Python检查现有密钥的最佳方法

Question

检查密钥是否存在的更有效/更快/更好的方法是哪种？

if 'subject' in request.POST:
    subject = request.POST['subject']
else:
    // handle error

要么

try:
    subject = request.POST['subject']
except KeyError:
    // handle error

Answer 1

后者(try/except)形式通常是更好的形式.

try块非常便宜但是捕获异常可能更昂贵.对dict的收容检查往往便宜,但并不比没有便宜.我怀疑效率会有平衡,具体取决于'subject'实际存在的频率.然而,这并不重要,因为过早的优化是无用的,分散注意力,浪费和无效.你会选择更好的解决方案.

如果代码实际上是形式的

if 'subject' in request.POST:
    subject = request.POST['subject']
else:
    subject = some_default

然后你真正想要的是request.POST.get('subject', some_default).

Answer 2

我使用 .get() 方法——这是更好的方法。

Python 2.5.2 (r252:60911, Jul 22 2009, 15:33:10)
[GCC 4.2.4 (Ubuntu 4.2.4-1ubuntu3)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import dis
>>> def f1(key, d):
...   if key in d:
...     return d[key]
...   else:
...     return "default"
...
>>> dis.dis(f1)
  2           0 LOAD_FAST                0 (key)
              3 LOAD_FAST                1 (d)
              6 COMPARE_OP               6 (in)
              9 JUMP_IF_FALSE           12 (to 24)
             12 POP_TOP

  3          13 LOAD_FAST                1 (d)
             16 LOAD_FAST                0 (key)
             19 BINARY_SUBSCR
             20 RETURN_VALUE
             21 JUMP_FORWARD             5 (to 29)
        >>   24 POP_TOP

  5          25 LOAD_CONST               1 ('default')
             28 RETURN_VALUE
        >>   29 LOAD_CONST               0 (None)
             32 RETURN_VALUE
>>> def f2(key, d):
...   return d.get(key, "default")
...
>>> dis.dis(f2)
  2           0 LOAD_FAST                1 (d)
              3 LOAD_ATTR                0 (get)
              6 LOAD_FAST                0 (key)
              9 LOAD_CONST               1 ('default')
             12 CALL_FUNCTION            2
             15 RETURN_VALUE
>>> def f3(key, d):
...   try:
...     return d[key]
...   except KeyError:
...     return "default"
...
>>> dis.dis(f3)
  2           0 SETUP_EXCEPT            12 (to 15)

  3           3 LOAD_FAST                1 (d)
              6 LOAD_FAST                0 (key)
              9 BINARY_SUBSCR
             10 RETURN_VALUE
             11 POP_BLOCK
             12 JUMP_FORWARD            23 (to 38)

  4     >>   15 DUP_TOP
             16 LOAD_GLOBAL              0 (KeyError)
             19 COMPARE_OP              10 (exception match)
             22 JUMP_IF_FALSE           11 (to 36)
             25 POP_TOP
             26 POP_TOP
             27 POP_TOP
             28 POP_TOP

  5          29 LOAD_CONST               1 ('default')
             32 RETURN_VALUE
             33 JUMP_FORWARD             2 (to 38)
        >>   36 POP_TOP
             37 END_FINALLY
        >>   38 LOAD_CONST               0 (None)
             41 RETURN_VALUE