Usage

metaframe allows placing hooks into the creation/initializaion of objects, enabling use cases like:

• Modification of args/kwargs on the fly
• Instance scanning/modification

Direct Inheritance

The package offers an already metaclassed base class supporting the infrastructure.

• MetaFrameBase

Intercepting Object Creation

An example from one of the tests included in the sources.

import metaframe as mf


class FrameTest(mf.MetaFrameBase):
    _KEY = 'ft'
    _VAL = True

    def __init__(self, *args, **kwargs):
        self._val = kwargs.get(self._KEY, False)

    def check_val(self):
        return self._val == self._VAL

    @classmethod
    def _new_pre(cls, *args, **kwargs):
        # Insert a kwarg
        kwargs[cls._KEY] = cls._VAL
        return cls, args, kwargs

Doing something with it:

ft = FrameTest()
print('ft.check_val:', ft.check_val())

Yields the following output:

ft.check_val: True

From the example:

• No kwargs were passed to FrameTest for instantiation

• During init self._KEY (‘ft’) was extracted from kwargs and assigned to self._val

• The kwargs were actually modified in the classmethod() where self._VAL was added with key ``self._KEY’‘

  And the modified kwargs were returned to be fed to object creation/initialization

• Hence check_val() returning True

Before initialization

The previous example can be extended to undo the effect achieved during object creation.

Let’s add a hook before init

    @classmethod
    def _init_pre(cls, obj, *args, **kwargs):
        # Remove the kwarg
        kwargs.pop(cls._KEY)
        return obj, args, kwargs

Doing something with it:

ft = FrameTest()
print('ft.check_val:', ft.check_val())

Yields the following output:

ft.check_val: False

The new code in _init_pre() removes the key self._KEY from the passed kwargs and returns them for object initialization.

After initialization

Redoing the effect by directly operating on the instance can be done after initialization.

The hook after __init__

    @classmethod
    def _init_post(cls, obj, *args, **kwargs):
        # change self._val ... to the expected value
        obj._val = obj._VAL
        return obj, args, kwargs

Repeating execution:

ft = FrameTest()
print('ft.check_val:', ft.check_val())

Yields the following output:

ft.check_val: True

In this case the post initialization hook has directly changed the value of attribute _val after object init.

Applying the metaclass

Instead of inheriting from MetaFrameBase a derived metaclass for your class can be created:

import metaframe as mf

class MyMetaClass(mf.MetaFrame):
    def _new_pre(cls, *args, **kwargs):
        # Insert a kwarg
        kwargs[cls._KEY] = cls._VAL
        return cls, args, kwargs

    def _init_pre(cls, obj, *args, **kwargs):
        # Remove the kwarg
        kwargs.pop(cls._KEY)
        return obj, args, kwargs

    def _init_post(cls, obj, *args, **kwargs):
        # change self._val ... to the expected value
        obj._val = obj._VAL
        return obj, args, kwargs

Now there is no need to declare the 3 hoods as classmethods because they are already being declared in the MetaClass.

The FrameTest class would now look like this:

class FrameTest(MyMetaClass.as_metaclass(object)):
    _KEY = 'ft'
    _VAL = True

    def __init__(self, *args, **kwargs):
        self._val = kwargs.get(self._KEY, False)

    def check_val(self):
        return self._val == self._VAL

The execution examples remain unchanged.

Alternatively, you can directly MetaFrame-enable a class applying MetaFrame as metaclass and defining the methods in the class as @classmethods:

class FrameTest(mf.MetaFrame.as_metaclass(object)):
    _KEY = 'ft'
    _VAL = True

  @classmethod
  def _new_pre(cls, *args, **kwargs):
      # Insert a kwarg
      kwargs[cls._KEY] = cls._VAL
      return cls, args, kwargs

  @classmethod
  def _init_pre(cls, obj, *args, **kwargs):
      # Remove the kwarg
      kwargs.pop(cls._KEY)
      return obj, args, kwargs

  @classmethod
  def _init_post(cls, obj, *args, **kwargs):
      # change self._val ... to the expected value
      obj._val = obj._VAL
      return obj, args, kwargs

    def __init__(self, *args, **kwargs):
        self._val = kwargs.get(self._KEY, False)

    def check_val(self):
        return self._val == self._VAL