字典和默认值

Question

假设connectionDetails是Python字典，那么像这样的重构代码的最佳，最优雅，最“ pythonic”的方法是什么？if "host" in connectionDetails:    host = connectionDetails["host"]else:    host = someDefaultValue

Answer 1

像这样：

host = connectionDetails.get('host', someDefaultValue)

Answer 2

您也可以这样使用defaultdict：

from collections import defaultdict

a = defaultdict(lambda: "default", key="some_value")

a["blabla"] => "default"

a["key"] => "some_value"

您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"

Answer 3

虽然这.get()是一个很好的习惯用法，但是它比if/else（比try/except大多数情况下可以预期字典中键的存在要慢）：

>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", 

... stmt="try:\n a=d[1]\nexcept KeyError:\n a=10")

0.07691968797894333

>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", 

... stmt="try:\n a=d[2]\nexcept KeyError:\n a=10")

0.4583777282275605

>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", 

... stmt="a=d.get(1, 10)")

0.17784020746671558

>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", 

... stmt="a=d.get(2, 10)")

0.17952161730158878

>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", 

... stmt="if 1 in d:\n a=d[1]\nelse:\n a=10")

0.10071221458065338

>>> timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", 

... stmt="if 2 in d:\n a=d[2]\nelse:\n a=10")

0.06966537335119938