Need instruction on how to use isinstance

[Steven W. Orr]

On 6/27/2010 10:25 PM, Stephen Hansen wrote:
> On 6/27/10 7:09 PM, Steven W. Orr wrote:
>> So, my question is, what value can I use as the 2nd arg to isinstance
>> to see if
>> foo is a function? And while I'm on the subject, what types does
>> isinstance not
>> support?
> 
> Does it have to be a function? -- There's quite a few things which are
> function-y enough that you really should accept them even if they may
> not be technically a /function/. Like a class instance with the __call__
> method defined wrapping a function as a decorator. Or a bound method,
> really.
> 
> In Python 2.x, you can use callable(fun) -- that goes away in Py3
> though. But personally, I think this is the wrong approach.

Ok, you just asked great questions, and your questions were based on what I
didn't tell you.

I'm trying to teach myself about how __metaclass__ can be used as a substitute
for LSD. ;-)

So, instead of writing a class that does typechecking on its arguments at
instantiation time, I'm writing a metaclass that allows the class to announce
what sort of values it specifically will be expecting after instantiation.

The model I'm using is something I've found that

http://cleverdevil.org/computing/78/

So, the basic idea is

>>> class Person(Enforcer):
    ...    name = Field(str)
    ...    age = Field(int)
    ...    ff = Field(function)

but there's no factory function called function.

Does this help?

> 
> I'm assuming you need to do some different sort of behavior based on if
> its a str, tuple or list-- okay.
> 
> But, personally-- at that point, I would just *assume* the argument is a
> function. And call it.
> 
> If it can't be called, its not a functiony-thing. You can either let
> that traceback propagate, or just raise a TypeError "expected arg to be
> a string, tuple, list or callable".
> 
> Then again, similarly I almost never want to test for if somethings a
> tuple or list. I'd rather use it as a sequence type and see if it works:
> while there's not as many 'sequency-like-things' out there as there are
> function-like-things, there's still quite a few.
> 
> So personally, I'd check if its a string (can it be unicode or regular?
> If so isinstance(x,basestring)).
> 
> Then I'd try to use it as a sequence, doing whatever I woulda done with
> it sequence-like. If that works, great. If not, I'd try to call it.
> 
> Etc.
> 
> Then again it does depend on just what you're *doing* with the arg being
> passed in.
> 


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steveo at syslang.net