Suggeted - allowing cache mechanism for files

[guruyaya]

Getting files from remote servers, can take quite some time. It may be benifitial to add an option to hold cache for remote requests. I'd suggest adding a cache_file and/or cache_dir to a smart_open, whether it's write or read. If it's a write. cache_file param will save the smart open locally with the file. cache_dir will use the sha256 of the resource as a name of the locally saved file. By default, if writing fails, it should just send a warning. It should also contain a cleanup date.
Writing a file, will be applied to both the remote and local resource. If the file exists, it will be deleted, to avoid permission issues. Reading a file will first look in the cache folder. If found, it will look at the creation date, and if it's too old, it will be deleted, and replaced with the remote file.

[stuaxo]

Some kinds of remote servers (such as s3) will support ETAG, it would make sense to store the ETAG too.

You can just send an http HEAD request to get info such as the etag.