Python的str.format()方法的默认kwarg值

Question

我希望尝试尽可能简单地保持现有字符串的多元化,并且想知道str.format()在寻找kwargs时是否有可能解释默认值.这是一个例子:

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"

干杯

Answer 1

作为PEP 3101,string.format(**other_dict)不可用.

如果索引或关键字引用了不存在的项,则应引发IndexError/KeyError.

解决问题的提示是Customizing Formatters,PEP 3101.那用string.Formatter.

我改进了以下示例PEP 3101:

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)

输出是

1 sheep has run away