Metaclass Recipe: Generating Bitfield Properties

Step 2

It would be nice if we had more control over the names of the properties being generated. One way to allow would be to let you specify the properties using attributes:

class Widget(object):
    __metaclass__ = __bitproperties__
    style = 0

    enabled       = bit_property(ENABLED)
    simple_border = bit_property(SIMPLE)
    sunken_border = bit_property(SUNKEN)
    raised_border = bit_property(RAISED)
    transparent_background = bit_property(TRANSPARENT)

Implement the bit_property class and finish implementing the __new__ magic method.

class bit_property(object):
    """define this class"""

class __bitproperties__(type):
    def __new__(meta, classname, bases, classDict):
        bitprops = (item for item in classDict.items()
                    if isinstance(item[1], bit_property))

        # Modify the classDict object here

        return type.__new__(meta, classname, bases, classDict)

ENABLED, SIMPLE, SUNKEN, RAISED, TRANSPARENT = 1, 2, 4, 8, 16

class Widget(object):
    __metaclass__ = __bitproperties__
    style = 0

    enabled       = bit_property(ENABLED)
    simple_border = bit_property(SIMPLE)
    sunken_border = bit_property(SUNKEN)
    raised_border = bit_property(RAISED)
    transparent_background = bit_property(TRANSPARENT)

w = Widget()
w.enabled = True
print w.style               # 1

w.sunken_border = True
print w.style               # 5

w.transparent_background = True
print w.style               # 21

w.sunken_border = False
print w.style               # 17

Expected output:

1
5
21
17

Hints

1. The definition of bit_property is pretty simple; all you need to provide is the __init__ magic method.

2. You can reuse the getproperty function from the previous step.

3. One of the lines in your solution will likely be:

   classDict[name] = getproperty(bitprop.enum)
   

Solution: solutions/bitfield2.py

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