Fix two S3 bugs

[mpenkov]

• Fix transport_params: default value for keyword argument is mutable #304
• Fix Custom S3 host is ignored when using the url format. #305

[luckydonald]

@mpenkov Hey, any feedback to the questions above?

[mpenkov]

Which questions are you referring to? I don't see any in this PR

[luckydonald]

I asked here (It says pending, so maybe that didn't go through):

    #
    # If the default region is not us-east-1, we need to construct our own
    # session.  This is because smart_open will create a session in the default
    # region, which _must_ be us-east-1 for minio to work.

Why is that? Is this specific to minio, or all custom S3 implementations?
Shouldn't that be automatic?

Needing to set that manually feels like it could cause bugs very easily, later on.
My assumption would be it'll be smart and "just work". Maybe smart_open can set the session accordingly?

[mpenkov]

I think it's a minio thing, but I don't use it, so my knowledge is limited.

I don't think it's worth including such details in smart_open library for several reasons:

• Minio is a nonstandard S3 implementation (it's an edge case)
• it's already possible to set and override the default S3 region via boto3 mechanisms
• In the worst case, it's possible to override the region via transport_params

[luckydonald]

How did you stumbled upon the fact that it needs to be us-east-1?
That's new to me, I didn't even know that, as it just worked^_TM for me.
Does that also apply to other S3 vendors out there, like wasabi etc?

[mpenkov]

Trial and error. That's another reason why I'm not confident about including it in the library.

No, I haven't tried other S3 vendors.