A bottom-up PCFG parser that uses dynamic programming to find the single most likely parse for a text. The ViterbiParser parser parses texts by filling in a "most likely constituent table". This table records the most probable tree representation for any given span and node value. In particular, it has an entry for every start index, end index, and node value, recording the most likely subtree that spans from the start index to the end index, and has the given node value.
The ViterbiParser parser fills in this table incrementally. It starts by filling in all entries for constituents that span one element of text (i.e., entries where the end index is one greater than the start index). After it has filled in all table entries for constituents that span one element of text, it fills in the entries for constitutants that span two elements of text. It continues filling in the entries for constituents spanning larger and larger portions of the text, until the entire table has been filled. Finally, it returns the table entry for a constituent spanning the entire text, whose node value is the grammar's start symbol.
In order to find the most likely constituent with a given span and node value, the ViterbiParser parser considers all productions that could produce that node value. For each production, it finds all children that collectively cover the span and have the node values specified by the production's right hand side. If the probability of the tree formed by applying the production to the children is greater than the probability of the current entry in the table, then the table is updated with this new tree.
A pseudo-code description of the algorithm used by ViterbiParser is:
| Method | __init__ |
Create a new ViterbiParser parser, that uses grammar to parse texts. |
| Method | __repr__ |
Undocumented |
| Method | grammar |
No summary |
| Method | parse |
When possible this list is sorted from most likely to least likely. |
| Method | trace |
Set the level of tracing output that should be generated when parsing a text. |
| Method | _add |
Find any constituents that might cover span, and add them to the most likely constituents table. |
| Method | _find |
No summary |
| Method | _match |
No summary |
| Method | _trace |
Undocumented |
| Method | _trace |
Print trace output indicating that a given production has been applied at a given location. |
| Instance Variable | _grammar |
The grammar used to parse sentences. |
| Instance Variable | _trace |
The level of tracing output that should be generated when parsing a text. |
Inherited from ParserI:
| Method | parse |
No summary |
| Method | parse |
No summary |
| Method | parse |
Apply self.parse() to each element of sents. :rtype: iter(iter(Tree)) |
Create a new ViterbiParser parser, that uses grammar to parse texts.
| Parameters | |
| grammar:PCFG | The grammar used to parse texts. |
| 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. |
nltk.parse.api.ParserI.parseWhen possible this list is sorted from most likely to least likely.
| Parameters | |
| tokens | Undocumented |
| sent:list(str) | The sentence to be parsed |
| Returns | |
| iter(Tree) | An iterator that generates parse trees for the sentence. |
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 | Undocumented |
Find any constituents that might cover span, and add them to the most likely constituents table.
| Parameters | |
| span:tuple(int, int) | The section of the text for which we are trying to find possible constituents. The span is specified as a pair of integers, where the first integer is the index of the first token that should be included in the constituent; and the second integer is the index of the first token that should not be included in the constituent. I.e., the constituent should cover text[span[0]:span[1]], where text is the text that we are parsing. |
| constituents:dict(tuple(int,int,Nonterminal) -> ProbabilisticToken or ProbabilisticTree) | The most likely constituents table. This table records the most probable tree representation for any given span and node value. In particular, constituents(s,e,nv) is the most likely ProbabilisticTree that covers text[s:e] and has a node value nv.symbol(), where text is the text that we are parsing. When _add_constituents_spanning is called, constituents should contain all possible constituents that are shorter than span. |
| tokens:list of tokens | The text we are parsing. This is only used for trace output. |
| Returns | |
| None | Undocumented |
| Parameters | |
| span:tuple(int, int) | The section of the text for which we are trying to find production instantiations. The span is specified as a pair of integers, where the first integer is the index of the first token that should be covered by the production instantiation; and the second integer is the index of the first token that should not be covered by the production instantiation. |
| constituents:dict(tuple(int,int,Nonterminal) -> ProbabilisticToken or ProbabilisticTree) | The most likely constituents table. This table records the most probable tree representation for any given span and node value. See the module documentation for more information. |
| Returns | |
| a list of the production instantiations that cover a given span of the text. A "production instantiation" is a tuple containing a production and a list of children, where the production's right hand side matches the list of children; and the children cover span. :rtype: list of pair of Production, (list of (ProbabilisticTree or token. | |
| Parameters | |
| rhs:list(Nonterminal or any) | The list specifying what kinds of children need to cover span. Each nonterminal in rhs specifies that the corresponding child should be a tree whose node value is that nonterminal's symbol. Each terminal in rhs specifies that the corresponding child should be a token whose type is that terminal. |
| span:tuple(int, int) | The section of the text for which we are trying to find child lists. The span is specified as a pair of integers, where the first integer is the index of the first token that should be covered by the child list; and the second integer is the index of the first token that should not be covered by the child list. |
| constituents:dict(tuple(int,int,Nonterminal) -> ProbabilisticToken or ProbabilisticTree) | The most likely constituents table. This table records the most probable tree representation for any given span and node value. See the module documentation for more information. |
| Returns | |
| list(list(ProbabilisticTree or token) | a set of all the lists of children that cover span and that match rhs. |