Loop while checking if element in a list in Python

Question

Let's say I have a simple piece of code like this:

for i in range(1000):
 if i in [150, 300, 500, 750]:
 print(i)

Does the list [150, 300, 500, 750] get created every iteration of the loop? Or can I assume that the interpreter (say, CPython 2.7) is smart enough to optimize this away?

• Kevin: Interesting related question: Tuple or list when using 'in' in an 'if' clause?. Goes into some detail about what CPython does under the hood.
• Jon Surrell: Unless you specify that you want to know about 1 specific interpreter, this is very difficult to answer. Can you rephrase "(say, CPython 2.7)" to specify you want to know about that interpreter exactly?

Answer 1

You can view the bytecode using dis.dis. Here's the output for CPython 2.7.11:

 2 0 SETUP_LOOP 40 (to 43)
 3 LOAD_GLOBAL 0 (range)
 6 LOAD_CONST 1 (1000)
 9 CALL_FUNCTION 1
 12 GET_ITER 
 >> 13 FOR_ITER 26 (to 42)
 16 STORE_FAST 0 (i)
 3 19 LOAD_FAST 0 (i)
 22 LOAD_CONST 6 ((150, 300, 500, 750))
 25 COMPARE_OP 6 (in)
 28 POP_JUMP_IF_FALSE 13
 4 31 LOAD_FAST 0 (i)
 34 PRINT_ITEM 
 35 PRINT_NEWLINE 
 36 JUMP_ABSOLUTE 13
 39 JUMP_ABSOLUTE 13
 >> 42 POP_BLOCK 
 >> 43 LOAD_CONST 0 (None)
 46 RETURN_VALUE 

Hence, the list creation is optimized to the loading of a constant tuple (byte 22). The list (which is in reality a tuple in this case) is not created anew on each iteration.