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

Question

如何定义 nltk.grammar.is_terminal() 使用的语法？无论我在什么对象上使用此方法，我总是得到 A true 作为返回值。但相反，我想检查一个名为 wordlist 的列表是否包含在 grammar.cfg 下安全的上下文无关语法中定义的作品。

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)

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

因为，首先，所有字符串都包含__hash__ 属性，它是一个散列字符串的函数，参见https://docs.python.org/2/reference/datamodel.html#object.hash

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

其次，所有字符串肯定不是 NLTK 中的 Nonterminal 对象，因为类 Nonterminal 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，字符串符合以下两个条件:(1) 具有 __hash__ 属性，并且 (2) 不是 Nonterminal 对象。因此，nltk.grammar.is_terminal() 总是为所有字符串返回 True。

然后如何让它返回 False，仅当您加载语法然后读取语法中的非终端对象时，可能仅当对象被专门创建或转换为非终端时，例如http://www.nltk.org/_modules/nltk/parse/pchart.html