dict comprehension

Steven Bethard steven.bethard at gmail.com
Sat Feb 2 19:06:54 CET 2008


Wildemar Wildenburger wrote:
> Arnaud Delobelle wrote:
>>> I believe both set and dict comprehensions will be in 3.0.
>>
>> Python 3.0a1+ (py3k:59330, Dec  4 2007, 18:44:39)
>> [GCC 4.0.1 (Apple Inc. build 5465)] on darwin
>> Type "help", "copyright", "credits" or "license" for more information.
>>>>> {x*x for x in range(10)}
>> {0, 1, 4, 81, 64, 9, 16, 49, 25, 36}
>>>>> {x:x*x for x in range(10)}
>> {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
>>
> OK, not bad. But I don't really see how this is better than the 
> generator approach.

It's more than twice as fast:

 >>> setup = "items = range(10)"
 >>> timeit.Timer("dict((x, x * x) for x in items)", setup).timeit()
6.0559464959932114
 >>> timeit.Timer("{x:x * x for x in items}", setup).timeit()
2.8347301821879682

It also doesn't build the unnecessary intermediate tuples:

 >>> def dict_genexp(items):
...     return dict((x, x * x) for x in items)
...
 >>> def dict_comp(items):
...     return {x:x * x for x in items}
...
 >>> dis.dis(dict_genexp.__code__.co_consts[1])
   2           0 LOAD_FAST                0 (.0)
         >>    3 FOR_ITER                21 (to 27)
               6 STORE_FAST               1 (x)
               9 LOAD_FAST                1 (x)
              12 LOAD_FAST                1 (x)
              15 LOAD_FAST                1 (x)
              18 BINARY_MULTIPLY
              19 BUILD_TUPLE              2
              22 YIELD_VALUE
              23 POP_TOP
              24 JUMP_ABSOLUTE            3
         >>   27 LOAD_CONST               0 (None)
              30 RETURN_VALUE
 >>> dis.dis(dict_comp.__code__.co_consts[1])
   2           0 BUILD_MAP                0
               3 DUP_TOP
               4 STORE_FAST               1 (_[1])
               7 LOAD_FAST                0 (.0)
         >>   10 FOR_ITER                21 (to 34)
              13 STORE_FAST               2 (x)
              16 LOAD_FAST                1 (_[1])
              19 LOAD_FAST                2 (x)
              22 LOAD_FAST                2 (x)
              25 BINARY_MULTIPLY
              26 ROT_TWO
              27 LOAD_FAST                2 (x)
              30 STORE_SUBSCR
              31 JUMP_ABSOLUTE           10
         >>   34 RETURN_VALUE

STeVe



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