On 08/11/2015 12:43, Marko Rauhamaa wrote:
BartC <b...@freeuk.com>:
On 08/11/2015 11:50, Marko Rauhamaa wrote:
========================================================================
import shutil
class Source:
def __init__(self):
self.remaining = "hello world"
def read(self, count):
if count <= 0:
return ""
chunk, self.remaining = self.remaining[:count], self.remaining[count:]
return chunk
class Dest:
def write(self, stuff):
print("<{}>".format(stuff))
shutil.copyfileobj(Source(), Dest())
========================================================================
OK, so here, it is necessary to resolve "copyfileobj" by seeing if
"shutil" is something that contains a name "copyfileobj" that happens
to be a method.
Is this the lookup you're talking about, and is it done using the
actual string "copyfileobj"? If so, does it need to be done every
single time this line is executed? It would extraordinarily
inefficient if that was the case.
No, what I'm talking about is that copyfileobj calls src.read() and
dst.write() without having any idea what kinds of objects src and dst
are. Thus, it will be difficult for a performance optimizer to get rid
of the dictionary lookups.
Sorry, you'll have to assume I'm very stupid.
What exactly is being looked up, and in what?
From what I can understand in your example:
* You are calling shutil.copyfileobj with two arguments, which happen to
be instances of classes Source and Dest.
* Let's say these are known as src and dst inside .copyfileobj.
* .copyfileobj then calls methods src.read() dst.write().
* Each of these method calls has the form A.B() which seems to me to be
little different from shutil.copyfileobj().
So to get back to what I was saying, does this lookup involving
searching for method B in object A, and if so, does it actually do a
search by name?
(Someone mentioned a precalculated hash, of "A", or of "src" or "dst",
use used, but still, it's looking something up in a table, and a hash
table lookup I believe still requires an string compare to check if
you've got the right entry.)
--
Bartc
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