Package nltk :: Package tag :: Module sequential :: Class ContextTagger

Class ContextTagger

         object --+        
                  |        
        api.TaggerI --+    
                      |    
SequentialBackoffTagger --+
                          |
                         ContextTagger

Known Subclasses:

An abstract base class for sequential backoff taggers that choose a tag for a token based on the value of its "context". Different subclasses are used to define different contexts.

A ContextTagger chooses the tag for a token by calculating the token's context, and looking up the corresponding tag in a table. This table can be constructed manually; or it can be automatically constructed based on a training corpus, using the _train() factory method.

Instance Methods

[hide private]

__init__(self, context_to_tag, backoff=None)

source code

(hashable)

context(self, tokens, index, history)
Returns: the context that should be used to look up the tag for the specified token; or None if the specified token should not be handled by this tagger. source code

str

choose_tag(self, tokens, index, history)
Decide which tag should be used for the specified token, and return that tag.

source code

size(self)
Returns: The number of entries in the table used by this tagger to map from contexts to tags.

source code

__repr__(self)
repr(x)

source code

_train(self, tagged_corpus, cutoff=1, verbose=False)
Initialize this ContextTagger's _context_to_tag table based on the given training data. source code

Inherited from SequentialBackoffTagger: tag, tag_one

Inherited from SequentialBackoffTagger (private): _get_backoff

Inherited from api.TaggerI: batch_tag

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

Deprecated

Inherited from SequentialBackoffTagger: tag_sents

Instance Variables

[hide private]

_context_to_tag
Dictionary mapping contexts to tags.

Inherited from SequentialBackoffTagger (private): _taggers

Properties

[hide private]

Inherited from SequentialBackoffTagger: backoff

Inherited from object: __class__

Method Details

[hide private]

init(self, context_to_tag, backoff=None)
(Constructor)

source code

Parameters:

context_to_tag - A dictionary mapping contexts to tags.
backoff - The backoff tagger that should be used for this tagger.

Overrides: SequentialBackoffTagger.__init__

context(self, tokens, index, history)

source code

Returns: (hashable): the context that should be used to look up the tag for the specified token; or None if the specified token should not be handled by this tagger.

choose_tag(self, tokens, index, history)

source code

Decide which tag should be used for the specified token, and return that tag. If this tagger is unable to determine a tag for the specified token, return None -- do not consult the backoff tagger. This method should be overridden by subclasses of SequentialBackoffTagger.

Parameters:

tokens - The list of words that are being tagged.
index - The index of the word whose tag should be returned.
history - A list of the tags for all words before index.

Returns: str

Overrides: SequentialBackoffTagger.choose_tag

(inherited documentation)

size(self)

source code

Returns:: The number of entries in the table used by this tagger to map from contexts to tags.

repr(self)
(Representation operator)

source code

repr(x)

Overrides: object.__repr__: (inherited documentation)

_train(self, tagged_corpus, cutoff=1, verbose=False)

source code

Initialize this ContextTagger's _context_to_tag table based on the given training data. In particular, for each context c in the training data, set _context_to_tag[c] to the most frequent tag for that context. However, exclude any contexts that are already tagged perfectly by the backoff tagger(s).

The old value of self._context_to_tag (if any) is discarded.

Parameters:

tagged_corpus - A tagged corpus. Each item should be a list of (word, tag) tuples.
cutoff - If the most likely tag for a context occurs fewer than cutoff times, then exclude it from the context-to-tag table for the new tagger.

Class ContextTagger

__init__(self, context_to_tag, backoff=None) (Constructor)

context(self, tokens, index, history)

choose_tag(self, tokens, index, history)

size(self)

__repr__(self) (Representation operator)

_train(self, tagged_corpus, cutoff=1, verbose=False)

init(self, context_to_tag, backoff=None)
(Constructor)

repr(self)
(Representation operator)