On 08/11/2015 11:50, Marko Rauhamaa wrote:
BartC <b...@freeuk.com>:
On 08/11/2015 11:02, Marko Rauhamaa wrote:
That elegant dynamism comes at a cost: method lookup is not a constant
memory offset. Rather, it is a dictionary lookup.
I've never understood why this seems to be necessary in Python. Why do
names have to be looked up? (I'm assuming this is searching by name in
some sort of table.)
When a module is compiled, while the compiler can't see the
definitions inside the imported modules, it /will/ know all the names
that appear in this module, so it can organise them into fixed tables.
Then the names can be referred to by index. (I think LOAD_FAST does
this.)
Modules are not the issue. Barely any functions are invoked from the
modules. Rather, almost all invocations are object methods. (Of course,
modules are objects, too.) Methods belong to objects that can be
literally anything.
Consider, for example,
shutil.copyfileobj(src, dst[, length])
The shutil module has absolutely no idea what kind of objects src and
dst are. An example program:
========================================================================
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.
--
Bartc
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