python字典能替換它發現的東西嗎？

Question

下正常工作：

line = "Hello %(firstname)s %(lastname)s, how are you?" 
print line % dict (firstname = "Mike", lastname="Kane") 

和打印：

您好邁克·凱恩，你怎麼樣？

但是，如果我沒有價值lastname，我還是希望這個工作：

line = "Hello %(firstname)s %(lastname)s, how are you?" 
print line % dict (firstname = "Mike") 

我希望它只是忽略姓鍵和打印：

邁克，你好嗎？

Answer 1

您可以使用defaultdict

from collections import defaultdict 

line = "Hello %(firstname)s %(lastname)s, how are you?" 
print line % defaultdict(str, firstname = "Mike") 

說明：第一個參數defaultdict初始化必須是返回默認值的調用。 str可調用，返回一個空字符串（嘗試str()）。

Answer 2

您可以使用作出這樣的遺漏值提供默認字典的一個子類：

>>> class Dict(dict): 
     def __missing__(self, key): 
      # supply a default value for a given key 
      return key 


>>> d = Dict(firstname = 'Mike') 
>>> print "Hello %(firstname)s %(lastname)s, how are you?" % d 
Hello Mike lastname, how are you? 

的技術讓你在返回什麼的完全控制。這裏有一些變種：

return '--missing--'  returns a default string 
return key.upper()   highlight the missing key 
return ''     return an empty string 

Answer 3

class MyDict(dict): 
    def __missing__(self, key): 
     return "" 
print("Hello %(firstname)s %(lastname)s, how are you?" 
     % MyDict(firstname = "Mike")) 

打印

Hello Mike , how are you?