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Parse natural language time expressions in python
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ctparse Bump version: 0.0.47 → 0.1.0 Mar 20, 2020
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README.rst

ctparse - Parse natural language time expressions in python

Travis Coverage PyPi Updates Documentation Status

This code is in early alpha stage. There can and will be potentially breaking changes right on the ``master`` branch

Background

The package ctparse is a pure python package to parse time expressions from natural language (i.e. strings). In many ways it builds on similar concepts as Facebook’s duckling package (https://github.com/facebook/duckling). However, for the time being it only targets times and only German and English text.

In principle ctparse can be used to detect time expressions in a text, however its main use case is the semantic interpretation of such expressions. Detecting time expressions in the first place can - to our experience - be done more efficiently (and precisely) using e.g. CRFs or other models targeted at this specific task.

ctparse is designed with the use case in mind where interpretation of time expressions is done under the following assumptions:

  • All expressions are relative to some pre-defined reference times
  • Unless explicitly specified in the time expression, valid resolutions are in the future relative to the reference time (i.e. 12.5. will be the next 12th of May, but 12.5.2012 should correctly resolve to the 12th of May 2012).
  • If in doubt, resolutions in the near future are more likely than resolutions in the far future (not implemented yet, but any resolution more than i.e. 3 month in the future is extremely unlikely).

The specific comtravo use-case is resolving time expressions in booking requests which almost always refer to some point in time within the next 4-8 weeks.

ctparse currently is language agnostic and supports German and English expressions. This might get an extension in the future. The main reason is that in real world communication more often than not people write in one language (their business language) but use constructs to express times that are based on their mother tongue and/or what they believe to be the way to express dates in the target language. This leads to text in German with English time expressions and vice-versa. Using a language detection upfront on the complete original text is for obvious no solution - rather it would make the problem worse.

Example

from ctparse import ctparse
from datetime import datetime

# Set reference time
ts = datetime(2018, 3, 12, 14, 30)
ctparse('May 5th 2:30 in the afternoon', ts=ts)

This should return a Time object represented as Time[0-29]{2018-05-05 14:30 (X/X)}, indicating that characters 0-29 were used in the resolution, that the resolved date time is the 5th of May 2018 at 14:30 and that this resolution is neither based on a day of week (first X) nor a part of day (second X).

Implementation

ctparse - as duckling - is a mixture of a rule and regular expression based system + some probabilistic modeling. In this sense it resembles a PCFG.

Rules

At the core ctparse is a collection of production rules over sequences of regular expressions and (intermediate) productions.

Productions are either of type Time or type Interval and can have certain predicates (e.g. whether a Time is a part of day like 'afternoon').

A typical rule than looks like this:

@rule(predicate('isDate'), dimension(Interval))

I.e. this rule is applicable when the intermediate production resulted in something that has a date, followed by something that is in interval (like e.g. in 'May 5th 9-10').

The actual production is a python function with the following signature:

@rule(predicate('isDate'), dimension(Interval))
def ruleDateInterval(ts, d, i):
  """
  param ts: datetime - the current refenrence time
  d: Time - a time that contains at least a full date
  i: Interval - some Interval
  """
  if not (i.t_from.isTOD and i.t_to.isTOD):
    return None
  return Interval(
    t_from=Time(year=d.year, month=d.month, day=d.day,
                hour=i.t_from.hour, minute=i.t_from.minute),
    t_to=Time(year=d.year, month=d.month, day=d.day,
              hour=i.t_to.hour, minute=i.t_to.minute))

This production will return a new interval at the date of predicate('isDate') spanning the time coded in dimension(Interval). If the latter does code for something else than a time of day (TOD), no production is returned, e.g. the rule matched but failed.

Technical Background

Some observations on the problem:

  • Each rule is a combination of regular expressions and productions.
  • Consequently, each production must originate in a sequence of regular expressions that must have matched (parts of) the text.
  • Hence, only subsequence of all regular expressions in all rules can lead to a successful production.

To this end the algorithm proceeds as follows:

  1. Input a string and a reference time
  2. Find all matches of all regular expressions from all rules in the input strings. Each regular expression is assigned an identifier.
  3. Find all distinct sequences of these matches where two matches do not overlap nor have a gap inbetween
  4. To each such subsequence apply all rules at all possible positions until no further rules can be applied - in which case one solution is produced

Obviously, not all sequences of matching expressions and not all sequences of rules applied on top lead to meaningful results. Here the PCFG kicks in:

  • Based on example data (corpus.py) a model is calibrated to predict how likely a production is to lead to a/the correct result. Instead of doing a breadth first search, the most promising productions are applied first.
  • Resolutions are produced until there are no more resolutions or a timeout is hit.
  • Based on the same model from all resolutions the highest scoring is returned.

Credits

This package was created with Cookiecutter and the audreyr/cookiecutter-pypackage project template.

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