From: Thomas Jollans on
On 07/23/2010 07:13 PM, Thomas Jollans wrote:
> On 07/23/2010 12:34 AM, wheres pythonmonks wrote:
>> 2. Is there a better way to loopup by id? I'm not very familiar with
>> sys.exc_info, but creating the id->name hash each time seems like
>> overkill.
> I just had the most horrendous idea. Really, looking up objects by ID,
> or even swapping two objects, isn't that difficult if you do some C
> black magic. Don't try this in front of the kids.
> I wrote a little module, called "hell":
> Python 3.1.2 (release31-maint, Jul 8 2010, 09:18:08)
> [GCC 4.4.4] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>>>> from hell import swap, getptr
>>>> dir
> <built-in function dir>
>>>> len
> <built-in function len>
>>>> swap(dir, len)
>>>> dir
> <built-in function len>
>>>> len
> <built-in function dir>
>>>> a = "this was a"
>>>> b = "this was b, hell yeah"
>>>> (a, b, id(a), id(b))
> ('this was a', 'this was b, hell yeah', 32417752, 32418144)
>>>> tpl = (a, b, id(a), id(b))
>>>> tpl
> ('this was a', 'this was b, hell yeah', 32417752, 32418144)
>>>> swap(a, b)
>>>> tpl
> ('this was b, hell yeah', 'this was a', 32417752, 32418144)
>>>> getptr(32417752)
> 'this was b, hell yeah'

The great thing about this is that it can illustrate, in a very perverse
manner, some lovely facets of Python. For example: combined hash and
equality checks when accessing sets and dicts:

Python 3.1.2 (release31-maint, Jul 8 2010, 09:18:08)
[GCC 4.4.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from hell import swap
>>> s1 = 'string1'
>>> s2 = 'string2'
>>> s = {s1, s2}
>>> swap(s1, s2)
>>> s
{'string1', 'string2'}
>>> s1 in s
>>> s2 in s
>>> s1 in list(s)
>>> s2 in list(s)

From: Terry Reedy on
On 7/23/2010 4:10 AM, Steven D'Aprano wrote:

> Using exec or friends to avoid the overhead of function calls is like
> pushing your car to work to avoid the overhead of having to get in and
> out of the car.

And, of course, exec *is* a function call (explicitly in 3.x, but
somewhere also in the innards of 2.x).

Thanks for the laugh of the day.

Terry Jan Reedy

From: Albert van der Horst on
In article <4c495b50$0$28634$c3e8da3(a)>,
Steven D'Aprano <steve(a)> wrote:
>On Thu, 22 Jul 2010 21:23:05 -0700, Stephen Hansen wrote:
>> On 7/22/10 7:47 PM, wheres pythonmonks wrote:
>>> The truth is that I don't intend to use these approaches in anything
>>> serious. However, I've been known to do some metaprogramming from time
>>> to time.
>> Depending on how you define "metaprogramming", Python is pretty
>> notoriously ill-suited towards the task (more, its basically considered
>> a feature it doesn't let you go there) -- or, it allows you to do vast
>> amounts of stuff with its few dark magic hooks. I've never had a
>> satisfying definition of metaprogramming that more then 50% of any group
>> agree with, so I'm not sure which you're looking for. :)
>I disagree strongly at your characterisation that Python is notorious for
>being ill-suited towards metaprogramming. I'd say the complete opposite
>-- what is considered dark and scary metaprogramming tasks in other
>languages is considered too ordinary to even mention in Python.
>If you have a class that defines a single member (or attribute in Python
>terminology) "spam", and you want to add a second "ham" to a specific
>instance, such a thing is either deep, dark metaprogramming in some
>languages, if not outright impossible. In Python it is not even
>instance.ham = "something" # *yawns*
>Recently there was a thread started by some Java or C++ refugee who was
>distressed about attribute access in Python, because it made
>metaprogramming frighteningly easy:
>My response at the time was: Python makes metaprogramming *easy*:
>I can't imagine how he would have reacted if we had showed him how easy
>it is to monkey-patch built-in functions...
>> But! What it doesn't let you do is get clever with syntax and write a
>> sort of "simplified" or domain specific language to achieve certain
>> sorts of repetitive tasks quickly. You always end up writing normal
>> Python that looks like all other Python.
>Exactly... 90% of the time that you think you want a DSL, Python beat you
>to it.
>It is true that other languages (like Lisp, or Forth) allow you to write
>your own syntax. That's incredibly powerful, but it also has serious
>costs. If you can define your own syntax, that means every piece of code
>you look at is potentially a different language.

The answer is, of course, discipline.
I'm a Forth programmer and I morf Forth into an assembler.
Now some sources become assembler sources and one should be
very adamant about it. I've written a Perl interpreter for
those sources, and a Python interpreter would be even easier.

If I'm doing Python, I don't miss macro possibilities, but
sometimes my code gets repetitive.
If I ever get bored by repetition, there is m4.

m4 < aap.pym >

(See my site below for Forth assemblers, and non-trivial m4 usage.)


Groetjes Albert

Economic growth -- being exponential -- ultimately falters.
albert(a)spe&ar& &=n