Issue 249 postponed configuration

[artslob]

Closes #249

[vmarkovtsev]

This is a big change that touches some of the most critical code and complicates the object lifecycle considerably. I am not even sure we want the deferred configuration in the first place.

[artslob]

Yeah, PR and issue are open for discussion to determine whether this PR will be useful. I just think that sometimes its not convenient nor beautiful to parse config at import time. I prefer factory pattern so made this PR.

[rafalp]

IMHO there's problem with our API design, but band-aiding it will not help, only make the problem harder to deal with later.

Core object wraps the process of strategy creation, but there's no way to extend this process without modifying internals themselves. This bites not only people who need to configure backend-specific extra options not supported by the core, but also those interested in developing support for other databases.

[artslob]

Can I do something to improve this PR? Or should I close it then?

[artslob]

Maybe create some proxy/wrapper around Database class? This way Database would not be changed:

class DatabaseWrapper:
    def __init__(self):
        self.db = None

    def configure(self, **all_rapams):
        self.db = Database(**all_rapams)

    def check_configured(self):
        if not self.db:
            raise ValueError

    def connect(self):
        # proxies all methods to inner object
        self.check_configured()
        return self.db.connect()


class Database:
    # this class would not be changed
    pass


db = DatabaseWrapper()


def main():
    params = parse_env_params()
    db.configure(**params)
    connect = db.connect()