Instance factory - am I doing this right?

[Laszlo Nagy]

This is just an interesting code pattern that I have recently used:

class CacheStorage(object):
"""Generic cache storage class."""
@classmethod
def get_factory(cls,*args,**kwargs):
"""Create factory for a given set of cache storage creation
parameters."""
class CacheStorageFactory(cls):
_construct_args = args
_construct_kwargs = kwargs
def __init__(self):
cls.__init__(self,
*self._construct_args,**self._construct_kwargs)
return CacheStorageFactory

Then, I can create subclasses like:

class GdbmCacheStorage(CacheStorage):
"""Gdbm cache storage class.

@param basedir: Base directory where gdbm files should be stored.
@param basename: Base name for logging and creating gdbm files.
"""
def __init__(self,basedir,basename):
..... blablabla place initialization code here

class MemoryCacheStorage(CacheStorage):
"""In-Memory cache storage class.

Please note that keys and values are always mashal-ed.
E.g. when you cache an object, it makes a deep copy.
"""
def __init__(self):
..... blablabla place initialization code here


And the finally, I can create a factory that can create cache storage
instances for storing data in gdbm in a given directory:

cache_factory = GdbmCacheStorage.get_factory("~gandalf/db","test")
print cache_factory # <class '__main__.CacheStorageFactory'>
print cache_factory()

OR I can create a factory that can create instances for storing data in
memory:


cache_factory = MemoryCacheStorage.get_factory()
print cache_factory # <class '__main__.CacheStorageFactory'>
print cache_factory() # <__main__.CacheStorageFactory object at 0x8250c6c>


Now, here is my question. Am I right in doing this? Or are there better
language tools to be used in Python for the same thing? This whole thing
about creating factories looks a bit odd for me. Is it Pythonic enough?

Thanks,

Laszlo

 

[eb303]

This is just an interesting code pattern that I have recently used:

class CacheStorage(object):
    """Generic cache storage class."""
    @classmethod
    def get_factory(cls,*args,**kwargs):
        """Create factory for a given set of cache storage creation
parameters."""
        class CacheStorageFactory(cls):
            _construct_args = args
            _construct_kwargs = kwargs
            def __init__(self):
                cls.__init__(self,
                    *self._construct_args,**self._construct_kwargs)
        return CacheStorageFactory

Then, I can create subclasses like:

class GdbmCacheStorage(CacheStorage):
    """Gdbm cache storage class.

    @param basedir: Base directory where gdbm files should be stored.
    @param basename: Base name for logging and creating gdbm files.
    """
    def __init__(self,basedir,basename):
          ..... blablabla place initialization code here

class MemoryCacheStorage(CacheStorage):
    """In-Memory cache storage class.

    Please note that keys and values are always mashal-ed.
    E.g. when you cache an object, it makes a deep copy.
    """
    def __init__(self):
          ..... blablabla place initialization code here

And the finally, I can create a factory that can create cache storage
instances for storing data in gdbm in a given directory:

cache_factory = GdbmCacheStorage.get_factory("~gandalf/db","test")
print cache_factory # <class '__main__.CacheStorageFactory'>
print cache_factory()

OR I can create a factory that can create instances for storing data in
memory:

cache_factory = MemoryCacheStorage.get_factory()
print cache_factory # <class '__main__.CacheStorageFactory'>
print cache_factory() # <__main__.CacheStorageFactory object at 0x8250c6c>

Now, here is my question. Am I right in doing this? Or are there better
language tools to be used in Python for the same thing? This whole thing
about creating factories looks a bit odd for me. Is it Pythonic enough?

Thanks,

   Laszlo

Seems you try to reinvent functools:

class GdbmCacheStorage(object):
def __init__(self,basedir,basename):
...
cache_factory = functools.partial(GdbmCacheStorage, "~gandalf/db",
"test")
print cache_factory()

Is it what you're after? I didn't see the point in creating a "cached
factory" for MemoryCacheStorage though, since it has no constructor
parameters to cache anyway. So doing 'cache_factory =
MemoryCacheStorage' in your example would do exactly the same thing as
what you did.

HTH
- Eric -