Created by: reyoung

This issue is a part of issue #1392 (closed) job and it is a design documentation of Fetcher.

The Cached Fetcher is a python package, which can get a dataset from an HTTP server, and open any file inside the dataset as a Python file-like object.

The basically interface of fetcher is

class Fetcher(object):
    def __init__(self, url,  md5=None, local_cached_dir=None):
        """
        :param url: url is the http address of this dataset package.
        :param md5: md5sum of this package. None if do not check md5 sum.
        :param local_cached_dir: the local directory which saved caches. if set 
        to None, then will pick a directory in `$HOME/.cache/cached_downloader/`
        base on url or filename.
        """
        pass

    def open(self, path=None):
        """
        Open a path inside the downloaded file.

        If the downloaded file is a tar package, it will open the `path` file 
        inside this package. If the downloaded file is a simple plain file like
        `XXX.json`, or a gzipped file like `XXX.json.gz`.

        The basic usage is\:

        ..  code-block:: python

            downloader = Fetcher(url="http://yann.lecun.com/exdb/mnist/train-images-idx3-ubyte.gz")
            with downloader.open() as train_images_file:
                ...
        """
        pass