nltk.parse.ShiftReduceParser

class nltk.parse.ShiftReduceParser

Bases: ParserI

A simple bottom-up CFG parser that uses two operations, “shift” and “reduce”, to find a single parse for a text.

ShiftReduceParser maintains a stack, which records the structure of a portion of the text. This stack is a list of strings and Trees that collectively cover a portion of the text. For example, while parsing the sentence “the dog saw the man” with a typical grammar, ShiftReduceParser will produce the following stack, which covers “the dog saw”:

[(NP: (Det: 'the') (N: 'dog')), (V: 'saw')]

ShiftReduceParser attempts to extend the stack to cover the entire text, and to combine the stack elements into a single tree, producing a complete parse for the sentence.

Initially, the stack is empty. It is extended to cover the text, from left to right, by repeatedly applying two operations:

• “shift” moves a token from the beginning of the text to the end of the stack.

• “reduce” uses a CFG production to combine the rightmost stack elements into a single Tree.

Often, more than one operation can be performed on a given stack. In this case, ShiftReduceParser uses the following heuristics to decide which operation to perform:

• Only shift if no reductions are available.

• If multiple reductions are available, then apply the reduction whose CFG production is listed earliest in the grammar.

Note that these heuristics are not guaranteed to choose an operation that leads to a parse of the text. Also, if multiple parses exists, ShiftReduceParser will return at most one of them.

See

nltk.grammar

__init__(grammar, trace=0)

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

Parameters

• grammar (Grammar) – 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.

grammar()

Returns

The grammar used by this parser.

parse(tokens)

Returns

An iterator that generates parse trees for the sentence. When possible this list is sorted from most likely to least likely.

Parameters

sent (list(str)) – The sentence to be parsed

Return type

iter(Tree)

trace(trace=2)

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.

Return type

None

parse_all(sent, *args, **kwargs)

Return type

list(Tree)

parse_one(sent, *args, **kwargs)

Return type

Tree or None

parse_sents(sents, *args, **kwargs)

Apply self.parse() to each element of sents. :rtype: iter(iter(Tree))