Python str.format() 方法的默认关键字参数值

Question

我想尽量让已有字符串的复数形式处理得简单一些，想知道能不能让 str.format() 在查找关键字参数时解释一个默认值。这里有个例子：

string = "{number_of_sheep} sheep {has} run away"
dict_compiled_somewhere_else = {'number_of_sheep' : 4, 'has' : 'have'}

string.format(**dict_compiled_somewhere_else)
# gives "4 sheep have run away"

other_dict = {'number_of_sheep' : 1}
string.format(**other_dict)
# gives a key error: u'has'
# What I'd like is for format to somehow default to the key, or perhaps have some way of defining the 'default' value for the 'has' key 
# I'd have liked: "1 sheep has run away"

谢谢

Answer 1

根据mskimm和Daniel的回答，这里有一个解决方案，它预先定义了单数和复数的词（同时纠正了mskimm的一些拼写错误）。

唯一的缺点是关键字参数number是硬编码的（所以我不能再使用number_of_sheep了）。

from string import Formatter

class Plural(Formatter):
    PLURALS = {'has' : 'have'}
    def __init__(self):
        super(Plural, self).__init__()

    def get_value(self, key, args, kwds):
        if isinstance(key, str):
            try:
                return kwds[key]
            except KeyError:
                if kwds.get('number', 1) == 1:
                    return key
                return self.PLURALS[key]
        return super(Plural, self).get_value(key, args, kwds)

string = "{number} sheep {has} run away"
fmt = Plural()
print fmt.format(string, **{'number' : 1})
print fmt.format(string, **{'number' : 2})

Answer 2

我看不出有什么好处。你还是得检查数量，因为通常你并不知道羊的确切数量。

class PluralVerb(object):
    EXCEPTIONS = {'have': 'has'}
    def __init__(self, plural):
        self.plural = plural

    def __format__(self, verb):
        if self.plural:
            return verb
        if verb in self.EXCEPTIONS:
            return self.EXCEPTIONS[verb]
        return verb+'s'

number_of_sheep = 4
print "{number_of_sheep} sheep {pl:run} away".format(number_of_sheep=number_of_sheep, pl=PluralVerb(number_of_sheep!=1))
print "{number_of_sheep} sheep {pl:have} run away".format(number_of_sheep=number_of_sheep, pl=PluralVerb(number_of_sheep!=1))

Answer 3

根据PEP 3101，string.format(**other_dict)这个功能是不可用的。

如果你用的索引或者关键字指向一个不存在的项目，就会出现IndexError或者KeyError的错误。

解决这个问题的一个提示在于自定义格式化器，也就是PEP 3101中提到的内容。这个方法使用了string.Formatter。

我对PEP 3101中的例子进行了改进：

from string import Formatter

class UnseenFormatter(Formatter):
    def get_value(self, key, args, kwds):
        if isinstance(key, str):
            try:
                return kwds[key]
            except KeyError:
                return key
        else:
            return Formatter.get_value(key, args, kwds)

string = "{number_of_sheep} sheep {has} run away"
other_dict = {'number_of_sheep' : 1}

fmt = UnseenFormatter()
print fmt.format(string, **other_dict)

输出结果是

1 sheep has run away