How to get static function fields in Python class

Question

I have a static class with a list of "colors" defined as fields. But each color is actually a time dependent function:

class Colors(object):
    @staticmethod
    def RED(t):
        return [255 * t, 0, 0]

Then, I have another class that will have these functions as fields:

class PathMaker(object):
    SUCCESS_COLOR = Colors.BLUE
    FAIL_COLOR = Colors.RED

But now, PathMaker.FAIL_COLOR is an unbound method, instead of a reference to Colors.RED.

Is there a better way of doing this? I could probably make SUCCESS_COLOR and FAIL_COLOR static methods, but that seems like a bad way of doing it.

you could switch to Python 3, which doesn't have the same method wrapping weirdness of 2.7. — Kevin
– Kevin, Commented Jul 10, 2014 at 17:51
Why do you think making FAIL_COLOR a staticmethod is a bad way? — BrenBarn
– BrenBarn, Commented Jul 10, 2014 at 17:55
General design advice: When something looks like a constant, it should be a constant. If it's not, don't make it look like one. — user3820547
– user3820547, Commented Jul 10, 2014 at 17:58
@Kevin: I'm not sure that will actually change anything here. — BrenBarn
– BrenBarn, Commented Jul 10, 2014 at 18:04
@BrenBarn, from my experiments, it seemed to make a difference: this Python 2 code gives a TypeError, but the same code in Python 3 produces the desired output. — Kevin
– Kevin, Commented Jul 10, 2014 at 18:06

BrenBarn · Accepted Answer · 2014-07-10 17:55:24Z

3

Wrapping the reference with staticmethod works:

class Colors(object):
    @staticmethod
    def RED(t):
        return [255 * t, 0, 0]

class PathMaker(object):
    FAIL_COLOR = staticmethod(Colors.RED)

>>> PathMaker.FAIL_COLOR(1)
[255, 0, 0]
>>> PathMaker.FAIL_COLOR(2)
[510, 0, 0]

The reason is that the staticmethod descriptor causes accessing Colors.RED to "unwrap" the staticmethod and return the underlying "raw" function (not a method). Without using staticmethod in PathMaker, PathMaker rewraps this function into its own method, just as it would if you defined a function directly in the class body. By using staticmethod(Colors.RED), you can rewrap the unwrapped function into a staticmethod.

answered Jul 10, 2014 at 17:55

BrenBarn

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