En Sun, 01 Mar 2009 13:20:28 -0200, John O'Hagan <resea...@johnohagan.com>
escribió:
Inspired by some recent threads here about using classes to extend the
behaviour of iterators, I'm trying to replace some some top-level
functions
aimed at doing such things with a class.
So far it's got a test for emptiness, a non-consuming peek-ahead method,
and
an extended next() which can return slices as well as the normal mode,
but
one thing I'm having a little trouble with is getting generator
expressions
to restart when exhausted. This code works for generator functions:
[...]
I'd like to do the same for generator expressions, something like:
genexp = (i for i in range(3))
regenexp = Regen(genexp, restart=True)
such that regenexp would behave like reg, i.e. restart when exhausted
(and
would only raise StopIteration if it's actually empty). However because
generator expressions aren't callable, the above approach won't work.
I'm afraid you can't do that. There is no way of "cloning" a generator:
py> g = (i for i in [1,2,3])
py> type(g)()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: cannot create 'generator' instances
py> g.gi_code = code
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: readonly attribute
py> import copy
py> copy.copy(g)
Traceback (most recent call last):
...
TypeError: object.__new__(generator) is not safe, use generator.__new__()
py> type(g).__new__
<built-in method __new__ of type object at 0x1E1CA560>
You can do that with a generator function because it acts as a "generator
factory", building a new generator when called. Even using the Python C
API, to create a generator one needs a frame object -- and there is no way
to create a frame object "on the fly" that I know of :(
py> import ctypes
py> PyGen_New = ctypes.pythonapi.PyGen_New
py> PyGen_New.argtypes = [ctypes.py_object]
py> PyGen_New.restype = ctypes.py_object
py> g = (i for i in [1,2,3])
py> g2 = PyGen_New(g.gi_frame)
py> g2.gi_code is g.gi_code
True
py> g2.gi_frame is g.gi_frame
True
py> g.next()
1
py> g2.next()
2
g and g2 share the same execution frame, so they're not independent. There
is no easy way to create a new frame in Python:
py> type(g.gi_frame)()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: cannot create 'frame' instances
One could try using PyFrame_New -- but that's way too magic for my taste...
--
Gabriel Genellina
--
http://mail.python.org/mailman/listinfo/python-list
