nltk.parse.shiftreduce module

class nltk.parse.shiftreduce.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

class nltk.parse.shiftreduce.SteppingShiftReduceParser

Bases: ShiftReduceParser

A ShiftReduceParser that allows you to setp through the parsing process, performing a single operation at a time. It also allows you to change the parser’s grammar midway through parsing a text.

The initialize method is used to start parsing a text. shift performs a single shift operation, and reduce performs a single reduce operation. step will perform a single reduce operation if possible; otherwise, it will perform a single shift operation. parses returns the set of parses that have been found by the parser.

Variables

_history – A list of (stack, remaining_text) pairs, containing all of the previous states of the parser. This history is used to implement the undo operation.

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.

initialize(tokens)

Start parsing a given text. This sets the parser’s stack to [] and sets its remaining text to tokens.

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)

parses()

Returns

An iterator of the parses that have been found by this parser so far.

Return type

iter(Tree)

reduce(production=None)

Use production to combine the rightmost stack elements into a single Tree. If production does not match the rightmost stack elements, then do nothing.

Returns

The production used to reduce the stack, if a reduction was performed. If no reduction was performed, return None.

Return type

Production or None

reducible_productions()

Returns

A list of the productions for which reductions are available for the current parser state.

Return type

list(Production)

remaining_text()

Returns

The portion of the text that is not yet covered by the stack.

Return type

list(str)

set_grammar(grammar)

Change the grammar used to parse texts.

Parameters

grammar (CFG) – The new grammar.

shift()

Move a token from the beginning of the remaining text to the end of the stack. If there are no more tokens in the remaining text, then do nothing.

Returns

True if the shift operation was successful.

Return type

bool

stack()

Returns

The parser’s stack.

Return type

list(str and Tree)

step()

Perform a single parsing operation. If a reduction is possible, then perform that reduction, and return the production that it is based on. Otherwise, if a shift is possible, then perform it, and return True. Otherwise, return False.

Returns

False if no operation was performed; True if a shift was performed; and the CFG production used to reduce if a reduction was performed.

Return type

Production or bool

undo()

Return the parser to its state before the most recent shift or reduce operation. Calling undo repeatedly return the parser to successively earlier states. If no shift or reduce operations have been performed, undo will make no changes.

Returns

true if an operation was successfully undone.

Return type

bool

nltk.parse.shiftreduce.demo()

A demonstration of the shift-reduce parser.