What is a function parameter =[] for?

[Steven D'Aprano]

On Thu, 26 Nov 2015 09:34 pm, Dave Farrance wrote:


> (Conversely, I see that unlike CPython, all PyPy's numbers have
> unchanging ids, even after exiting PyPy and restarting, so it seems that
> PyPy's numerical ids are "faked".)
> 
> [PyPy 2.6.1 with GCC 4.9.2] on linux2
>>>>> id(1+2j)
> 679900119843984469027190799480815353863L
> 
> [PyPy 2.6.1 with GCC 4.9.2] on linux2
>>>>> id(1+2j)
> 679900119843984469027190799480815353863L


I'm pretty sure that they are faked.

I remember seeing a question on the pypy-dev mailing list asking whether it
was important that object IDs remained stable. They had a problem where the
JIT compiler would take a Python list, convert it to a low-level array of
machine-sized ints for fast processing, and then recreate the list. This
all happened in the background without the user knowing, and the PyPy devs
needed to know if it was important that the object ID of the new list
matched the old list.

The language guarantees that the id of an object won't change for the
lifespan of the object, and consequently PyPy has to fake them.


-- 
Steven