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Is it possible to emulate Scala features in Python?

I want to create lightweight interfaces using methods that I can connect to classes. Here is a quick example in Scala:

class DB {
  def find(id: String) = ...
}

trait Transformation extends DB {
  def transform(obj: String): String

  override def find(id: String) =
    transform(super.find(id))
}

trait Cache extends DB {
  val cache = Cache()
  override def find(id: String) = {
    ...
    if (cache.contains(id))
       cache.find(id)
    else {
       cache.set(id, super.find(id))
       cache.get(id)
    }
  }   
}

Using these classes (attributes), we can instantiate DB classes using Transformation, Cache, or both. Note that Transformation has an abstract method transformation that still needs to be implemented in specific classes.

new DB() with Transformation {
  def transform(obj: String): obj.toLower()
}
new DB() with Cache
new DB() with Transformation with Cache {
  def transform(obj: String): obj.toLower()
}

Is there a way to achieve something like this in Python? I know there is a Traits package for Python, but its purpose seems to be different.

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4 answers

The simplest solution is probably just to create another subclass.

# assuming sensible bases:
class DB(object):
    ...

class Transformation(object):
    def transform(self, obj):
        ...

    def get(self, id):
        return self.transform(super(Transformation, self).get(id))

class Cache(object):
    def __init__(self, *args, **kwargs):
        self.cache = Cache()
        super(Cache, self).__init__(*args, **kwargs)
    def get(self, id):
        if id in self.cache:
            return self.cache.get(id)
        else:
            self.cache.set(id, super(Cache, self).get(id))
            return self.cache.get(id)

class DBwithTransformation(Transformation, DB):
    # empty body
    pass

, type .

class DBwithTransformation(Transformation, DB):
    pass

db = DBwithTransformation(arg1, arg2, ...)


db = type("DB", (Transformation, DB), {})(arg1, arg2, ...)

, Scala.

- python , (DB). , mixin .

. ; , , ( , object).

+8

Scala . abc:

import abc

class Transformation(DB):
     __metaclass__ = abc.ABCMeta

     @abc.abstractmethod
     def transform(self, obj):
         pass

     def find(self, id):
         return self.transform(super(Transformation, self).get(id))

Transformation .

, abc, NotImplementedError , . ABCMeta .

PS: Python 3 super ( !).

+3

. "" , abuse().

http://twitter.imtqy.com/scala_school/basics.html#trait

Car = type('Car', (object,), {'brand': ''})
Shiny = type('Shiny', (object,), {'refraction': 0})

BMW = type('BMW', (Car, Shiny,), {'brand': 'BMW', 'refraction': 100})
my_bmw = BMW()

print my_bmw.brand, my_bmw.refraction

BMW,

def bmw_init(self, refraction):
    self.refraction = refraction

BMW = type('BMW', (Car, Shiny,), {'brand': 'BMW', '__init__': bmw_init})
c1, c2 = BMW(10), BMW(100)
print c1.refraction, c2.refraction
0

Python Traits Enthought. , , , Python.

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