> List comprehensions are preferred to map and filter functions.In fact,
> filter is a syntactic sugar to list comprehension.
>
>
You're right in saying list comprehensions are preferred to map and filter -
not just from a readability standpoint but also from a performance
standpoint. (we all hate that .0001 of sec right? - no seriously)
However filter is *not* a syntactic sugar of list comprehension.
>>> l = [1,2,3,4,5,6,7,8]
>>> def foo(a):
... return [x for x in a if x % 2]
...
>>> def bar(a):
... return filter(lambda x: x%2, a)
...
>>> foo(l)
[1, 3, 5, 7]
>>> bar(l)
[1, 3, 5, 7]
>>> dis.dis(foo)
2 0 BUILD_LIST 0
3 DUP_TOP
4 LOAD_ATTR 0 (append)
7 STORE_FAST 1 (_[1])
10 LOAD_FAST 0 (a)
13 GET_ITER
>> 14 FOR_ITER 31 (to 48)
17 STORE_FAST 2 (x)
20 LOAD_FAST 2 (x)
23 LOAD_CONST 1 (2)
26 BINARY_MODULO
27 JUMP_IF_FALSE 14 (to 44)
30 POP_TOP
31 LOAD_FAST 1 (_[1])
34 LOAD_FAST 2 (x)
37 CALL_FUNCTION 1
40 POP_TOP
41 JUMP_ABSOLUTE 14
>> 44 POP_TOP
45 JUMP_ABSOLUTE 14
>> 48 DELETE_FAST 1 (_[1])
51 RETURN_VALUE
52 LOAD_CONST 0 (None)
55 RETURN_VALUE
>>> dis.dis(bar)
2 0 LOAD_GLOBAL 0 (filter)
3 LOAD_CONST 1 (<code object <lambda> at
0x2a955e4d50, file "<stdin>", line 2>)
6 MAKE_FUNCTION 0
9 LOAD_FAST 0 (a)
12 CALL_FUNCTION 2
15 RETURN_VALUE
16 LOAD_CONST 0 (None)
19 RETURN_VALUE
The alleged performance gain of list comprehension comes from the fact that
`filter` involve a function call (here= lambda). I'm sure this can be easily
proved using python -m timeit
Then what is syntatic sugar ?
This.
>>> def foo(a):
... a += 1
... return a
...
>>> def bar(a):
... a = a+1
... return a
...
>>> foo(10)
11
>>> bar(10)
11
>>> dis.dis(foo)
2 0 LOAD_FAST 0 (a)
3 LOAD_CONST 1 (1)
6 INPLACE_ADD
7 STORE_FAST 0 (a)
3 10 LOAD_FAST 0 (a)
13 RETURN_VALUE
14 LOAD_CONST 0 (None)
17 RETURN_VALUE
>>> dis.dis(bar)
2 0 LOAD_FAST 0 (a)
3 LOAD_CONST 1 (1)
6 BINARY_ADD
7 STORE_FAST 0 (a)
3 10 LOAD_FAST 0 (a)
13 RETURN_VALUE
14 LOAD_CONST 0 (None)
17 RETURN_VALUE
/jeff
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