resolves #27 port field in creds object

[wahani]

• resolves Bug: specify the port in a creds object #27 by deprecating the creds argument and using dicts as db_url
• addresses Passing SSL certificates to mysql and postgresql #12 by allowing the credentials dict to specify further arguments passed to connect_args

Notes

• I had to refactor the sub-class (mysql) quite a bit, so that I can take advantage of this new logic. I think this makes it a bit less complicated.
• I noticed that we can remove the connection_class argument from the Database class, and instead do something like:

class Database:
    connection_class = DefaultConnection
    def __init__(self, ...):
        ...

This still allows us to override the Connection class in any derived sub-class and brings two advantages:

• (a): we can remove it from the public interface. The Database class is for the user and the argument does not make sense. A user is not supposed to even now about the Connection class; that's an implementation detail.
• (b): when we derive the subclass, we do not have to override the complete init method, and also do not need to keep track of upstream changes. Currently a new argument to init forces us to modify all derived sub-classes which override init. Yes, currently only one, but I have made several errors due to the suptle change in the call to the super init method already. My impression is, that defining that global constant may be more transparent.

[wahani]

@Cattes I added you as a reviewer to get some feedback if the handling of SSL certificates would work for you in this version. Also see my comment here: #12 (comment)

[Cattes]

I am not so familiar with dict.pop and argument passing but it looks ok as far as I can see.
Do I understand the new test fixture correctly, that you define a dict with an addtional argument local_infile = 0 which is passed on to the underlying db driver? And this results in a test failure when you try to send data not coming from a file? Is local_infile an argument of the pymysql driver?

[wahani]

With this test, I only check that additional fields in the credentials are properly handed over as connection_args to sqlalchemy.create_engine. local_infile is set to 1 by default for dbrequests.mysql, and I simply check, that I can override it. Instead of your proposition in #12, you may now simply add ssl to the creds object, and it should be used. Along with all additional parameters optionally passed down with connect_args.

[phainom]

LGTM.

[wahani]

@phainom from #34 (comment) do you have an opinion about:

I noticed that we can remove the connection_class argument from the Database class, and instead do something like:

[phainom]

@phainom from #34 (comment) do you have an opinion about:

I noticed that we can remove the connection_class argument from the Database class, and instead do something like:

Sorry, overlooked it. In the subclass you would override the constant with a global keyword?

I agree the current implementation complicates things, if there is a better solution and there is no namespace confusion I am all for it.

[wahani]

In the subclass you would override the constant with a global keyword?

Yes, in the sub-class you can simply override the constant with some import. I will test if it works and solve it directly together with the other issues in this PR.