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""" |
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A utilitiy class for keeping track of intermediate parse states |
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""" |
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from collections import namedtuple |
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class TreeStack(namedtuple('TreeStack', ['value', 'parent', 'length'])): |
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""" |
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A stack which can branch in several directions, as long as you |
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keep track of the branching heads |
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An example usage is when K constituents are removed at once |
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to create a new constituent, and then the LSTM which tracks the |
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values of the constituents is updated starting from the Kth |
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output of the LSTM with the new value. |
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We don't simply keep track of a single stack object using a deque |
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because versions of the parser which use a beam will want to be |
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able to branch in different directions from the same base stack |
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Another possible usage is if an oracle is used for training |
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in a manner where some fraction of steps are non-gold steps, |
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but we also want to take a gold step from the same state. |
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Eg, parser gets to state X, wants to make incorrect transition T |
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instead of gold transition G, and so we continue training both |
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X+G and X+T. If we only represent the state X with standard |
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python stacks, it would not be possible to track both of these |
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states at the same time without copying the entire thing. |
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Value can be as transition, a word, or a partially built constituent |
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Implemented as a namedtuple to make it a bit more efficient |
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""" |
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def pop(self): |
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return self.parent |
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def push(self, value): |
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return TreeStack(value, self, self.length+1) |
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def __iter__(self): |
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stack = self |
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while stack.parent is not None: |
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yield stack.value |
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stack = stack.parent |
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yield stack.value |
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def __reversed__(self): |
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items = list(iter(self)) |
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for item in reversed(items): |
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yield item |
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def __str__(self): |
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return "TreeStack(%s)" % ", ".join([str(x) for x in self]) |
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def __len__(self): |
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return self.length |
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