[FIXED] creating a partial-like object with dynamic arguments

Issue

I'm trying to create a partial function but with dynamic arguments that are stored as class attributes and changed accordingly. Something like the following code:

from functools import partial

def foo(*args, msg):
    print(msg)

class Bar:
    def __init__(self, msg):
        self.msg = msg
        self.functions = dict()
        self.functions['foo'] = partial(foo, msg=self.msg)

    def foo_method(self, *args):
        return self.functions['foo'](*args)

b =Bar('1')
b.foo_method()
b.msg = '2'
b.foo_method()

Only, of course, both statements will print '1' as the partial object fixes the arguments. The only alternative I found was changing the attribute to a property and manually changing the partial attributes with the setter:

class Bar:
    def __init__(self, msg):
        self._msg = None
        self.functions = dict()
        self.functions['foo'] = partial(foo)
        self.msg = msg

    def foo_method(self, *args):
        return self.functions['foo'](*args)

    @property
    def msg(self):
        return self._msg

    @msg.setter
    def msg(self, msg):
        self._msg = msg
        self.functions['foo'].keywords['msg'] = msg

I would like to know if there is a more "pythonic" / efficient way to do this, since I really don't need to use properties except for this workaround.

Solution

You can use lambda instead of partial for deferred (or often referred to as "lazy") evaluation of the arguments, so that self.msg is not evaluated until the function is called:

class Bar:
    def __init__(self, msg):
        self.msg = msg
        self.functions = dict()
        self.functions['foo'] = lambda *args: foo(*args, msg=self.msg)

    def foo_method(self, *args):
        return self.functions['foo'](*args)

Answered By - blhsing