.. Copyright (C) 2001-2020 NLTK Project
.. For license information, see LICENSE.TXT

======================
Information Extraction
======================

Information Extraction standardly consists of three subtasks:

#. Named Entity Recognition

#. Relation Extraction

#. Template Filling

Named Entities
~~~~~~~~~~~~~~

The IEER corpus is marked up for a variety of Named Entities. A `Named
Entity`:dt: (more strictly, a Named Entity mention) is a name of an
entity belonging to a specified class. For example, the Named Entity
classes in IEER include PERSON, LOCATION, ORGANIZATION, DATE and so
on. Within NLTK, Named Entities are represented as subtrees within a
chunk structure: the class name is treated as node label, while the
entity mention itself appears as the leaves of the subtree. This is
illustrated below, where we have show an extract of the chunk
representation of document NYT_19980315.064:

    >>> from nltk.corpus import ieer
    >>> docs = ieer.parsed_docs('NYT_19980315')
    >>> tree = docs[1].text
    >>> print(tree) # doctest: +ELLIPSIS
    (DOCUMENT
    ...
      ``It's
      a
      chance
      to
      think
      about
      first-level
      questions,''
      said
      Ms.
      (PERSON Cohn)
      ,
      a
      partner
      in
      the
      (ORGANIZATION McGlashan &AMP; Sarrail)
      firm
      in
      (LOCATION San Mateo)
      ,
      (LOCATION Calif.)
      ...)

Thus, the Named Entity mentions in this example are *Cohn*, *McGlashan &AMP;
Sarrail*, *San Mateo* and *Calif.*.

The CoNLL2002 Dutch and Spanish data is treated similarly, although in
this case, the strings are also POS tagged.

    >>> from nltk.corpus import conll2002
    >>> for doc in conll2002.chunked_sents('ned.train')[27]:
    ...     print(doc)
    ('Het', 'Art')
    (ORG Hof/N van/Prep Cassatie/N)
    ('verbrak', 'V')
    ('het', 'Art')
    ('arrest', 'N')
    ('zodat', 'Conj')
    ('het', 'Pron')
    ('moest', 'V')
    ('worden', 'V')
    ('overgedaan', 'V')
    ('door', 'Prep')
    ('het', 'Art')
    ('hof', 'N')
    ('van', 'Prep')
    ('beroep', 'N')
    ('van', 'Prep')
    (LOC Antwerpen/N)
    ('.', 'Punc')

Relation Extraction
~~~~~~~~~~~~~~~~~~~

Relation Extraction standardly consists of identifying specified
relations between Named Entities. For example, assuming that we can
recognize ORGANIZATIONs and LOCATIONs in text, we might want to also
recognize pairs *(o, l)* of these kinds of entities such that *o* is
located in *l*.

The `sem.relextract` module provides some tools to help carry out a
simple version of this task. The `tree2semi_rel()` function splits a chunk
document into a list of two-member lists, each of which consists of a
(possibly empty) string followed by a `Tree` (i.e., a Named Entity):

    >>> from nltk.sem import relextract
    >>> pairs = relextract.tree2semi_rel(tree)
    >>> for s, tree in pairs[18:22]:
    ...     print('("...%s", %s)' % (" ".join(s[-5:]),tree))
    ("...about first-level questions,'' said Ms.", (PERSON Cohn))
    ("..., a partner in the", (ORGANIZATION McGlashan &AMP; Sarrail))
    ("...firm in", (LOCATION San Mateo))
    ("...,", (LOCATION Calif.))

The function `semi_rel2reldict()` processes triples of these pairs, i.e.,
pairs of the form ``((string1, Tree1), (string2, Tree2), (string3,
Tree3))`` and outputs a dictionary (a `reldict`) in which ``Tree1`` is
the subject of the relation, ``string2`` is the filler
and ``Tree3`` is the object of the relation. ``string1`` and ``string3`` are
stored as left and right context respectively.

    >>> reldicts = relextract.semi_rel2reldict(pairs)
    >>> for k, v in sorted(reldicts[0].items()):
    ...     print(k, '=>', v) # doctest: +ELLIPSIS
    filler => of messages to their own ``Cyberia'' ...
    lcon => transactions.'' Each week, they post
    objclass => ORGANIZATION
    objsym => white_house
    objtext => White House
    rcon => for access to its planned
    subjclass => CARDINAL
    subjsym => hundreds
    subjtext => hundreds
    untagged_filler => of messages to their own ``Cyberia'' ...

The next example shows some of the values for two `reldict`\ s
corresponding to the ``'NYT_19980315'`` text extract shown earlier.

    >>> for r in reldicts[18:20]:
    ...     print('=' * 20)
    ...     print(r['subjtext'])
    ...     print(r['filler'])
    ...     print(r['objtext'])
    ====================
    Cohn
    , a partner in the
    McGlashan &AMP; Sarrail
    ====================
    McGlashan &AMP; Sarrail
    firm in
    San Mateo

The function `relextract()` allows us to filter the `reldict`\ s
according to the classes of the subject and object named entities. In
addition, we can specify that the filler text has to match a given
regular expression, as illustrated in the next example. Here, we are
looking for pairs of entities in the IN relation, where IN has
signature <ORG, LOC>.

    >>> import re
    >>> IN = re.compile(r'.*\bin\b(?!\b.+ing\b)')
    >>> for fileid in ieer.fileids():
    ...     for doc in ieer.parsed_docs(fileid):
    ...         for rel in relextract.extract_rels('ORG', 'LOC', doc, corpus='ieer', pattern = IN):
    ...             print(relextract.rtuple(rel))  # doctest: +ELLIPSIS
    [ORG: 'Christian Democrats'] ', the leading political forces in' [LOC: 'Italy']
    [ORG: 'AP'] ') _ Lebanese guerrillas attacked Israeli forces in southern' [LOC: 'Lebanon']
    [ORG: 'Security Council'] 'adopted Resolution 425. Huge yellow banners hung across intersections in' [LOC: 'Beirut']
    [ORG: 'U.N.'] 'failures in' [LOC: 'Africa']
    [ORG: 'U.N.'] 'peacekeeping operation in' [LOC: 'Somalia']
    [ORG: 'U.N.'] 'partners on a more effective role in' [LOC: 'Africa']
    [ORG: 'AP'] ') _ A bomb exploded in a mosque in central' [LOC: 'San`a']
    [ORG: 'Krasnoye Sormovo'] 'shipyard in the Soviet city of' [LOC: 'Gorky']
    [ORG: 'Kelab Golf Darul Ridzuan'] 'in' [LOC: 'Perak']
    [ORG: 'U.N.'] 'peacekeeping operation in' [LOC: 'Somalia']
    [ORG: 'WHYY'] 'in' [LOC: 'Philadelphia']
    [ORG: 'McGlashan &AMP; Sarrail'] 'firm in' [LOC: 'San Mateo']
    [ORG: 'Freedom Forum'] 'in' [LOC: 'Arlington']
    [ORG: 'Brookings Institution'] ', the research group in' [LOC: 'Washington']
    [ORG: 'Idealab'] ', a self-described business incubator based in' [LOC: 'Los Angeles']
    [ORG: 'Open Text'] ', based in' [LOC: 'Waterloo']
    ...

The next example illustrates a case where the patter is a disjunction
of roles that a PERSON can occupy in an ORGANIZATION.

    >>> roles = """
    ... (.*(
    ... analyst|
    ... chair(wo)?man|
    ... commissioner|
    ... counsel|
    ... director|
    ... economist|
    ... editor|
    ... executive|
    ... foreman|
    ... governor|
    ... head|
    ... lawyer|
    ... leader|
    ... librarian).*)|
    ... manager|
    ... partner|
    ... president|
    ... producer|
    ... professor|
    ... researcher|
    ... spokes(wo)?man|
    ... writer|
    ... ,\sof\sthe?\s*  # "X, of (the) Y"
    ... """
    >>> ROLES = re.compile(roles, re.VERBOSE)
    >>> for fileid in ieer.fileids():
    ...     for doc in ieer.parsed_docs(fileid):
    ...         for rel in relextract.extract_rels('PER', 'ORG', doc, corpus='ieer', pattern=ROLES):
    ...             print(relextract.rtuple(rel)) # doctest: +ELLIPSIS
    [PER: 'Kivutha Kibwana'] ', of the' [ORG: 'National Convention Assembly']
    [PER: 'Boban Boskovic'] ', chief executive of the' [ORG: 'Plastika']
    [PER: 'Annan'] ', the first sub-Saharan African to head the' [ORG: 'United Nations']
    [PER: 'Kiriyenko'] 'became a foreman at the' [ORG: 'Krasnoye Sormovo']
    [PER: 'Annan'] ', the first sub-Saharan African to head the' [ORG: 'United Nations']
    [PER: 'Mike Godwin'] ', chief counsel for the' [ORG: 'Electronic Frontier Foundation']
    ...

In the case of the CoNLL2002 data, we can include POS tags in the
query pattern. This example also illustrates how the output can be
presented as something that looks more like a clause in a logical language.

    >>> de = """
    ... .*
    ... (
    ... de/SP|
    ... del/SP
    ... )
    ... """
    >>> DE = re.compile(de, re.VERBOSE)
    >>> rels = [rel for doc in conll2002.chunked_sents('esp.train')
    ...         for rel in relextract.extract_rels('ORG', 'LOC', doc, corpus='conll2002', pattern = DE)]
    >>> for r in rels[:10]:
    ...     print(relextract.clause(r, relsym='DE'))    # doctest: +NORMALIZE_WHITESPACE
    DE('tribunal_supremo', 'victoria')
    DE('museo_de_arte', 'alcorc\xf3n')
    DE('museo_de_bellas_artes', 'a_coru\xf1a')
    DE('siria', 'l\xedbano')
    DE('uni\xf3n_europea', 'pek\xedn')
    DE('ej\xe9rcito', 'rogberi')
    DE('juzgado_de_instrucci\xf3n_n\xfamero_1', 'san_sebasti\xe1n')
    DE('psoe', 'villanueva_de_la_serena')
    DE('ej\xe9rcito', 'l\xedbano')
    DE('juzgado_de_lo_penal_n\xfamero_2', 'ceuta')
    >>> vnv = """
    ... (
    ... is/V|
    ... was/V|
    ... werd/V|
    ... wordt/V
    ... )
    ... .*
    ... van/Prep
    ... """
    >>> VAN = re.compile(vnv, re.VERBOSE)
    >>> for doc in conll2002.chunked_sents('ned.train'):
    ...     for r in relextract.extract_rels('PER', 'ORG', doc, corpus='conll2002', pattern=VAN):
    ...         print(relextract.clause(r, relsym="VAN"))
    VAN("cornet_d'elzius", 'buitenlandse_handel')
    VAN('johan_rottiers', 'kardinaal_van_roey_instituut')
    VAN('annie_lennox', 'eurythmics')
