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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