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278 lines
11 KiB
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278 lines
11 KiB
Plaintext
//- 💫 DOCS > USAGE > TOKENIZER
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include ../../_includes/_mixins
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p
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| Tokenization is the task of splitting a text into meaningful segments,
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| called #[em tokens]. The input to the tokenizer is a unicode text, and
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| the output is a #[+api("doc") #[code Doc]] object. To construct a
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| #[code Doc] object, you need a #[+api("vocab") #[code Vocab]] instance,
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| a sequence of #[code word] strings, and optionally a sequence of
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| #[code spaces] booleans, which allow you to maintain alignment of the
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| tokens into the original string.
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+aside("spaCy's data model")
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| The main point to keep in mind is that spaCy's #[code Doc] doesn't
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| copy or refer to the original string. The string is reconstructed from
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| the tokens when required.
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+h(2, "101") Tokenizer 101
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include _spacy-101/_tokenization
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+h(3, "101-data") Tokenizer data
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p
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| #[strong Global] and #[strong language-specific] tokenizer data is
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| supplied via the language data in #[+src(gh("spaCy", "spacy/lang")) spacy/lang].
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| The tokenizer exceptions define special cases like "don't" in English,
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| which needs to be split into two tokens: #[code {ORTH: "do"}] and
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| #[code {ORTH: "n't", LEMMA: "not"}]. The prefixes, suffixes and infixes
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| mosty define punctuation rules – for example, when to split off periods
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| (at the end of a sentence), and when to leave token containing periods
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| intact (abbreviations like "U.S.").
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+image
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include ../../assets/img/docs/language_data.svg
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.u-text-right
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+button("/assets/img/docs/language_data.svg", false, "secondary").u-text-tag View large graphic
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+infobox
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| For more details on the language-specific data, see the
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| usage workflow on #[+a("/docs/usage/adding-languages") adding languages].
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+h(2, "special-cases") Adding special case tokenization rules
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p
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| Most domains have at least some idiosyncracies that require custom
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| tokenization rules. This could be very certain expressions, or
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| abbreviations only used in this specific field.
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+aside("Language data vs. custom tokenization")
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| Tokenization rules that are specific to one language, but can be
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| #[strong generalised across that language] should ideally live in the
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| language data in #[+src(gh("spaCy", "spacy/lang")) spacy/lang] – we
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| always appreciate pull requests! Anything that's specific to a domain or
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| text type – like financial trading abbreviations, or Bavarian youth slang
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| – should be added as a special case rule to your tokenizer instance. If
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| you're dealing with a lot of customisations, it might make sense to create
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| an entirely custom subclass.
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p
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| Here's how to add a special case rule to an existing
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| #[+api("tokenizer") #[code Tokenizer]] instance:
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+code.
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import spacy
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from spacy.symbols import ORTH, LEMMA, POS
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nlp = spacy.load('en')
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doc = nlp(u'gimme that') # phrase to tokenize
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assert [w.text for w in doc] == [u'gimme', u'that'] # current tokenization
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# add special case rule
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special_case = [{ORTH: u'gim', LEMMA: u'give', POS: u'VERB'}, {ORTH: u'me'}]
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nlp.tokenizer.add_special_case(u'gimme', special_case)
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assert [w.text for w in nlp(u'gimme that')] == [u'gim', u'me', u'that']
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assert [w.lemma_ for w in nlp(u'gimme that')] == [u'give', u'me', u'that']
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p
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| The special case doesn't have to match an entire whitespace-delimited
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| substring. The tokenizer will incrementally split off punctuation, and
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| keep looking up the remaining substring:
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+code.
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assert 'gimme' not in [w.text for w in nlp(u'gimme!')]
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assert 'gimme' not in [w.text for w in nlp(u'("...gimme...?")')]
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p
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| The special case rules have precedence over the punctuation splitting:
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+code.
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special_case = [{ORTH: u'...gimme...?', LEMMA: u'give', TAG: u'VB'}]
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nlp.tokenizer.add_special_case(u'...gimme...?', special_case)
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assert len(nlp(u'...gimme...?')) == 1
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p
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| Because the special-case rules allow you to set arbitrary token
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| attributes, such as the part-of-speech, lemma, etc, they make a good
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| mechanism for arbitrary fix-up rules. Having this logic live in the
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| tokenizer isn't very satisfying from a design perspective, however, so
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| the API may eventually be exposed on the
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| #[+api("language") #[code Language]] class itself.
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+h(2, "how-tokenizer-works") How spaCy's tokenizer works
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p
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| spaCy introduces a novel tokenization algorithm, that gives a better
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| balance between performance, ease of definition, and ease of alignment
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| into the original string.
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p
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| After consuming a prefix or infix, we consult the special cases again.
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| We want the special cases to handle things like "don't" in English, and
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| we want the same rule to work for "(don't)!". We do this by splitting
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| off the open bracket, then the exclamation, then the close bracket, and
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| finally matching the special-case. Here's an implementation of the
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| algorithm in Python, optimized for readability rather than performance:
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+code.
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def tokenizer_pseudo_code(text, find_prefix, find_suffix,
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find_infixes, special_cases):
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tokens = []
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for substring in text.split(' '):
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suffixes = []
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while substring:
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if substring in special_cases:
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tokens.extend(special_cases[substring])
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substring = ''
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elif find_prefix(substring) is not None:
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split = find_prefix(substring)
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tokens.append(substring[:split])
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substring = substring[split:]
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elif find_suffix(substring) is not None:
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split = find_suffix(substring)
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suffixes.append(substring[split:])
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substring = substring[:split]
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elif find_infixes(substring):
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infixes = find_infixes(substring)
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offset = 0
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for match in infixes:
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tokens.append(substring[i : match.start()])
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tokens.append(substring[match.start() : match.end()])
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offset = match.end()
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substring = substring[offset:]
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else:
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tokens.append(substring)
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substring = ''
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tokens.extend(suffixes)
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return tokens
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p
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| The algorithm can be summarized as follows:
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+list("numbers")
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+item Iterate over space-separated substrings
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+item
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| Check whether we have an explicitly defined rule for this substring.
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| If we do, use it.
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+item Otherwise, try to consume a prefix.
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+item
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| If we consumed a prefix, go back to the beginning of the loop, so
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| that special-cases always get priority.
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+item If we didn't consume a prefix, try to consume a suffix.
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+item
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| If we can't consume a prefix or suffix, look for "infixes" — stuff
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| like hyphens etc.
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+item Once we can't consume any more of the string, handle it as a single token.
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+h(2, "native-tokenizers") Customizing spaCy's Tokenizer class
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p
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| Let's imagine you wanted to create a tokenizer for a new language or
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| specific domain. There are four things you would need to define:
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+list("numbers")
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+item
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| A dictionary of #[strong special cases]. This handles things like
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| contractions, units of measurement, emoticons, certain
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| abbreviations, etc.
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+item
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| A function #[code prefix_search], to handle
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| #[strong preceding punctuation], such as open quotes, open brackets,
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| etc
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+item
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| A function #[code suffix_search], to handle
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| #[strong succeeding punctuation], such as commas, periods, close
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| quotes, etc.
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+item
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| A function #[code infixes_finditer], to handle non-whitespace
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| separators, such as hyphens etc.
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p
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| You shouldn't usually need to create a #[code Tokenizer] subclass.
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| Standard usage is to use #[code re.compile()] to build a regular
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| expression object, and pass its #[code .search()] and
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| #[code .finditer()] methods:
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+code.
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import re
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from spacy.tokenizer import Tokenizer
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prefix_re = re.compile(r'''[\[\("']''')
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suffix_re = re.compile(r'''[\]\)"']''')
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def create_tokenizer(nlp):
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return Tokenizer(nlp.vocab, prefix_search=prefix_re.search,
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suffix_search=suffix_re.search)
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nlp = spacy.load('en', tokenizer=create_tokenizer)
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p
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| If you need to subclass the tokenizer instead, the relevant methods to
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| specialize are #[code find_prefix], #[code find_suffix] and
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| #[code find_infix].
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+h(2, "custom-tokenizer") Hooking an arbitrary tokenizer into the pipeline
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p
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| You can pass a custom tokenizer using the #[code make_doc] keyword, when
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| you're creating the pipeline:
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+code.
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nlp = spacy.load('en', make_doc=my_tokenizer)
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p
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| However, this approach often leaves us with a chicken-and-egg problem.
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| To construct the tokenizer, we usually want attributes of the #[code nlp]
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| pipeline. Specifically, we want the tokenizer to hold a reference to the
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| pipeline's vocabulary object. Let's say we have the following class as
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| our tokenizer:
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+code.
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import spacy
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from spacy.tokens import Doc
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class WhitespaceTokenizer(object):
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def __init__(self, nlp):
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self.vocab = nlp.vocab
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def __call__(self, text):
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words = text.split(' ')
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# All tokens 'own' a subsequent space character in this tokenizer
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spaces = [True] * len(word)
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return Doc(self.vocab, words=words, spaces=spaces)
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p
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| As you can see, we need a #[code vocab] instance to construct this — but
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| we won't get the #[code vocab] instance until we get back the #[code nlp]
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| object from #[code spacy.load()]. The simplest solution is to build the
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| object in two steps:
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+code.
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nlp = spacy.load('en')
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nlp.make_doc = WhitespaceTokenizer(nlp)
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p
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| You can instead pass the class to the #[code create_make_doc] keyword,
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| which is invoked as callback once the #[code nlp] object is ready:
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+code.
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nlp = spacy.load('en', create_make_doc=WhitespaceTokenizer)
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p
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| Finally, you can of course create your own subclasses, and create a bound
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| #[code make_doc] method. The disadvantage of this approach is that spaCy
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| uses inheritance to give each language-specific pipeline its own class.
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| If you're working with multiple languages, a naive solution will
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| therefore require one custom class per language you're working with.
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| This might be at least annoying. You may be able to do something more
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| generic by doing some clever magic with metaclasses or mixins, if that's
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| the sort of thing you're into.
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