hack to auto-define variables?

[Michal Wallace]

I'm trying to come up with a way to make something like this happen:

>>> x
None

WITHOUT saying "x=None":

>>> x = "blablalskdjflaskdjf"
>>> del x
>>> x
None


Bascially, I'd like to make variables just pop into existence under
certain circumstances.

My first thought was to replace __dict__ (or __dict__.__getitem__) but
this raises an error.

Then I thought maybe I could throw an exception, assign the variable,
and then find some way to make that exception disappear.. (Like
whoever mentioned signals vs exceptions earlier).. But I haven't
found a way to make that happen.

Then I found a way I could do what I wanted for a module that I'm
importing, but not the top level module:


## a module that isn't a module....
## must call with "from badboy import *" though.. :/

class _BadBoy:
    def __init__(self):
        self.__dict__["__attrs__"] = {}
        self.__dict__["__default__"] = None
    def __getattr__(self, name):
        return self.__dict__["__attrs__"].get(
            name, self.__dict__["__default__"])
    def __setattr__(self, name, value):
        self.__dict__["__attrs__"][name] = value
    def __str__(self):
        return "BADBOY!"

try:
    raise "this is just a hack!"
except:
    _frame = sys.exc_info()[2].tb_frame.f_back
    _frame.f_locals["badboy"] = _BadBoy()


#####

>>> from badboy import * # because import is an assignment
>>> badboy.name
None


#####


I've got some other ideas, but they all seem like too much work:

  - write a C module that, when you import it, replaces the locals()
    of the calling frame with something that has a __getitem__..

  - write a wrapper script, and just define my own __globals__..
    maybe give scripts like this a special extension, and create
    an ihook...

  - preprocess the python file and define variables myself..

  - hack python itself, and make __dict__ writable..

  - set sys.settrace() to some evil function that will look
    ahead for uses of undeclared variables and declare them
    before they cause an error.. (Which sounds like a lot of
    fun, but I suspect it would kill performance)

  - some kind of bytecodehack?


Anyone have any ideas? (besides "just declare your variables!")

Cheers,

- Michal
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