python - condense pyqtproperties

Question

I'm writing a script in Python and have a bit of a problem:

class LightDMUser(QObject):
  def __init__(self, user):
    super(LightDMUser, self).__init__()
    self.user = user

  @pyqtProperty(QVariant)
  def background(self):      return self.user.get_background()

  @pyqtProperty(QVariant)
  def display_name(self):    return self.user.get_display_name()

  @pyqtProperty(QVariant)
  def has_messages(self):    return self.user.get_has_messages()

  @pyqtProperty(QVariant)
  def home_directory(self):  return self.user.get_home_directory()

  @pyqtProperty(QVariant)
  def image(self):           return self.user.get_image()

  @pyqtProperty(QVariant)
  def language(self):        return self.user.get_language()

  @pyqtProperty(QVariant)
  def layout(self):          return self.user.get_layout()

  @pyqtProperty(QVariant)
  def layouts(self):         return self.user.get_layouts()

  @pyqtProperty(QVariant)
  def logged_in(self):       return self.user.get_logged_in()

  @pyqtProperty(QVariant)
  def name(self):            return self.user.get_name()

  @pyqtProperty(QVariant)
  def real_name(self):       return self.user.get_real_name()

  @pyqtProperty(QVariant)
  def session(self):         return self.user.get_session()

As you can see, this code is horribly redundant. I tried condensing it like this:

class LightDMUser(QObject):
  attributes = ['background', 'display_name', 'has_messages', 'home_directory', 'image', 'language', 'layout', 'layouts', 'logged_in', 'name', 'real_name', 'session']

  def __init__(self, user):
    super(LightDMUser, self).__init__()
    self.user = user

    for attribute in self.attributes:
      setattr(self, attribute, pyqtProperty(QVariant, getattr(self.user, 'get_' + attribute)))

PyQt4, however, expects the class methods to be present for the class itself, not an instance. Moving the setattr code out of the __init__ block didn't work either because self wasn't defined for the class, so I don't really know what to do.

Can anyone see a way to condense this code?

Answer 1

There are number of ways to do it: class decorator, metaclass, Mixin.

Common helper function:

def set_pyqtproperties(klass, properties, proxy='user'):
    def make_prop(prop):        
        def property_(self):
            return getattr(getattr(self, proxy), 'get_' + prop)
        property_.__name__ = prop
        return property_

    if isinstance(properties, basestring):
       properties = properties.split()
    for prop in properties:
         setattr(klass, prop, pyqtProperty(QVariant, make_prop(prop)))

Class decorator

def set_properties(properties):
    def decorator(klass):
        set_pyqtproperties(klass, properties)
        return klass
    return decorator

Usage

@set_properties("display background")
class LightDMUser(QObject): pass

if there is no support for class decorators then you could try:

class LightDMUser(QObject): 
    pass
LightDMUser = set_properties("display background")(LightDMUser)

Metaclass

def set_properties_meta(properties):
    def meta(name, bases, attrs):
        cls = type(name, bases, attrs)
        set_pyqtproperties(cls, properties)
        return cls
    return meta

Usage

class LightDMUser(QObject):
    __metaclass__ =  set_properties_meta("display background")

Note: you could reuse the same metaclass if you set the list of properties as a class attribute:

def MetaClass(name, bases, attrs):
    cls = type(name, bases, attrs)
    set_pyqtproperties(cls, attrs.get('properties', ''))
    return cls

class LightDMUser(QObject):
    properties = "display background"
    __metaclass__ = MetaClass

Also you could manipulate attrs directly: attrs[name] = value before calling type() instead of setattr(cls, name, value).

The above assumes that QObject.__class__ is type.

Mixin

def properties_mixin(classname, properties):
    #note: create a new class by whatever means necessary
    # e.g., even using exec() as namedtuple does
    # http://hg.python.org/cpython/file/3.2/Lib/collections.py#l235

    # reuse class decorator here
    return set_properties(properties)(type(classname, (), {}))

Usage

PropertiesMixin = properties_mixin('PropertiesMixin', 'display background')
class LightDMUser(PropertiesMixin, QObject): pass

I haven't tried any of it. The code is here to show the amount and the kind of code it might require to implement the feature.