Metaclass Recipe: Generating Bitfield Properties

Step 1

Finish implementing the inner function getproperty so that we can use bitprop

class __bitproperties__(type):
    def __new__(meta, classname, bases, classDict):
        for name in classDict['bit_properties']:
            def getproperty(enum):
                # Change the code here
                return property(lambda self: None,
                                lambda self, value: None)

            enum = eval(name.upper())
            classDict[name] = getproperty(enum)

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

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

class Widget(object):
    __metaclass__ = __bitproperties__
    style = 0
    bit_properties = ['enabled', 'simple', 'sunken', 'raised', 'transparent']

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

w.sunken = True
print w.style               # 5

w.transparent = True
print w.style               # 21

w.sunken = False
print w.style               # 17

Expected output:

1
5
21
17

Hints

1. The getter method for the property looks like this:

   def get(self):
       return bool(self.style & enum)
   

2. The setter method for the property looks like this:

   def set(self, v):
       if v:
           self.style = self.style | enum
       else:
           self.style = self.style ^ enum
   

3. Inside of getproperty, you need to invoke the built-in function property.

Solution: solutions/bitfield1.py

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