Possible to assign a class attribute that's a simple function python

Question

I'd like to hold reference to callback functions in a singleton. However, python does some magic (either at the point of assignment or at the point of calling), that means the function requires an instance object as the first parameter -- I want to be able to store a simple function. How can I do this?

Assigning to X.__dict__ has the same effect.

def default(): print "default"

class X:
    func = None

    @staticmethod
    def setup(f=default):
        X.func = f

    @staticmethod
    def doit():
        if X.func is None: X.setup()
        # !!!!!!!
        # TypeError: unbound method g() must be called with X instance as first argument (got nothing instead)
        X.func()


def g(): print "g"
X.setup(g)
X.doit()

Why does X exist at all? Why did you define a class for this, especially if you're not going to make instances of the class? — user2357112
– user2357112, Commented Jan 25, 2018 at 18:47
Possible duplicate of storing unbound python functions in a class object — internet_user
– internet_user, Commented Jan 25, 2018 at 18:49
And why are you using an old-style class? More to the point, why are you using Python 2? — PM 2Ring
– PM 2Ring, Commented Jan 25, 2018 at 18:52

Pobe · Accepted Answer · 2018-01-25 18:59:09Z

2

First,

If you do :

X.func() after X.setup(). It won't work. The problem is with the setup function.

The first problem is your f=default. This syntax does not work in python. You cannot assign a variable to a default parameter.

Concerning your problem itself, what happen is that your class X does not "bound" with function g (or default).

You need

def default(): print "default"

class X:
    func = None

    @staticmethod
    def setup(f):
        if f is None:
            X.func = staticmethod(default)
        else:
            X.func = staticmethod(f)

    @staticmethod
    def doit():
        if X.func is None: X.setup()
        X.func()

def g(): print "g"
X.setup(g)
X.doit()

edit: Use python 3 please

answered Jan 25, 2018 at 18:59

Pobe

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