I am going trough the docs of Python 3.X, I have doubt about List Comprehension speed of execution and how it exactly work.
Let's take the following example:
Listing 1
...
L = range(0,10)
L = [x ** 2 for x in L]
...
Now in my knowledge this return a new listing, and it's equivalent to write down:
Listing 2
...
res = []
for x in L:
res.append(x ** 2)
...
The main difference is the speed of execution if I am correct. Listing 1 is supposed to be performed at C language speed inside the interpreter, meanwhile Listing 2 is not.
But Listing 2 is what the list comprehension does internally (not sure), so why Listing 1 is executed at C Speed inside the interpreter & Listing 2 is not? Both are converted to byte code before being processed, or am I missing something?
Look at the actual bytecode that is produced. I've put the two fragments of code into fuctions called f1 and f2.
The comprehension does this:
3 15 LOAD_CONST 3 (<code object <listcomp> at 0x7fbf6c1b59c0, file "<stdin>", line 3>)
18 LOAD_CONST 4 ('f1.<locals>.<listcomp>')
21 MAKE_FUNCTION 0
24 LOAD_FAST 0 (L)
27 GET_ITER
28 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
31 STORE_FAST 0 (L)
Notice there is no loop in the bytecode. The loop happens in C.
Now the for loop does this:
4 21 SETUP_LOOP 31 (to 55)
24 LOAD_FAST 0 (L)
27 GET_ITER
>> 28 FOR_ITER 23 (to 54)
31 STORE_FAST 2 (x)
34 LOAD_FAST 1 (res)
37 LOAD_ATTR 1 (append)
40 LOAD_FAST 2 (x)
43 LOAD_CONST 3 (2)
46 BINARY_POWER
47 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
50 POP_TOP
51 JUMP_ABSOLUTE 28
>> 54 POP_BLOCK
In contrast to the comprehension, the loop is clearly here in the bytecode. So the loop occurs in python.
The bytecodes are different, and the first should be faster.