Emulating Object Oriented Programming with dict and function.

emulating_oop.py

"""Emulating Object Oriented Programming with dict and function.

### 0. reference
1) This code was written by refering below content.
   Python のクラスシステム - http://www.shido.info/py/python7.html

### 1. emulating 3 concepts, class, instance and method.
1) class           -> dict
2) instance        -> dict
3) method          -> function

### 2. emulating 3 built-in functions.
1) setattr         -> setattr_
2) getattr         -> getattr_
3) type            -> type_
https://docs.python.jp/3/library/functions.html#type
https://docs.python.jp/3/library/functions.html#setattr
https://docs.python.jp/3/library/functions.html#getattr

### 3. creating 1 new function for convinience.
1) instantiate      -> instantiate_

### 4. consideration
The complexity of getattr_() function in this code shows
that attribue reference is one of the most important factor
in object oriented programming language.
"""


#
#
#
def main():
    print('### 0. You have got two same results.')
    print()

    print('### 1. instance object')
    print('          executing with builtin functions')
    sample_code(setattr, getattr, type, instantiate)
    print()

    print('### 2. emulating dict object')
    print('          executing with emulating functions')
    sample_code(setattr_, getattr_, type_, instantiate_)
    print()


#
#
#
def sample_code(setattr, getattr, type, instantiate):
    #
    # 1. Define class.
    #
    def __init__(self, name, task):
        setattr(self, 'name', name)
        setattr(self, 'task', task)

    def work(self, hour):
        name = getattr(self, 'name')
        task = getattr(self, 'task')
        max_hour = getattr(self, 'max_hour')
        return '%s %s for %d hours of %d hours.'\
            % (name, task, hour, max_hour)

    ItWorker = type(
        'ItWorker',
        (object, ),
        {'max_hour': 7, '__init__': __init__, 'work': work}
    )

    #
    # 2. Instantiate object.
    #
    henley = instantiate(ItWorker, 'Henley', 'makes web site')
    thomas = instantiate(ItWorker, 'Thomas', task='checks up server')
    gates = instantiate(ItWorker, name='Gates', task='writes programs')

    #
    # 3.
    #

    print('# (1) Refer instance attributes.')
    print(getattr(henley, 'max_hour'))
    print(getattr(thomas, 'name'))
    print(getattr(gates, 'task'))

    print('# (2) Overwrite instance attributes.')
    setattr(henley, 'max_hour', 12)
    setattr(thomas, 'name', 'James')
    setattr(gates, 'task', 'desings web page')
    print(getattr(henley, 'max_hour'))
    print(getattr(thomas, 'name'))
    print(getattr(gates, 'task'))

    print('# (3) Refer class attributes.')
    # print(getattr(henley, 'max_hour'))  # class attribute was overwritten.
    print(getattr(thomas, 'max_hour'))
    print(getattr(gates, 'max_hour'))

    print('# (4) Call methods.')
    print(getattr(henley, 'work')(3))
    print(getattr(thomas, 'work')(4))
    print(getattr(gates, 'work')(6))

    print('# (5) Change methods dynamically.')

    def holiday(self, days):
        name = getattr(self, 'name')
        return '%s enjoys vacation for %d days' % (name, days)
    setattr(ItWorker, 'work', holiday)

    print(getattr(henley, 'work')(3))
    print(getattr(thomas, 'work')(4))
    print(getattr(gates, 'work')(6))


def instantiate(cls, *args, **kwargs):
    self = cls(*args, **kwargs)
    return self


#
# Using dict and function(closure),
#   we can emulate instance object, class object and method.
#
def setattr_(obj: dict, attr_name, value):
    """Set a value to the attribute."""
    obj[attr_name] = value


def getattr_(obj: dict, attr_name):
    """Get a value from the attribute."""
    # instance attribute
    if attr_name in obj:
        value = obj[attr_name]
        return value
    # class attribute
    elif attr_name in obj['__class__']:
        value = obj['__class__'][attr_name]
        # value
        if not callable(value):
            return value
        # method
        else:
            def method(*args, **kwargs):
                return value(obj, *args, **kwargs)
            return method
    else:
        raise AttributeError("object has no attribute '%s'" % attr_name)


def type_(name: str, bases: tuple, class_name_space: dict) -> dict:
    """Create a new class object."""
    cls = {}
    cls['__name__'] = name
    cls['__bases__'] = bases  # inheritance is not implemented.
    cls.update(class_name_space)
    return cls


def instantiate_(cls: dict, *args, **kwargs) -> dict:
    """Create a new instance object."""
    self = {}
    self['__class__'] = cls
    getattr_(self, '__init__')(*args, **kwargs)
    return self


#
#
#
if __name__ == '__main__':
    main()