Python String template dict KeyError，如何设置默认值

Question

我想试试:

"%(name)s, %(age)s" % {"name":'Bob'}`

和控制台 Bob, 20 或 Bob,。

但是，这会引发:

Traceback (most recent call last):`
  File "<pyshell#4>", line 1, in <module>`
    "%(name)s, %(age)s" % {"name":'a'}`
KeyError: 'age'`

如何设置默认值？事实上，当我输入 "%(name)s, %(age)s"% {"name":'Bob'} 时，我想得到 Bob, don' t 提高 Except.

Answer 1

你可以这样继承 dict:

class Default(dict):
    def __init__(self, default, *args, **kwargs):
        self.default = default
        dict.__init__(self, *args, **kwargs)
    def __missing__(self, key):
        return self.default

#用法:

print "%(name)s, %(age)s" % Default('', {"name":'Bob'})
print "%(name)s, %(age)s" % Default('', name='Bob')

上面两行打印Bob,

See it working online
See version of this code written in Python 3 working online

编辑

我重新邀请了collections.defaultdict .

from collections import defaultdict

print "%(name)s, %(age)s" % defaultdict(lambda: '', {"name":'Bob'})
print "%(name)s, %(age)s" % defaultdict(lambda: '', name='Bob')

See it working online