[Python-ideas] Explicit variable capture list

[M.-A. Lemburg]

On 21.01.2016 09:48, Victor Stinner wrote:
> The difference between "def hello(print=print): ..." and Serhiy's
> const idea (or my optimization) is that "def hello(print=print): ..."
> changes the signature of the function which can be a serious issue in
> an API.
> 
> Note: The other optimization "local_print = print" in the function is
> only useful for loops (when the builtin is loaded multiple times) and
> it still loads the builtin once per function call, whereas my
> optimization uses a constant and so no lookup is required anymore.
> 
> Then guards are used to disable the optimization if builtins are
> modified. See the PEP 510 for an explanation on that part.

I ran performance tests on these optimization tricks (and
others) in 2014. See this talk:

http://www.egenix.com/library/presentations/PyCon-UK-2014-When-performance-matters/

(slides 33ff.)

The keyword trick doesn't really pay off in terms of added
performance vs. danger of introducing weird bugs.

Still, it would be great to have a way to say "please look
this symbol up at compile time and stick the result in a local
variable" (which is basically what the keyword trick does),
only in a form that's easier to detect when reading the code
and doesn't change the function signature.

A decorator could help with this (by transforming the byte
code and localizing the symbols), e.g.

@localize(len)
def f(seq):
    z = 0
    for x in seq:
       if x:
           z += len(x)
    return z

but the more we move language features to decorators, the
less readable the code will get by having long tails of
decorators on many functions (we don't really want our
functions to resemble snakes, do we ? :-)).

So perhaps it is indeed time for a new keyword to localize
symbols in a function or module, say:

# module scope localization, applies to all code objects in
# this module:
localize len

def f(seq):
    ...

or:

def f(seq):
    # Localize len in this function, since we need it in
    # tight loops
    localize len
    ...

All that said, I don't really believe that this is a high
priority feature request. The gained performance win is
not all that great and only becomes relevant when used
in tight loops.

-- 
Marc-Andre Lemburg
eGenix.com

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