[Python-ideas] Method chaining notation

[Chris Angelico]

On Tue, Feb 25, 2014 at 6:59 AM, Masklinn <masklinn at masklinn.net> wrote:
>     a = foo()
>     a.bar()
>
> compiles to:
>
>               0 LOAD_*                   0 (foo)
>               3 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
>               6 STORE_FAST               0 (a)
>
>               9 LOAD_FAST                0 (a)
>              12 LOAD_ATTR                1 (bar)
>              15 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
>              18 POP_TOP
>
> the pair (6, 9) is a noop and could trivially be removed (in the absence
> of jumps around). According to [0] a patch implementing this (although
> without taking care of jumps) was rejected:

Possible reason for rejection: The optimizer would have to be sure
that a wasn't used anywhere else.

a = foo()
a.bar()
a.spam()

  2           0 LOAD_GLOBAL              0 (foo)
              3 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
              6 STORE_FAST               0 (a)

  3           9 LOAD_FAST                0 (a)
             12 LOAD_ATTR                1 (bar)
             15 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
             18 POP_TOP

  4          19 LOAD_FAST                0 (a)
             22 LOAD_ATTR                2 (spam)
             25 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
             28 POP_TOP

The subsequent LOAD_FAST of a depends on the STORE_FAST having been
done. In the specific case mentioned in your link, he was looking for
a RETURN_VALUE opcode, so that would be safe. (But if there really is
code like he's seeing, I'd look at tidying it up on the Python source
level. Just return the value directly. No need for "single exit point"
in Python code.)

ChrisA