instance/class methods: having my cake and eating it too

class Decimal:

format = '%.0f' # default format is for integer printing

def GetFormat(self):
return self.format

def SeeFormat(self):
print self.GetFormat()

def SetDigits(cls, digits):
digits = min(digits, 5)
cls.format = "%%.%uf" % digits

SetDigits = classmethod(SetDigits)

class Weight(Decimal):

Decimal.SetDigits(3)

>>> x = Weight()
>>> x.SeeFormat()
%.3f

But I also want to be able to override this class attribute with an
instance attribute that is set using an instance method:

def InstanceSetDigits(self, digits):
digits = min(digits, 5)
self.format = "%%.%uf" % digits

class MyContainer:

def __init__(self):
self.wt = Weight()
self.wt.InstanceSetDigits(2)

>>> y = MyContainer()
>>> y.wt.SeeFormat()
%.2f

This works, but having both SetDigits and InstanceSetDigits is
redundant code and makes for a confusing API. Is there any way I can
define ONE method that works like a class method when called from the
base class object and like an instance method when called from an
instance?

Sure.
Classes deriving from object can completely customize
the binding process by implementing a __get__ method:

class _BoundMethod:
# Helper class.
def __init__(self, func, first):
self.func = func
self.first = first

def __call__(self, *args):
return self.func(self.first, *args)

class unimethod(object):
# universal method: binds to either a class or an instance
def __init__(self, func):
self.func = func

def __get__(self, inst, type=None):
if inst is None:
# bind to the class
return _BoundMethod(self.func, type)
else:
# bind to the instance
return _BoundMethod(self.func, inst)

Then you can do:

class Decimal:
...
def SetDigits(cls_or_inst, ...):
....

SetDigits = unimethod(SetDigits)

-----

Thomas

Thank you for this answer. It is *exactly* what I was looking for. I
had previously studied PEP 252 and other documentation related to it,
but somehow I don't think I ever would have come up with this solution
on my own. Now that I have seen your working example, however, I
understand better how descriptors work, and I thank you very much.

Donnal

Although, unfortunately, still some important stuff is missing,
see the 'Additional Topics' section at the end.

Thomas