nltk.grammar.is_terminal('str')总是返回true？

Question

如何定义nltk.grammar.is_terminal()使用的语法？无论我在哪个对象上强制转换此方法，我都会得到一个true作为返回值。但是，我想要检查一个名为wordlist的列表是否包含在grammar.cfg下使用上下文无关文法定义的结果。

Answer 1

查看https://github.com/nltk/nltk/blob/develop/nltk/grammar.py上的代码

def is_nonterminal(item):
    """
    :return: True if the item is a ``Nonterminal``.
    :rtype: bool
    """
    return isinstance(item, Nonterminal)


def is_terminal(item):
    """
    Return True if the item is a terminal, which currently is
    if it is hashable and not a ``Nonterminal``.
    :rtype: bool
    """
    return hasattr(item, '__hash__') and not isinstance(item, Nonterminal)

尽管我不确定应该如何使用这些函数，但是对于任何字符串输入，is_terminal()的默认值始终是True。

首先，因为所有字符串都包含__hash__属性，所以它是一个对字符串进行散列的函数，请参见https://docs.python.org/2/reference/datamodel.html#object.

>>> astring = 'foo bar'
>>> astring.__hash__
<method-wrapper '__hash__' of str object at 0x7f06bb0cbcc0>
>>> astring.__hash__()
8194924035431162904

其次，在NLTK中，所有字符串肯定不是Nonterminal对象，因为Nonterminal类是：

class Nonterminal(object):
    """
    A non-terminal symbol for a context free grammar.  ``Nonterminal``
    is a wrapper class for node values; it is used by ``Production``
    objects to distinguish node values from leaf values.
    The node value that is wrapped by a ``Nonterminal`` is known as its
    "symbol".  Symbols are typically strings representing phrasal
    categories (such as ``"NP"`` or ``"VP"``).  However, more complex
    symbol types are sometimes used (e.g., for lexicalized grammars).
    Since symbols are node values, they must be immutable and
    hashable.  Two ``Nonterminals`` are considered equal if their
    symbols are equal.
    :see: ``CFG``, ``Production``
    :type _symbol: any
    :ivar _symbol: The node value corresponding to this
        ``Nonterminal``.  This value must be immutable and hashable.
    """

因此，一个字符串需要同时满足(1)具有__hash__属性和(2)不是Nonterminal对象这两个条件。因此，对于所有字符串，nltk.grammar.is_terminal()总是返回True。

那么，只有当您加载语法，然后读取语法中的非终结符对象时，可能仅当某个对象被专门创建或强制转换为非终结符时，我才能使其返回False，例如http://www.nltk.org/_modules/nltk/parse/pchart.html