Package nltk_lite :: Package parse :: Module pchart :: Class BottomUpChartParse
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Class BottomUpChartParse

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object --+        
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    ParseI --+    
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 AbstractParse --+
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                BottomUpChartParse
Known Subclasses:
InsideParse, LongestParse, RandomParse, UnsortedParse

An abstract bottom-up parser for PCFGs that uses a Chart to record partial results. BottomUpChartParse maintains a queue of edges that can be added to the chart. This queue is initialized with edges for each token in the text that is being parsed. BottomUpChartParse inserts these edges into the chart one at a time, starting with the most likely edges, and proceeding to less likely edges. For each edge that is added to the chart, it may become possible to insert additional edges into the chart; these are added to the queue. This process continues until enough complete parses have been generated, or until the queue is empty.

The sorting order for the queue is not specified by BottomUpChartParse. Different sorting orders will result in different search strategies. The sorting order for the queue is defined by the method sort_queue; subclasses are required to provide a definition for this method.

Instance Methods [hide private]
 
__init__(self, grammar, beam_size=0, trace=0)
Create a new BottomUpChartParse, that uses grammar to parse texts.
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None
trace(self, trace=2)
Set the level of tracing output that should be generated when parsing a text.
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list of Tree
get_parse_list(self, tokens)
Returns: A list of the parse trees for the sentence.
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_setprob(self, tree, prod_probs) source code
None
sort_queue(self, queue, chart)
Sort the given queue of Edges, placing the edge that should be tried first at the beginning of the queue.
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_prune(self, queue, chart)
Discard items in the queue if the queue is longer than the beam.
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Inherited from AbstractParse: get_parse, grammar, parse

Inherited from ParseI: get_parse_dict, get_parse_probs

Inherited from object: __delattr__, __getattribute__, __hash__, __new__, __reduce__, __reduce_ex__, __repr__, __setattr__, __str__

Instance Variables [hide private]
PCFG _grammar
The grammar used to parse sentences.
int _trace
The level of tracing output that should be generated when parsing a text.
Properties [hide private]

Inherited from object: __class__

Method Details [hide private]

__init__(self, grammar, beam_size=0, trace=0)
(Constructor)

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Create a new BottomUpChartParse, that uses grammar to parse texts.

Parameters:
  • grammar (PCFG) - The grammar used to parse texts.
  • beam_size (int) - The maximum length for the parser's edge queue.
  • trace (int) - The level of tracing that should be used when parsing a text. 0 will generate no tracing output; and higher numbers will produce more verbose tracing output.
Overrides: AbstractParse.__init__

trace(self, trace=2)

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Set the level of tracing output that should be generated when parsing a text.

Parameters:
  • trace (int) - The trace level. A trace level of 0 will generate no tracing output; and higher trace levels will produce more verbose tracing output.
Returns: None

get_parse_list(self, tokens)

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Returns: list of Tree
A list of the parse trees for the sentence. When possible, this list should be sorted from most likely to least likely.
Overrides: AbstractParse.get_parse_list

sort_queue(self, queue, chart)

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Sort the given queue of Edges, placing the edge that should be tried first at the beginning of the queue. This method will be called after each Edge is added to the queue.

Parameters:
  • queue (list of Edge) - The queue of Edges to sort. Each edge in this queue is an edge that could be added to the chart by the fundamental rule; but that has not yet been added.
  • chart (Chart) - The chart being used to parse the text. This chart can be used to provide extra information for sorting the queue.
Returns: None