Don't import all keyrings unless necessary

[jyn514]

If a keyring is set in configuration or environment variables,
use that one and don't load all available keyrings.

Partially addresses #403.

[jyn514]

@mitya57 this has been sitting around for a while, are there any changes I should make?

[mitya57]

Sorry for the delay. Thinking more about this, filter() is lazy in Python 3, so it does not actually do any iteration when you assign its result to a variable.

Maybe you are still using Python 2? We no longer support it…

[jyn514]

Filter is lazy, but get_all_keyrings() is eager and is called before it is passed to filter. I am using Python3, I can show you the time speedups of you want.

[jyn514]

If you want to keep the existing code structure, maybe get_all_keyrings could be turned into a generator so it is evaluated lazily.

[mitya57]

That makes sense. Thank you! Maybe in future we should make get_all_keyring lazier too (and add the missing s to its name).