On 2022-10-17 16:43, David Lowry-Duda wrote:
One can use the `dis` module and investigate the generated python
bytecode. For me, I get
# file "dis1.py"
thing = 123
for i in range(10):
if "hi" == str(thing):
print("found")
break
The bytecode is then
1 0 LOAD_CONST 0 (123)
2 STORE_NAME 0 (thing)
2 4 LOAD_NAME 1 (range)
6 LOAD_CONST 1 (10)
8 CALL_FUNCTION 1
10 GET_ITER
>> 12 FOR_ITER 28 (to 42)
14 STORE_NAME 2 (i)
3 16 LOAD_CONST 2 ('hi')
18 LOAD_NAME 3 (str)
20 LOAD_NAME 0 (thing)
22 CALL_FUNCTION 1
24 COMPARE_OP 2 (==)
26 POP_JUMP_IF_FALSE 12
4 28 LOAD_NAME 4 (print)
30 LOAD_CONST 3 ('found')
32 CALL_FUNCTION 1
34 POP_TOP
5 36 POP_TOP
38 JUMP_ABSOLUTE 42
40 JUMP_ABSOLUTE 12
>> 42 LOAD_CONST 4 (None)
44 RETURN_VALUE
I note that line 22 calls the function str repeatedly, and no
optimization is done here.
# file "dis2.py"
thing = 123
strthing = str(thing)
for i in range(10):
if "hi" == strthing:
print("found")
break
This generates bytecode
1 0 LOAD_CONST 0 (123)
2 STORE_NAME 0 (thing)
2 4 LOAD_NAME 1 (str)
6 LOAD_NAME 0 (thing)
8 CALL_FUNCTION 1
10 STORE_NAME 2 (strthing)
3 12 LOAD_NAME 3 (range)
14 LOAD_CONST 1 (10)
16 CALL_FUNCTION 1
18 GET_ITER
>> 20 FOR_ITER 24 (to 46)
22 STORE_NAME 4 (i)
4 24 LOAD_CONST 2 ('hi')
26 LOAD_NAME 2 (strthing)
28 COMPARE_OP 2 (==)
30 POP_JUMP_IF_FALSE 20
5 32 LOAD_NAME 5 (print)
34 LOAD_CONST 3 ('found')
36 CALL_FUNCTION 1
38 POP_TOP
6 40 POP_TOP
42 JUMP_ABSOLUTE 46
44 JUMP_ABSOLUTE 20
>> 46 LOAD_CONST 4 (None)
48 RETURN_VALUE
In short, it seems the cpython interpreter doesn't (currently) perform
this sort of optimization.
It can't optimise that because, say, 'print' could've been bound to a
function that rebinds 'str'.
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