bound vs unbound functions
Kent Johnson
kent3737 at yahoo.com
Sat Jan 29 21:42:12 EST 2005
Michael Tobis wrote:
> I'm trying to do metaprogramming. I'm sure I've got this all wrong
> wrong wrong, but somehow my approach hasn't yet hit a brick wall.
>
> Anyway, I'd like to dynamically add a method to an instance at
> instantiation time. Something like
>
> ######
> In [71]: class quux(object):
> ....: def __init__(self,stuff):
> ....: template = "def foo(self,b): print b + %s" % stuff
> ....: exec(template)
> ....: self.bazz = foo
> ....:
>
>
> In [72]: q = quux(5)
>
> In [73]: q.bazz(4)
> ---------------------------------------------------------------------------
> TypeError Traceback (most recent call
> last)
>
> /Users/tobis/PyNSol/<console>
>
> TypeError: foo() takes exactly 2 arguments (1 given)
>
> In [74]: q.bazz("not much",4)
> 9
> ########
The thread Steve quoted suggests using new.instancemethod():
import new
class quux(object):
def __init__(self,stuff):
template = "def foo(self,b): print b + %s" % stuff
exec(template)
self.bazz = new.instancemethod(foo, self, quux)
There is no need for exec; you can define foo() directly as a nested function:
class quux(object):
def __init__(self,stuff):
def foo(self,b):
print b + stuff
self.bazz = new.instancemethod(foo, self, quux)
Of course for this simple example you can just remember stuff as an attribute:
class quux(object):
def __init__(self,stuff):
self.stuff = stuff
def bazz(self, b):
print b + self.stuff
Kent
>
> So the straightforward question is why, even though bazz is a method of
> class quux, it doesn't have that extra call parameter 'self'. Is this a
> problem? If I actually need a reference to self is it OK to do:
>
> In [76]: q.bazz(q,4)
>
> ?
>
> The more vague question is why do people despise 'exec', and how should
> I do this sort of thing instead?
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