假设connectionDetails是一个Python字典,那么像这样重构代码的最佳、最优雅、最“Python”的方式是什么呢?
if "host" in connectionDetails:
host = connectionDetails["host"]
else:
host = someDefaultValue
假设connectionDetails是一个Python字典,那么像这样重构代码的最佳、最优雅、最“Python”的方式是什么呢?
if "host" in connectionDetails:
host = connectionDetails["host"]
else:
host = someDefaultValue
当前回答
你也可以像这样使用defaultdict:
from collections import defaultdict
a = defaultdict(lambda: "default", key="some_value")
a["blabla"] => "default"
a["key"] => "some_value"
你可以传递任何普通函数而不是lambda:
from collections import defaultdict
def a():
return 4
b = defaultdict(a, key="some_value")
b['absent'] => 4
b['key'] => "some_value"
其他回答
(这是一个迟来的回答)
另一种方法是继承dict类的子类并实现__missing__()方法,如下所示:
class ConnectionDetails(dict):
def __missing__(self, key):
if key == 'host':
return "localhost"
raise KeyError(key)
例子:
>>> connection_details = ConnectionDetails(port=80)
>>> connection_details['host']
'localhost'
>>> connection_details['port']
80
>>> connection_details['password']
Traceback (most recent call last):
File "python", line 1, in <module>
File "python", line 6, in __missing__
KeyError: 'password'
你也可以像这样使用defaultdict:
from collections import defaultdict
a = defaultdict(lambda: "default", key="some_value")
a["blabla"] => "default"
a["key"] => "some_value"
你可以传递任何普通函数而不是lambda:
from collections import defaultdict
def a():
return 4
b = defaultdict(a, key="some_value")
b['absent'] => 4
b['key'] => "some_value"
对于多个不同的默认值,请尝试以下方法:
connectionDetails = { "host": "www.example.com" }
defaults = { "host": "127.0.0.1", "port": 8080 }
completeDetails = {}
completeDetails.update(defaults)
completeDetails.update(connectionDetails)
completeDetails["host"] # ==> "www.example.com"
completeDetails["port"] # ==> 8080
是这样的:
host = connectionDetails.get('host', someDefaultValue)
这并不是我们要问的问题,但是在python字典中有一个方法:dict.setdefault
host = connectionDetails.setdefault('host',someDefaultValue)
然而,这个方法将connectionDetails['host']的值设置为someDefaultValue,如果key host还没有定义,这与问题不同。