API server code health improvements

[melange396]

The covidcast API server code works well, but it can be improved with an eye toward extensibility and other future changes. A number of points of potential refinement exist, including:

• Long and/or complex ternaries and comprehensions are pervasive and should be reduced and/or simplified -- code golf is not good for readability/maintainability
• Many class, method, and variable names are not descriptive or are misleading, and should be clarified
• Documentation in the code for explaining algorithms and implementation choices is very thin and should be expanded. Undocumented code understandability is further exacerbated by naming choices.
• Type annotation/hinting is clunky and verbose, and often not necessary.

[melange396]

here are some more concrete examples:

• flask.request and request argument value handling:

  • flask.request should not be referenced outside of endpoint handlers
  • (or other event receivers, like those methods decorated by before_request or teardown_appcontext, etc)
  • request makes sense in the endpoint context, but not really beyond that...
  • this means things in _params.py like parse_source_signal_pairs() and parse_time_pairs() and etc should take an argument instead of pulling request out of thin air
    • we should them into more pure functions, so that it is easier to know when an argument that is part of a web/api request is being accessed
  • these methods that retrieve such request values are inconsistently named and located
  • methods currently called parse_blah() do more than just parse, they also 'get' and 'validate' or even 'enforce'
  • there are parse_x() methods defined in src/server/endpoints/covidcast.py that should be in with the rest of the parse_* methods in _params.py
  • there are extract_x() methods in _validate.py that have near identical behaviors and should be moved into the same file as the other similar methods (and renamed)
  • im sure there are more that i cannot currently find / remember

• _handle_lag_issues_as_of() in src/server/endpoints/covidcast.py:

  • called thrice (only in the same file) with all "None" arguments, which is a complete no-op. a call is also commented out once. these can all be removed.
  • called once with all 3 arguments, and called once with just the as_of argument (which is arguably the most complicated thing this does)
  • argument order does not even match the name!
  • args are mutexed! at least evaluation matches arg order (but obviously not name order)
  • if a lag and an issue are specified, the lag is not added to the WHERE (its a strange use case, but args should be respected or an error should be produced)
  • if you specify a lag or issue, as_of is invalidated (again, possibly strange usage, but inconsistent none the less)

• set_order() in src/server/_query.py:

  • needlessly overcomplicated, and DESCending is never even used in the codebase

• get_related_signals() in src/server/endpoints/covidcast_utils/model.py is never used

• src/server/_db.py is pretty much never used

  • its one method has no usage elsewhere in the repository
  • the rest of it is configuration/options stuff that would better go in _common.py or _config.py

• TimePair (and other similarly named classes/type-aliases/etc) have horrible names...

  • their names should give some affordance to their nature and/or their usage
  • a "TimePair" is not a pair of times!

• a number of class definitions are related but scattered around (and might even have some overlap), and could potentially be integrated:

  • _params.py: GeoPair _params.py: SourceSignalPair _params.py: TimePair
  • endpoints/covidcast_meta.py: SourceSignal
  • endpoints/covidcast_utils/model.py: TimeType endpoints/covidcast_utils/model.py: DataSignal endpoints/covidcast_utils/model.py: DataSource
  • maybe/probably others too???