"Antoon Pardon" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> I disagree. The += version only evaluates the index once, but still has
> to find the object twice.
No it does not *have* to find the object twice and no it does *not* find
the object twice. From the viewpoint of the interpreter, the purpose of
abbreviating 'objectexpression = objectexpression op arg' as
'objectexpression op=arg' is to avoid unnecessary recalculation
> But as far as I can interpret what is happening from the printed lines
Your print get/set examples from your black-box testing miss the following
key point. While the interpreter still has to *use* the object(s) twice,
once to get and once to set, it no longer has to *calculate* the objects
twice. If we open the box and look at the compiled pycode, we get, for
example:
>>> dis(compile('x[i][j+k]=x[i][j+k]+1', '', 'single'))
1 0 LOAD_NAME 0 (x)
3 LOAD_NAME 1 (i)
6 BINARY_SUBSCR
7 LOAD_NAME 2 (j)
10 LOAD_NAME 3 (k)
13 BINARY_ADD
14 BINARY_SUBSCR
15 LOAD_CONST 0 (1)
18 BINARY_ADD
19 LOAD_NAME 0 (x)
22 LOAD_NAME 1 (i)
25 BINARY_SUBSCR
26 LOAD_NAME 2 (j)
29 LOAD_NAME 3 (k)
32 BINARY_ADD
33 STORE_SUBSCR
>>> dis(compile('x[i][j+k]+=1', '', 'single'))
1 0 LOAD_NAME 0 (x)
3 LOAD_NAME 1 (i)
6 BINARY_SUBSCR
7 LOAD_NAME 2 (j)
10 LOAD_NAME 3 (k)
13 BINARY_ADD
14 DUP_TOPX 2
17 BINARY_SUBSCR
18 LOAD_CONST 0 (1)
21 INPLACE_ADD
22 ROT_THREE
23 STORE_SUBSCR
[blank lines added for clarity]
Both forms call binary_subscr to get the number to add to 1 and both call
store_subscr to set the result back in the list. But in the first case,
the expressions for both the source-target list and the index of the value
for the addition are duplicated and re-evaluated. In the second, the
*results* of the first evaluation are duplicated (on the stack) and saved
for the storage operation.
For understanding the detailed operation of CPython, dis is a great help
Terry Jan Reedy
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