':', '-' (in the block context).
99        # In the block context, this flag also signifies if a block collection
100        # may start at the current position.
101        self.allow_simple_key = True
102
103        # Keep track of possible simple keys. This is a dictionary. The key
104        # is `flow_level`; there can be no more that one possible simple key
105        # for each level. The value is a SimpleKey record:
106        #   (token_number, required, index, line, column, mark)
107        # A simple key may start with ALIAS, ANCHOR, TAG, SCALAR(flow),
108        # '[', or '{' tokens.
109        self.possible_simple_keys = {}
110
111    # Public methods.
112
113    def check_token(self, *choices):
114        # Check if the next token is one of the given types.
115        while self.need_more_tokens():
116            self.fetch_more_tokens()
117        if self.tokens:
118            if not choices:
119                return True
120            for choice in choices:
121                if isinstance(self.tokens[0], choice):
122                    return True
123        return False
124
125    def peek_token(self):
126        # Return the next token, but do not delete if from the queue.
127        while self.need_more_tokens():
128            self.fetch_more_tokens()
129        if self.tokens:
130            return self.tokens[0]
131
132    def get_token(self):
133        # Return the next token.
134        while self.need_more_tokens():
135            self.fetch_more_tokens()
136        if self.tokens:
137            self.tokens_taken += 1
138            return self.tokens.pop(0)
139
140    def __iter__(self):
141        # Iterator protocol.
142        while self.need_more_tokens():
143            self.fetch_more_tokens()
144        while self.tokens:
145            self.tokens_taken += 1
146            yield self.tokens.pop(0)
147            while self.need_more_tokens():
148                self.fetch_more_tokens()
149
150    # Private methods.
151
152    def need_more_tokens(self):
153        if self.done:
154            return False
155        if not self.tokens:
156            return True
157        # The current token may be a potential simple key, so we
158        # need to look further.
159        self.stale_possible_simple_keys()
160        if self.next_possible_simple_key() == self.tokens_taken:
161            return True
162
163    def fetch_more_tokens(self):
164
165        # Eat whitespaces and comments until we reach the next token.
166        self.scan_to_next_token()
167
168        # Remove obsolete possible simple keys.
169        self.stale_possible_simple_keys()
170
171        # Compare the current indentation and column. It may add some tokens
172        # and decrease the current indentation level.
173        self.unwind_indent(self.column)
174
175        # Peek the next character.
176        ch = self.peek()
177
178        # Is it the end of stream?
179        if ch == u'\0':
180            return self.fetch_stream_end()
181
182        # Is it a directive?
183        if ch == u'%' and self.check_directive():
184            return self.fetch_directive()
185
186        # Is it the document start?
187        if ch == u'-' and self.check_document_start():
188            return self.fetch_document_start()
189
190        # Is it the document end?
191        if ch == u'.' and self.check_document_end():
192            return self.fetch_document_end()
193
194        # TODO: support for BOM within a stream.
195        #if ch == u'\uFEFF':
196        #    return self.fetch_bom()    <-- issue BOMToken
197
198        # Note: the order of the following checks is NOT significant.
199
200        # Is it the flow sequence start indicator?
201        if ch == u'[':
202            return self.fetch_flow_sequence_start()
203
204        # Is it the flow mapping start indicator?
205        if ch == u'{':
206            return self.fetch_flow_mapping_start()
207
208        # Is it the flow sequence end indicator?
209        if ch == u']':
210            return self.fetch_flow_sequence_end()
211
212        # Is it the flow mapping end indicator?
213        if ch == u'}':
214            return self.fetch_flow_mapping_end()
215
216        # Is it the flow entry indicator?
217        if ch == u',':
218            return self.fetch_flow_entry()
219
220        # Is it the block entry indicator?
221        if ch == u'-' and self.check_block_entry():
222            return self.fetch_block_entry()
223
224        # Is it the key indicator?
225        if ch == u'?' and self.check_key():
226            return self.fetch_key()
227
228        # Is it the value indicator?
229        if ch == u':' and self.check_value():
230            return self.fetch_value()
231
232        # Is it an alias?
233        if ch == u'*':
234            return self.fetch_alias()
235
236        # Is it an anchor?
237        if ch == u'&':
238            return self.fetch_anchor()
239
240        # Is it a tag?
241        if ch == u'!':
242            return self.fetch_tag()
243
244        # Is it a literal scalar?
245        if ch == u'|' and not self.flow_level:
246            return self.fetch_literal()
247
248        # Is it a folded scalar?
249        if ch == u'>' and not self.flow_level:
250            return self.fetch_folded()
251
252        # Is it a single quoted scalar?
253        if ch == u'\'':
254            return self.fetch_single()
255
256        # Is it a double quoted scalar?
257        if ch == u'\"':
258            return self.fetch_double()
259
260        # It must be a plain scalar then.
261        if self.check_plain():
262            return self.fetch_plain()
263
264        # No? It's an error. Let's produce a nice error message.
265        raise ScannerError("while scanning for the next token", None,
266                "found character %r that cannot start any token"
267                % ch.encode('utf-8'), self.get_mark())
268
269    # Simple keys treatment.
270
271    def next_possible_simple_key(self):
272        # Return the number of the nearest possible simple key. Actually we
273        # don't need to loop through the whole dictionary. We may replace it
274        # with the following code:
275        #   if not self.possible_simple_keys:
276        #       return None
277        #   return self.possible_simple_keys[
278        #           min(self.possible_simple_keys.keys())].token_number
279        min_token_number = None
280        for level in self.possible_simple_keys:
281            key = self.possible_simple_keys[level]
282            if min_token_number is None or key.token_number < min_token_number:
283                min_token_number = key.token_number
284        return min_token_number
285
286    def stale_possible_simple_keys(self):
287        # Remove entries that are no longer possible simple keys. According to
288        # the YAML specification, simple keys
289        # - should be limited to a single line,
290        # - should be no longer than 1024 characters.
291        # Disabling this procedure will allow simple keys of any length and
292        # height (may cause problems if indentation is broken though).
293        for level in self.possible_simple_keys.keys():
294            key = self.possible_simple_keys[level]
295            if key.line != self.line  \
296                    or self.index-key.index > 1024:
297                if key.required:
298                    raise ScannerError("while scanning a simple key", key.mark,
299                            "could not found expected ':'", self.get_mark())
300                del self.possible_simple_keys[level]
301
302    def save_possible_simple_key(self):
303        # The next token may start a simple key. We check if it's possible
304        # and save its position. This function is called for
305        #   ALIAS, ANCHOR, TAG, SCALAR(flow), '[', and '{'.
306
307        # Check if a simple key is required at the current position.
308        required = not self.flow_level and self.indent == self.column
309
310        # A simple key is required only if it is the first token in the current
311        # line. Therefore it is always allowed.
312        assert self.allow_simple_key or not required
313
314        # The next token might be a simple key. Let's save it's number and
315        # position.
316        if self.allow_simple_key:
317            self.remove_possible_simple_key()
318            token_number = self.tokens_taken+len(self.tokens)
319            key = SimpleKey(token_number, required,
320                    self.index, self.line, self.column, self.get_mark())
321            self.possible_simple_keys[self.flow_level] = key
322
323    def remove_possible_simple_key(self):
324        # Remove the saved possible key position at the current flow level.
325        if self.flow_level in self.possible_simple_keys:
326            key = self.possible_simple_keys[self.flow_level]
327           
328            if key.required:
329                raise ScannerError("while scanning a simple key", key.mark,
330                        "could not found expected ':'", self.get_mark())
331
332            del self.possible_simple_keys[self.flow_level]
333
334    # Indentation functions.
335
336    def unwind_indent(self, column):
337
338        ## In flow context, tokens should respect indentation.
339        ## Actually the condition should be `self.indent >= column` according to
340        ## the spec. But this condition will prohibit intuitively correct
341        ## constructions such as
342        ## key : {
343        ## }
344        #if self.flow_level and self.indent > column:
345        #    raise ScannerError(None, None,
346        #            "invalid intendation or unclosed '[' or '{'",
347        #            self.get_mark())
348
349        # In the flow context, indentation is ignored. We make the scanner less
350        # restrictive then specification requires.
351        if self.flow_level:
352            return
353
354        # In block context, we may need to issue the BLOCK-END tokens.
355        while self.indent > column:
356            mark = self.get_mark()
357            self.indent = self.indents.pop()
358            self.tokens.append(BlockEndToken(mark, mark))
359
360    def add_indent(self, column):
361        # Check if we need to increase indentation.
362        if self.indent < column:
363            self.indents.append(self.indent)
364            self.indent = column
365            return True
366        return False
367
368    # Fetchers.
369
370    def fetch_stream_start(self):
371        # We always add STREAM-START as the first token and STREAM-END as the
372        # last token.
373
374        # Read the token.
375        mark = self.get_mark()
376       
377        # Add STREAM-START.
378        self.tokens.append(StreamStartToken(mark, mark,
379            encoding=self.encoding))
380       
381
382    def fetch_stream_end(self):
383
384        # Set the current intendation to -1.
385        self.unwind_indent(-1)
386
387        # Reset everything (not really needed).
388        self.allow_simple_key = False
389        self.possible_simple_keys = {}
390
391        # Read the token.
392        mark = self.get_mark()
393       
394        # Add STREAM-END.
395        self.tokens.append(StreamEndToken(mark, mark))
396
397        # The steam is finished.
398        self.done = True
399
400    def fetch_directive(self):
401       
402        # Set the current intendation to -1.
403        self.unwind_indent(-1)
404
405        # Reset simple keys.
406        self.remove_possible_simple_key()
407        self.allow_simple_key = False
408
409        # Scan and add DIRECTIVE.
410        self.tokens.append(self.scan_directive())
411
412    def fetch_document_start(self):
413        self.fetch_document_indicator(DocumentStartToken)
414
415    def fetch_document_end(self):
416        self.fetch_document_indicator(DocumentEndToken)
417
418    def fetch_document_indicator(self, TokenClass):
419
420        # Set the current intendation to -1.
421        self.unwind_indent(-1)
422
423        # Reset simple keys. Note that there could not be a block collection
424        # after '---'.
425        self.remove_possible_simple_key()
426        self.allow_simple_key = False
427
428        # Add DOCUMENT-START or DOCUMENT-END.
429        start_mark = self.get_mark()
430        self.forward(3)
431        end_mark = self.get_mark()
432        self.tokens.append(TokenClass(start_mark, end_mark))
433
434    def fetch_flow_sequence_start(self):
435        self.fetch_flow_collection_start(FlowSequenceStartToken)
436
437    def fetch_flow_mapping_start(self):
438        self.fetch_flow_collection_start(FlowMappingStartToken)
439
440    def fetch_flow_collection_start(self, TokenClass):
441
442        # '[' and '{' may start a simple key.
443        self.save_possible_simple_key()
444
445        # Increase the flow level.
446        self.flow_level += 1
447
448        # Simple keys are allowed after '[' and '{'.
449        self.allow_simple_key = True
450
451        # Add FLOW-SEQUENCE-START or FLOW-MAPPING-START.
452        start_mark = self.get_mark()
453        self.forward()
454        end_mark = self.get_mark()
455        self.tokens.append(TokenClass(start_mark, end_mark))
456
457    def fetch_flow_sequence_end(self):
458        self.fetch_flow_collection_end(FlowSequenceEndToken)
459
460    def fetch_flow_mapping_end(self):
461        self.fetch_flow_collection_end(FlowMappingEndToken)
462
463    def fetch_flow_collection_end(self, TokenClass):
464
465        # Reset possible simple key on the current level.
466        self.remove_possible_simple_key()
467
468        # Decrease the flow level.
469        self.flow_level -= 1
470
471        # No simple keys after ']' or '}'.
472        self.allow_simple_key = False
473
474        # Add FLOW-SEQUENCE-END or FLOW-MAPPING-END.
475        start_mark = self.get_mark()
476        self.forward()
477        end_mark = self.get_mark()
478        self.tokens.append(TokenClass(start_mark, end_mark))
479
480    def fetch_flow_entry(self):
481
482        # Simple keys are allowed after ','.
483        self.allow_simple_key = True
484
485        # Reset possible simple key on the current level.
486        self.remove_possible_simple_key()
487
488        # Add FLOW-ENTRY.
489        start_mark = self.get_mark()
490        self.forward()
491        end_mark = self.get_mark()
492        self.tokens.append(FlowEntryToken(start_mark, end_mark))
493
494    def fetch_block_entry(self):
495
496        # Block context needs additional checks.
497        if not self.flow_level:
498
499            # Are we allowed to start a new entry?
500            if not self.allow_simple_key:
501                raise ScannerError(None, None,
502                        "sequence entries are not allowed here",
503                        self.get_mark())
504
505            # We may need to add BLOCK-SEQUENCE-START.
506            if self.add_indent(self.column):
507                mark = self.get_mark()
508                self.tokens.append(BlockSequenceStartToken(mark, mark))
509
510        # It's an error for the block entry to occur in the flow context,
511        # but we let the parser detect this.
512        else:
513            pass
514
515        # Simple keys are allowed after '-'.
516        self.allow_simple_key = True
517
518        # Reset possible simple key on the current level.
519        self.remove_possible_simple_key()
520
521        # Add BLOCK-ENTRY.
522        start_mark = self.get_mark()
523        self.forward()
524        end_mark = self.get_mark()
525        self.tokens.append(BlockEntryToken(start_mark, end_mark))
526
527    def fetch_key(self):
528       
529        # Block context needs additional checks.
530        if not self.flow_level:
531
532            # Are we allowed to start a key (not nessesary a simple)?
533            if not self.allow_simple_key:
534                raise ScannerError(None, None,
535                        "mapping keys are not allowed here",
536                        self.get_mark())
537
538            # We may need to add BLOCK-MAPPING-START.
539            if self.add_indent(self.column):
540                mark = self.get_mark()
541                self.tokens.append(BlockMappingStartToken(mark, mark))
542
543        # Simple keys are allowed after '?' in the block context.
544        self.allow_simple_key = not self.flow_level
545
546        # Reset possible simple key on the current level.
547        self.remove_possible_simple_key()
548
549        # Add KEY.
550        start_mark = self.get_mark()
551        self.forward()
552        end_mark = self.get_mark()
553        self.tokens.append(KeyToken(start_mark, end_mark))
554
555    def fetch_value(self):
556
557        # Do we determine a simple key?
558        if self.flow_level in self.possible_simple_keys:
559
560            # Add KEY.
561            key = self.possible_simple_keys[self.flow_level]
562            del self.possible_simple_keys[self.flow_level]
563            self.tokens.insert(key.token_number-self.tokens_taken,
564                    KeyToken(key.mark, key.mark))
565
566            # If this key starts a new block mapping, we need to add
567            # BLOCK-MAPPING-START.
568            if not self.flow_level:
569                if self.add_indent(key.column):
570                    self.tokens.insert(key.token_number-self.tokens_taken,
571                            BlockMappingStartToken(key.mark, key.mark))
572
573            # There cannot be two simple keys one after another.
574            self.allow_simple_key = False
575
576        # It must be a part of a complex key.
577        else:
578           
579            # Block context needs additional checks.
580            # (Do we really need them? They will be catched by the parser
581            # anyway.)
582            if not self.flow_level:
583
584                # We are allowed to start a complex value if and only if
585                # we can start a simple key.
586                if not self.allow_simple_key:
587                    raise ScannerError(None, None,
588                            "mapping values are not allowed here",
589                            self.get_mark())
590
591            # If this value starts a new block mapping, we need to add
592            # BLOCK-MAPPING-START.  It will be detected as an error later by
593            # the parser.
594            if not self.flow_level:
595                if self.add_indent(self.column):
596                    mark = self.get_mark()
597                    self.tokens.append(BlockMappingStartToken(mark, mark))
598
599            # Simple keys are allowed after ':' in the block context.
600            self.allow_simple_key = not self.flow_level
601
602            # Reset possible simple key on the current level.
603            self.remove_possible_simple_key()
604
605        # Add VALUE.
606        start_mark = self.get_mark()
607        self.forward()
608        end_mark = self.get_mark()
609        self.tokens.append(ValueToken(start_mark, end_mark))
610
611    def fetch_alias(self):
612
613        # ALIAS could be a simple key.
614        self.save_possible_simple_key()
615
616        # No simple keys after ALIAS.
617        self.allow_simple_key = False
618
619        # Scan and add ALIAS.
620        self.tokens.append(self.scan_anchor(AliasToken))
621
622    def fetch_anchor(self):
623
624        # ANCHOR could start a simple key.
625        self.save_possible_simple_key()
626
627        # No simple keys after ANCHOR.
628        self.allow_simple_key = False
629
630        # Scan and add ANCHOR.
631        self.tokens.append(self.scan_anchor(AnchorToken))
632
633    def fetch_tag(self):
634
635        # TAG could start a simple key.
636        self.save_possible_simple_key()
637
638        # No simple keys after TAG.
639        self.allow_simple_key = False
640
641        # Scan and add TAG.
642        self.tokens.append(self.scan_tag())
643
644    def fetch_literal(self):
645        self.fetch_block_scalar(style='|')
646
647    def fetch_folded(self):
648        self.fetch_block_scalar(style='>')
649
650    def fetch_block_scalar(self, style):
651
652        # A simple key may follow a block scalar.
653        self.allow_simple_key = True
654
655        # Reset possible simple key on the current level.
656        self.remove_possible_simple_key()
657
658        # Scan and add SCALAR.
659        self.tokens.append(self.scan_block_scalar(style))
660
661    def fetch_single(self):
662        self.fetch_flow_scalar(style='\'')
663
664    def fetch_double(self):
665        self.fetch_flow_scalar(style='"')
666
667    def fetch_flow_scalar(self, style):
668
669        # A flow scalar could be a simple key.
670        self.save_possible_simple_key()
671
672        # No simple keys after flow scalars.
673        self.allow_simple_key = False
674
675        # Scan and add SCALAR.
676        self.tokens.append(self.scan_flow_scalar(style))
677
678    def fetch_plain(self):
679
680        # A plain scalar could be a simple key.
681        self.save_possible_simple_key()
682
683        # No simple keys after plain scalars. But note that `scan_plain` will
684        # change this flag if the scan is finished at the beginning of the
685        # line.
686        self.allow_simple_key = False
687
688        # Scan and add SCALAR. May change `allow_simple_key`.
689        self.tokens.append(self.scan_plain())
690
691    # Checkers.
692
693    def check_directive(self):
694
695        # DIRECTIVE:        ^ '%' ...
696        # The '%' indicator is already checked.
697        if self.column == 0:
698            return True
699
700    def check_document_start(self):
701
702        # DOCUMENT-START:   ^ '---' (' '|'\n')
703        if self.column == 0:
704            if self.prefix(3) == u'---'  \
705                    and self.peek(3) in u'\0 \t\r\n\x85\u2028\u2029':
706                return True
707
708    def check_document_end(self):
709
710        # DOCUMENT-END:     ^ '...' (' '|'\n')
711        if self.column == 0:
712            if self.prefix(3) == u'...'  \
713                    and self.peek(3) in u'\0 \t\r\n\x85\u2028\u2029':
714                return True
715
716    def check_block_entry(self):
717
718        # BLOCK-ENTRY:      '-' (' '|'\n')
719        return self.peek(1) in u'\0 \t\r\n\x85\u2028\u2029'
720
721    def check_key(self):
722
723        # KEY(flow context):    '?'
724        if self.flow_level:
725            return True
726
727        # KEY(block context):   '?' (' '|'\n')
728        else:
729            return self.peek(1) in u'\0 \t\r\n\x85\u2028\u2029'
730
731    def check_value(self):
732
733        # VALUE(flow context):  ':'
734        if self.flow_level:
735            return True
736
737        # VALUE(block context): ':' (' '|'\n')
738        else:
739            return self.peek(1) in u'\0 \t\r\n\x85\u2028\u2029'
740
741    def check_plain(self):
742
743        # A plain scalar may start with any non-space character except:
744        #   '-', '?', ':', ',', '[', ']', '{', '}',
745        #   '#', '&', '*', '!', '|', '>', '\'', '\"',
746        #   '%', '@', '`'.
747        #
748        # It may also start with
749        #   '-', '?', ':'
750        # if it is followed by a non-space character.
751        #
752        # Note that we limit the last rule to the block context (except the
753        # '-' character) because we want the flow context to be space
754        # independent.
755        ch = self.peek()
756        return ch not in u'\0 \t\r\n\x85\u2028\u2029-?:,[]{}#&*!|>\'\"%@`'  \
757                or (self.peek(1) not in u'\0 \t\r\n\x85\u2028\u2029'
758                        and (ch == u'-' or (not self.flow_level and ch in u'?:')))
759
760    # Scanners.
761
762    def scan_to_next_token(self):
763        # We ignore spaces, line breaks and comments.
764        # If we find a line break in the block context, we set the flag
765        # `allow_simple_key` on.
766        # The byte order mark is stripped if it's the first character in the
767        # stream. We do not yet support BOM inside the stream as the
768        # specification requires. Any such mark will be considered as a part
769        # of the document.
770        #
771        # TODO: We need to make tab handling rules more sane. A good rule is
772        #   Tabs cannot precede tokens
773        #   BLOCK-SEQUENCE-START, BLOCK-MAPPING-START, BLOCK-END,
774        #   KEY(block), VALUE(block), BLOCK-ENTRY
775        # So the checking code is
776        #   if <TAB>:
777        #       self.allow_simple_keys = False
778        # We also need to add the check for `allow_simple_keys == True` to
779        # `unwind_indent` before issuing BLOCK-END.
780        # Scanners for block, flow, and plain scalars need to be modified.
781
782        if self.index == 0 and self.peek() == u'\uFEFF':
783            self.forward()
784        found = False
785        while not found:
786            while self.peek() == u' ':
787                self.forward()
788            if self.peek() == u'#':
789                while self.peek() not in u'\0\r\n\x85\u2028\u2029':
790                    self.forward()
791            if self.scan_line_break():
792                if not self.flow_level:
793                    self.allow_simple_key = True
794            else:
795                found = True
796
797    def scan_directive(self):
798        # See the specification for details.
799        start_mark = self.get_mark()
800        self.forward()
801        name = self.scan_directive_name(start_mark)
802        value = None
803        if name == u'YAML':
804            value = self.scan_yaml_directive_value(start_mark)
805            end_mark = self.get_mark()
806        elif name == u'TAG':
807            value = self.scan_tag_directive_value(start_mark)
808            end_mark = self.get_mark()
809        else:
810            end_mark = self.get_mark()
811            while self.peek() not in u'\0\r\n\x85\u2028\u2029':
812                self.forward()
813        self.scan_directive_ignored_line(start_mark)
814        return DirectiveToken(name, value, start_mark, end_mark)
815
816    def scan_directive_name(self, start_mark):
817        # See the specification for details.
818        length = 0
819        ch = self.peek(length)
820        while u'0' <= ch <= u'9' or u'A' <= ch <= 'Z' or u'a' <= ch <= 'z'  \
821                or ch in u'-_':
822            length += 1
823            ch = self.peek(length)
824        if not length:
825            raise ScannerError("while scanning a directive", start_mark,
826                    "expected alphabetic or numeric character, but found %r"
827                    % ch.encode('utf-8'), self.get_mark())
828        value = self.prefix(length)
829        self.forward(length)
830        ch = self.peek()
831        if ch not in u'\0 \r\n\x85\u2028\u2029':
832            raise ScannerError("while scanning a directive", start_mark,
833                    "expected alphabetic or numeric character, but found %r"
834                    % ch.encode('utf-8'), self.get_mark())
835        return value
836
837    def scan_yaml_directive_value(self, start_mark):
838        # See the specification for details.
839        while self.peek() == u' ':
840            self.forward()
841        major = self.scan_yaml_directive_number(start_mark)
842        if self.peek() != '.':
843            raise ScannerError("while scanning a directive", start_mark,
844                    "expected a digit or '.', but found %r"
845                    % self.peek().encode('utf-8'),
846                    self.get_mark())
847        self.forward()
848        minor = self.scan_yaml_directive_number(start_mark)
849        if self.peek() not in u'\0 \r\n\x85\u2028\u2029':
850            raise ScannerError("while scanning a directive", start_mark,
851                    "expected a digit or ' ', but found %r"
852                    % self.peek().encode('utf-8'),
853                    self.get_mark())
854        return (major, minor)
855
856    def scan_yaml_directive_number(self, start_mark):
857        # See the specification for details.
858        ch = self.peek()
859        if not (u'0' <= ch <= '9'):
860            raise ScannerError("while scanning a directive", start_mark,
861                    "expected a digit, but found %r" % ch.encode('utf-8'),
862                    self.get_mark())
863        length = 0
864        while u'0' <= self.peek(length) <= u'9':
865            length += 1
866        value = int(self.prefix(length))
867        self.forward(length)
868        return value
869
870    def scan_tag_directive_value(self, start_mark):
871        # See the specification for details.
872        while self.peek() == u' ':
873            self.forward()
874        handle = self.scan_tag_directive_handle(start_mark)
875        while self.peek() == u' ':
876            self.forward()
877        prefix = self.scan_tag_directive_prefix(start_mark)
878        return (handle, prefix)
879
880    def scan_tag_directive_handle(self, start_mark):
881        # See the specification for details.
882        value = self.scan_tag_handle('directive', start_mark)
883        ch = self.peek()
884        if ch != u' ':
885            raise ScannerError("while scanning a directive", start_mark,
886                    "expected ' ', but found %r" % ch.encode('utf-8'),
887                    self.get_mark())
888        return value
889
890    def scan_tag_directive_prefix(self, start_mark):
891        # See the specification for details.
892        value = self.scan_tag_uri('directive', start_mark)
893        ch = self.peek()
894        if ch not in u'\0 \r\n\x85\u2028\u2029':
895            raise ScannerError("while scanning a directive", start_mark,
896                    "expected ' ', but found %r" % ch.encode('utf-8'),
897                    self.get_mark())
898        return value
899
900    def scan_directive_ignored_line(self, start_mark):
901        # See the specification for details.
902        while self.peek() == u' ':
903            self.forward()
904        if self.peek() == u'#':
905            while self.peek() not in u'\0\r\n\x85\u2028\u2029':
906                self.forward()
907        ch = self.peek()
908        if ch not in u'\0\r\n\x85\u2028\u2029':
909            raise ScannerError("while scanning a directive", start_mark,
910                    "expected a comment or a line break, but found %r"
911                        % ch.encode('utf-8'), self.get_mark())
912        self.scan_line_break()
913
914    def scan_anchor(self, TokenClass):
915        # The specification does not restrict characters for anchors and
916        # aliases. This may lead to problems, for instance, the document:
917        #   [ *alias, value ]
918        # can be interpteted in two ways, as
919        #   [ "value" ]
920        # and
921        #   [ *alias , "value" ]
922        # Therefore we restrict aliases to numbers and ASCII letters.
923        start_mark = self.get_mark()
924        indicator = self.peek()
925        if indicator == '*':
926            name = 'alias'
927        else:
928            name = 'anchor'
929        self.forward()
930        length = 0
931        ch = self.peek(length)
932        while u'0' <= ch <= u'9' or u'A' <= ch <= 'Z' or u'a' <= ch <= 'z'  \
933                or ch in u'-_':
934            length += 1
935            ch = self.peek(length)
936        if not length:
937            raise ScannerError("while scanning an %s" % name, start_mark,
938                    "expected alphabetic or numeric character, but found %r"
939                    % ch.encode('utf-8'), self.get_mark())
940        value = self.prefix(length)
941        self.forward(length)
942        ch = self.peek()
943        if ch not in u'\0 \t\r\n\x85\u2028\u2029?:,]}%@`':
944            raise ScannerError("while scanning an %s" % name, start_mark,
945                    "expected alphabetic or numeric character, but found %r"
946                    % ch.encode('utf-8'), self.get_mark())
947        end_mark = self.get_mark()
948        return TokenClass(value, start_mark, end_mark)
949
950    def scan_tag(self):
951        # See the specification for details.
952        start_mark = self.get_mark()
953        ch = self.peek(1)
954        if ch == u'<':
955            handle = None
956            self.forward(2)
957            suffix = self.scan_tag_uri('tag', start_mark)
958            if self.peek() != u'>':
959                raise ScannerError("while parsing a tag", start_mark,
960                        "expected '>', but found %r" % self.peek().encode('utf-8'),
961                        self.get_mark())
962            self.forward()
963        elif ch in u'\0 \t\r\n\x85\u2028\u2029':
964            handle = None
965            suffix = u'!'
966            self.forward()
967        else:
968            length = 1
969            use_handle = False
970            while ch not in u'\0 \r\n\x85\u2028\u2029':
971                if ch == u'!':
972                    use_handle = True
973                    break
974                length += 1
975                ch = self.peek(length)
976            handle = u'!'
977            if use_handle:
978                handle = self.scan_tag_handle('tag', start_mark)
979            else:
980                handle = u'!'
981                self.forward()
982            suffix = self.scan_tag_uri('tag', start_mark)
983        ch = self.peek()
984        if ch not in u'\0 \r\n\x85\u2028\u2029':
985            raise ScannerError("while scanning a tag", start_mark,
986                    "expected ' ', but found %r" % ch.encode('utf-8'),
987                    self.get_mark())
988        value = (handle, suffix)
989        end_mark = self.get_mark()
990        return TagToken(value, start_mark, end_mark)
991
992    def scan_block_scalar(self, style):
993        # See the specification for details.
994
995        if style == '>':
996            folded = True
997        else:
998            folded = False
999
1000        chunks = []
1001        start_mark = self.get_mark()
1002
1003        # Scan the header.
1004        self.forward()
1005        chomping, increment = self.scan_block_scalar_indicators(start_mark)
1006        self.scan_block_scalar_ignored_line(start_mark)
1007
1008        # Determine the indentation level and go to the first non-empty line.
1009        min_indent = self.indent+1
1010        if min_indent < 1:
1011            min_indent = 1
1012        if increment is None:
1013            breaks, max_indent, end_mark = self.scan_block_scalar_indentation()
1014            indent = max(min_indent, max_indent)
1015        else:
1016            indent = min_indent+increment-1
1017            breaks, end_mark = self.scan_block_scalar_breaks(indent)
1018        line_break = u''
1019
1020        # Scan the inner part of the block scalar.
1021        while self.column == indent and self.peek() != u'\0':
1022            chunks.extend(breaks)
1023            leading_non_space = self.peek() not in u' \t'
1024            length = 0
1025            while self.peek(length) not in u'\0\r\n\x85\u2028\u2029':
1026                length += 1
1027            chunks.append(self.prefix(length))
1028            self.forward(length)
1029            line_break = self.scan_line_break()
1030            breaks, end_mark = self.scan_block_scalar_breaks(indent)
1031            if self.column == indent and self.peek() != u'\0':
1032
1033                # Unfortunately, folding rules are ambiguous.
1034                #
1035                # This is the folding according to the specification:
1036               
1037                if folded and line_break == u'\n'   \
1038                        and leading_non_space and self.peek() not in u' \t':
1039                    if not breaks:
1040                        chunks.append(u' ')
1041                else:
1042                    chunks.append(line_break)
1043               
1044                # This is Clark Evans's interpretation (also in the spec
1045                # examples):
1046                #
1047                #if folded and line_break == u'\n':
1048                #    if not breaks:
1049                #        if self.peek() not in ' \t':
1050                #            chunks.append(u' ')
1051                #        else:
1052                #            chunks.append(line_break)
1053                #else:
1054                #    chunks.append(line_break)
1055            else:
1056                break
1057
1058        # Chomp the tail.
1059        if chomping is not False:
1060            chunks.append(line_break)
1061        if chomping is True:
1062            chunks.extend(breaks)
1063
1064        # We are done.
1065        return ScalarToken(u''.join(chunks), False, start_mark, end_mark,
1066                style)
1067
1068    def scan_block_scalar_indicators(self, start_mark):
1069        # See the specification for details.
1070        chomping = None
1071        increment = None
1072        ch = self.peek()
1073        if ch in u'+-':
1074            if ch == '+':
1075                chomping = True
1076            else:
1077                chomping = False
1078            self.forward()
1079            ch = self.peek()
1080            if ch in u'0123456789':
1081                increment = int(ch)
1082                if increment == 0:
1083                    raise ScannerError("while scanning a block scalar", start_mark,
1084                            "expected indentation indicator in the range 1-9, but found 0",
1085                            self.get_mark())
1086                self.forward()
1087        elif ch in u'0123456789':
1088            increment = int(ch)
1089            if increment == 0:
1090                raise ScannerError("while scanning a block scalar", start_mark,
1091                        "expected indentation indicator in the range 1-9, but found 0",
1092                        self.get_mark())
1093            self.forward()
1094            ch = self.peek()
1095            if ch in u'+-':
1096                if ch == '+':
1097                    chomping = True
1098                else:
1099                    chomping = False
1100                self.forward()
1101        ch = self.peek()
1102        if ch not in u'\0 \r\n\x85\u2028\u2029':
1103            raise ScannerError("while scanning a block scalar", start_mark,
1104                    "expected chomping or indentation indicators, but found %r"
1105                        % ch.encode('utf-8'), self.get_mark())
1106        return chomping, increment
1107
1108    def scan_block_scalar_ignored_line(self, start_mark):
1109        # See the specification for details.
1110        while self.peek() == u' ':
1111            self.forward()
1112        if self.peek() == u'#':
1113            while self.peek() not in u'\0\r\n\x85\u2028\u2029':
1114                self.forward()
1115        ch = self.peek()
1116        if ch not in u'\0\r\n\x85\u2028\u2029':
1117            raise ScannerError("while scanning a block scalar", start_mark,
1118                    "expected a comment or a line break, but found %r"
1119                        % ch.encode('utf-8'), self.get_mark())
1120        self.scan_line_break()
1121
1122    def scan_block_scalar_indentation(self):
1123        # See the specification for details.
1124        chunks = []
1125        max_indent = 0
1126        end_mark = self.get_mark()
1127        while self.peek() in u' \r\n\x85\u2028\u2029':
1128            if self.peek() != u' ':
1129                chunks.append(self.scan_line_break())
1130                end_mark = self.get_mark()
1131            else:
1132                self.forward()
1133                if self.column > max_indent:
1134                    max_indent = self.column
1135        return chunks, max_indent, end_mark
1136
1137    def scan_block_scalar_breaks(self, indent):
1138        # See the specification for details.
1139        chunks = []
1140        end_mark = self.get_mark()
1141        while self.column < indent and self.peek() == u' ':
1142            self.forward()
1143        while self.peek() in u'\r\n\x85\u2028\u2029':
1144            chunks.append(self.scan_line_break())
1145            end_mark = self.get_mark()
1146            while self.column < indent and self.peek() == u' ':
1147                self.forward()
1148        return chunks, end_mark
1149
1150    def scan_flow_scalar(self, style):
1151        # See the specification for details.
1152        # Note that we loose indentation rules for quoted scalars. Quoted
1153        # scalars don't need to adhere indentation because " and ' clearly
1154        # mark the beginning and the end of them. Therefore we are less
1155        # restrictive then the specification requires. We only need to check
1156        # that document separators are not included in scalars.
1157        if style == '"':
1158            double = True
1159        else:
1160            double = False
1161        chunks = []
1162        start_mark = self.get_mark()
1163        quote = self.peek()
1164        self.forward()
1165        chunks.extend(self.scan_flow_scalar_non_spaces(double, start_mark))
1166        while self.peek() != quote:
1167            chunks.extend(self.scan_flow_scalar_spaces(double, start_mark))
1168            chunks.extend(self.scan_flow_scalar_non_spaces(double, start_mark))
1169        self.forward()
1170        end_mark = self.get_mark()
1171        return ScalarToken(u''.join(chunks), False, start_mark, end_mark,
1172                style)
1173
1174    ESCAPE_REPLACEMENTS = {
1175        u'0':   u'\0',
1176        u'a':   u'\x07',
1177        u'b':   u'\x08',
1178        u't':   u'\x09',
1179        u'\t':  u'\x09',
1180        u'n':   u'\x0A',
1181        u'v':   u'\x0B',
1182        u'f':   u'\x0C',
1183        u'r':   u'\x0D',
1184        u'e':   u'\x1B',
1185        u' ':   u'\x20',
1186        u'\"':  u'\"',
1187        u'\\':  u'\\',
1188        u'N':   u'\x85',
1189        u'_':   u'\xA0',
1190        u'L':   u'\u2028',
1191        u'P':   u'\u2029',
1192    }
1193
1194    ESCAPE_CODES = {
1195        u'x':   2,
1196        u'u':   4,
1197        u'U':   8,
1198    }
1199
1200    def scan_flow_scalar_non_spaces(self, double, start_mark):
1201        # See the specification for details.
1202        chunks = []
1203        while True:
1204            length = 0
1205            while self.peek(length) not in u'\'\"\\\0 \t\r\n\x85\u2028\u2029':
1206                length += 1
1207            if length:
1208                chunks.append(self.prefix(length))
1209                self.forward(length)
1210            ch = self.peek()
1211            if not double and ch == u'\'' and self.peek(1) == u'\'':
1212                chunks.append(u'\'')
1213                self.forward(2)
1214            elif (double and ch == u'\'') or (not double and ch in u'\"\\'):
1215                chunks.append(ch)
1216                self.forward()
1217            elif double and ch == u'\\':
1218                self.forward()
1219                ch = self.peek()
1220                if ch in self.ESCAPE_REPLACEMENTS:
1221                    chunks.append(self.ESCAPE_REPLACEMENTS[ch])
1222                    self.forward()
1223                elif ch in self.ESCAPE_CODES:
1224                    length = self.ESCAPE_CODES[ch]
1225                    self.forward()
1226                    for k in range(length):
1227                        if self.peek(k) not in u'0123456789ABCDEFabcdef':
1228                            raise ScannerError("while scanning a double-quoted scalar", start_mark,
1229                                    "expected escape sequence of %d hexdecimal numbers, but found %r" %
1230                                        (length, self.peek(k).encode('utf-8')), self.get_mark())
1231                    code = int(self.prefix(length), 16)
1232                    chunks.append(unichr(code))
1233                    self.forward(length)
1234                elif ch in u'\r\n\x85\u2028\u2029':
1235                    self.scan_line_break()
1236                    chunks.extend(self.scan_flow_scalar_breaks(double, start_mark))
1237                else:
1238                    raise ScannerError("while scanning a double-quoted scalar", start_mark,
1239                            "found unknown escape character %r" % ch.encode('utf-8'), self.get_mark())
1240            else:
1241                return chunks
1242
1243    def scan_flow_scalar_spaces(self, double, start_mark):
1244        # See the specification for details.
1245        chunks = []
1246        length = 0
1247        while self.peek(length) in u' \t':
1248            length += 1
1249        whitespaces = self.prefix(length)
1250        self.forward(length)
1251        ch = self.peek()
1252        if ch == u'\0':
1253            raise ScannerError("while scanning a quoted scalar", start_mark,
1254                    "found unexpected end of stream", self.get_mark())
1255        elif ch in u'\r\n\x85\u2028\u2029':
1256            line_break = self.scan_line_break()
1257            breaks = self.scan_flow_scalar_breaks(double, start_mark)
1258            if line_break != u'\n':
1259                chunks.append(line_break)
1260            elif not breaks:
1261                chunks.append(u' ')
1262            chunks.extend(breaks)
1263        else:
1264            chunks.append(whitespaces)
1265        return chunks
1266
1267    def scan_flow_scalar_breaks(self, double, start_mark):
1268        # See the specification for details.
1269        chunks = []
1270        while True:
1271            # Instead of checking indentation, we check for document
1272            # separators.
1273            prefix = self.prefix(3)
1274            if (prefix == u'---' or prefix == u'...')   \
1275                    and self.peek(3) in u'\0 \t\r\n\x85\u2028\u2029':
1276                raise ScannerError("while scanning a quoted scalar", start_mark,
1277                        "found unexpected document separator", self.get_mark())
1278            while self.peek() in u' \t':
1279                self.forward()
1280            if self.peek() in u'\r\n\x85\u2028\u2029':
1281                chunks.append(self.scan_line_break())
1282            else:
1283                return chunks
1284
1285    def scan_plain(self):
1286        # See the specification for details.
1287        # We add an additional restriction for the flow context:
1288        #   plain scalars in the flow context cannot contain ',', ':' and '?'.
1289        # We also keep track of the `allow_simple_key` flag here.
1290        # Indentation rules are loosed for the flow context.
1291        chunks = []
1292        start_mark = self.get_mark()
1293        end_mark = start_mark
1294        indent = self.indent+1
1295        # We allow zero indentation for scalars, but then we need to check for
1296        # document separators at the beginning of the line.
1297        #if indent == 0:
1298        #    indent = 1
1299        spaces = []
1300        while True:
1301            length = 0
1302            if self.peek() == u'#':
1303                break
1304            while True:
1305                ch = self.peek(length)
1306                if ch in u'\0 \t\r\n\x85\u2028\u2029'   \
1307                        or (not self.flow_level and ch == u':' and
1308                                self.peek(length+1) in u'\0 \t\r\n\x28\u2028\u2029') \
1309                        or (self.flow_level and ch in u',:?[]{}'):
1310                    break
1311                length += 1
1312            # It's not clear what we should do with ':' in the flow context.
1313            if (self.flow_level and ch == u':'
1314                    and self.peek(length+1) not in u'\0 \t\r\n\x28\u2028\u2029,[]{}'):
1315                self.forward(length)
1316                raise ScannerError("while scanning a plain scalar", start_mark,
1317                    "found unexpected ':'", self.get_mark(),
1318                    "Please check http://pyyaml.org/wiki/YAMLColonInFlowContext for details.")
1319            if length == 0:
1320                break
1321            self.allow_simple_key = False
1322            chunks.extend(spaces)
1323            chunks.append(self.prefix(length))
1324            self.forward(length)
1325            end_mark = self.get_mark()
1326            spaces = self.scan_plain_spaces(indent, start_mark)
1327            if not spaces or self.peek() == u'#' \
1328                    or (not self.flow_level and self.column < indent):
1329                break
1330        return ScalarToken(u''.join(chunks), True, start_mark, end_mark)
1331
1332    def scan_plain_spaces(self, indent, start_mark):
1333        # See the specification for details.
1334        # The specification is really confusing about tabs in plain scalars.
1335        # We just forbid them completely. Do not use tabs in YAML!
1336        chunks = []
1337        length = 0
1338        while self.peek(length) in u' ':
1339            length += 1
1340        whitespaces = self.prefix(length)
1341        self.forward(length)
1342        ch = self.peek()
1343        if ch in u'\r\n\x85\u2028\u2029':
1344            line_break = self.scan_line_break()
1345            self.allow_simple_key = True
1346            prefix = self.prefix(3)
1347            if (prefix == u'---' or prefix == u'...')   \
1348                    and self.peek(3) in u'\0 \t\r\n\x85\u2028\u2029':
1349                return
1350            breaks = []
1351            while self.peek() in u' \r\n\x85\u2028\u2029':
1352                if self.peek() == ' ':
1353                    self.forward()
1354                else:
1355                    breaks.append(self.scan_line_break())
1356                    prefix = self.prefix(3)
1357                    if (prefix == u'---' or prefix == u'...')   \
1358                            and self.peek(3) in u'\0 \t\r\n\x85\u2028\u2029':
1359                        return
1360            if line_break != u'\n':
1361                chunks.append(line_break)
1362            elif not breaks:
1363                chunks.append(u' ')
1364            chunks.extend(breaks)
1365        elif whitespaces:
1366            chunks.append(whitespaces)
1367        return chunks
1368
1369    def scan_tag_handle(self, name, start_mark):
1370        # See the specification for details.
1371        # For some strange reasons, the specification does not allow '_' in
1372        # tag handles. I have allowed it anyway.
1373        ch = self.peek()
1374        if ch != u'!':
1375            raise ScannerError("while scanning a %s" % name, start_mark,
1376                    "expected '!', but found %r" % ch.encode('utf-8'),
1377                    self.get_mark())
1378        length = 1
1379        ch = self.peek(length)
1380        if ch != u' ':
1381            while u'0' <= ch <= u'9' or u'A' <= ch <= 'Z' or u'a' <= ch <= 'z'  \
1382                    or ch in u'-_':
1383                length += 1
1384                ch = self.peek(length)
1385            if ch != u'!':
1386                self.forward(length)
1387                raise ScannerError("while scanning a %s" % name, start_mark,
1388                        "expected '!', but found %r" % ch.encode('utf-8'),
1389                        self.get_mark())
1390            length += 1
1391        value = self.prefix(length)
1392        self.forward(length)
1393        return value
1394
1395    def scan_tag_uri(self, name, start_mark):
1396        # See the specification for details.
1397        # Note: we do not check if URI is well-formed.
1398        chunks = []
1399        length = 0
1400        ch = self.peek(length)
1401        while u'0' <= ch <= u'9' or u'A' <= ch <= 'Z' or u'a' <= ch <= 'z'  \
1402                or ch in u'-;/?:@&=+$,_.!~*\'()[]%':
1403            if ch == u'%':
1404                chunks.append(self.prefix(length))
1405                self.forward(length)
1406                length = 0
1407                chunks.append(self.scan_uri_escapes(name, start_mark))
1408            else:
1409                length += 1
1410            ch = self.peek(length)
1411        if length:
1412            chunks.append(self.prefix(length))
1413            self.forward(length)
1414            length = 0
1415        if not chunks:
1416            raise ScannerError("while parsing a %s" % name, start_mark,
1417                    "expected URI, but found %r" % ch.encode('utf-8'),
1418                    self.get_mark())
1419        return u''.join(chunks)
1420
1421    def scan_uri_escapes(self, name, start_mark):
1422        # See the specification for details.
1423        bytes = []
1424        mark = self.get_mark()
1425        while self.peek() == u'%':
1426            self.forward()
1427            for k in range(2):
1428                if self.peek(k) not in u'0123456789ABCDEFabcdef':
1429                    raise ScannerError("while scanning a %s" % name, start_mark,
1430                            "expected URI escape sequence of 2 hexdecimal numbers, but found %r" %
1431                                (self.peek(k).encode('utf-8')), self.get_mark())
1432            bytes.append(chr(int(self.prefix(2), 16)))
1433            self.forward(2)
1434        try:
1435            value = unicode(''.join(bytes), 'utf-8')
1436        except UnicodeDecodeError, exc:
1437            raise ScannerError("while scanning a %s" % name, start_mark, str(exc), mark)
1438        return value
1439
1440    def scan_line_break(self):
1441        # Transforms:
1442        #   '\r\n'      :   '\n'
1443        #   '\r'        :   '\n'
1444        #   '\n'        :   '\n'
1445        #   '\x85'      :   '\n'
1446        #   '\u2028'    :   '\u2028'
1447        #   '\u2029     :   '\u2029'
1448        #   default     :   ''
1449        ch = self.peek()
1450        if ch in u'\r\n\x85':
1451            if self.prefix(2) == u'\r\n':
1452                self.forward(2)
1453            else:
1454                self.forward()
1455            return u'\n'
1456        elif ch in u'\u2028\u2029':
1457            self.forward()
1458            return ch
1459        return u''
1460
1461#try:
1462#    import psyco
1463#    psyco.bind(Scanner)
1464#except ImportError:
1465#    pass
1466
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