Python and the need for speed

Steven D'Aprano steve at pearwood.info
Thu Apr 13 04:08:14 EDT 2017


On Wed, 12 Apr 2017 16:30:38 -0700, bart4858 wrote:

> (Although I think Python would have difficulty in turning x+=1 into a
> single opcode, if using normal object references and a shared object
> model.)

You know, since Python actually exists and isn't just a hypothetical 
language, we can find out what it actually does, not just guess :-)


>>> import dis
>>> code = compile("x += 1", "", "single")
>>> dis.dis(code)
  1           0 LOAD_NAME                0 (x)
              3 LOAD_CONST               0 (1)
              6 INPLACE_ADD
              7 STORE_NAME               0 (x)
             10 LOAD_CONST               1 (None)
             13 RETURN_VALUE


There's an op-code for looking up the name 'x', another to push the 
constant 1 on the stack, an op-code for "INPLACE_ADD", followed by an op-
code for STORE_NAME again.

In principle, we could replace the LOAD_CONST and INPLACE_ADD with a 
single op-code that combines the two. Whether that would speed anything 
up is another question.

Is it possible to skip the STORE_NAME op-code? If you knew *for sure* 
that the target (x) was a mutable object which implemented += using an in-
place mutation, then you could, but the only built-in where that applies 
is list so even if you could guarantee x was a list, it hardly seems 
worth the bother.



-- 
Steve


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