fix(performance): lazily load expensive modules within aurweb.db

Closes #374

Signed-off-by: Kevin Morris <kevr@0cost.org>

aurweb/db.py

-import functools
-import hashlib
-import math
-import os
-import re
-
-from typing import Iterable, NewType
-
-import sqlalchemy
-
-from sqlalchemy import create_engine, event
-from sqlalchemy.engine.base import Engine
-from sqlalchemy.engine.url import URL
-from sqlalchemy.orm import Query, Session, SessionTransaction, scoped_session, sessionmaker
-
-import aurweb.config
-import aurweb.util
-
+# Supported database drivers.
 DRIVERS = {
     "mysql": "mysql+mysqldb"
 }
 
-# Some types we don't get access to in this module.
-Base = NewType("Base", "aurweb.models.declarative_base.Base")
-
 
 def make_random_value(table: str, column: str, length: int):
     """ Generate a unique, random value for a string column in a table.
 
     :return: A unique string that is not in the database
     """
+    import aurweb.util
     string = aurweb.util.make_random_string(length)
     while query(table).filter(column == string).first():
         string = aurweb.util.make_random_string(length)
@@ -52,6 +33,10 @@ def test_name() -> str:
 
     :return: Unhashed database name
     """
+    import os
+
+    import aurweb.config
+
     db = os.environ.get("PYTEST_CURRENT_TEST",
                         aurweb.config.get("database", "name"))
     return db.split(":")[0]
@@ -70,7 +55,10 @@ def name() -> str:
     dbname = test_name()
     if not dbname.startswith("test/"):
         return dbname
+
+    import hashlib
     sha1 = hashlib.sha1(dbname.encode()).hexdigest()
+
     return "db" + sha1
 
 
@@ -78,12 +66,13 @@ def name() -> str:
 _sessions = dict()
 
 
-def get_session(engine: Engine = None) -> Session:
+def get_session(engine=None):
     """ Return aurweb.db's global session. """
     dbname = name()
 
     global _sessions
     if dbname not in _sessions:
+        from sqlalchemy.orm import scoped_session, sessionmaker
 
         if not engine:  # pragma: no cover
             engine = get_engine()
@@ -106,13 +95,17 @@ def pop_session(dbname: str) -> None:
     _sessions.pop(dbname)
 
 
-def refresh(model: Base) -> Base:
-    """ Refresh the session's knowledge of `model`. """
+def refresh(model):
+    """
+    Refresh the session's knowledge of `model`.
+
+    :returns: Passed in `model`
+    """
     get_session().refresh(model)
     return model
 
 
-def query(Model: Base, *args, **kwargs) -> Query:
+def query(Model, *args, **kwargs):
     """
     Perform an ORM query against the database session.
 
@@ -124,7 +117,7 @@ def query(Model: Base, *args, **kwargs) -> Query:
     return get_session().query(Model).filter(*args, **kwargs)
 
 
-def create(Model: Base, *args, **kwargs) -> Base:
+def create(Model, *args, **kwargs):
     """
     Create a record and add() it to the database session.
 
@@ -135,7 +128,7 @@ def create(Model: Base, *args, **kwargs) -> Base:
     return add(instance)
 
 
-def delete(model: Base) -> None:
+def delete(model) -> None:
     """
     Delete a set of records found by Query.filter(*args, **kwargs).
 
@@ -144,8 +137,9 @@ def delete(model: Base) -> None:
     get_session().delete(model)
 
 
-def delete_all(iterable: Iterable) -> None:
+def delete_all(iterable) -> None:
     """ Delete each instance found in `iterable`. """
+    import aurweb.util
     session_ = get_session()
     aurweb.util.apply_all(iterable, session_.delete)
 
@@ -155,23 +149,29 @@ def rollback() -> None:
     get_session().rollback()
 
 
-def add(model: Base) -> Base:
+def add(model):
     """ Add `model` to the database session. """
     get_session().add(model)
     return model
 
 
-def begin() -> SessionTransaction:
+def begin():
     """ Begin an SQLAlchemy SessionTransaction. """
     return get_session().begin()
 
 
-def get_sqlalchemy_url() -> URL:
+def get_sqlalchemy_url():
     """
     Build an SQLAlchemy URL for use with create_engine.
 
     :return: sqlalchemy.engine.url.URL
     """
+    import sqlalchemy
+
+    from sqlalchemy.engine.url import URL
+
+    import aurweb.config
+
     constructor = URL
 
     parts = sqlalchemy.__version__.split('.')
@@ -209,13 +209,17 @@ def get_sqlalchemy_url() -> URL:
 
 def sqlite_regexp(regex, item) -> bool:  # pragma: no cover
     """ Method which mimics SQL's REGEXP for SQLite. """
+    import re
     return bool(re.search(regex, str(item)))
 
 
-def setup_sqlite(engine: Engine) -> None:  # pragma: no cover
+def setup_sqlite(engine) -> None:  # pragma: no cover
     """ Perform setup for an SQLite engine. """
+    from sqlalchemy import event
+
     @event.listens_for(engine, "connect")
     def do_begin(conn, record):
+        import functools
         create_deterministic_function = functools.partial(
             conn.create_function,
             deterministic=True
@@ -227,7 +231,7 @@ def setup_sqlite(engine: Engine) -> None:  # pragma: no cover
 _engines = dict()
 
 
-def get_engine(dbname: str = None, echo: bool = False) -> Engine:
+def get_engine(dbname: str = None, echo: bool = False):
     """
     Return the SQLAlchemy engine for `dbname`.
 
@@ -238,6 +242,8 @@ def get_engine(dbname: str = None, echo: bool = False) -> Engine:
     :param echo: Flag passed through to sqlalchemy.create_engine
     :return: SQLAlchemy Engine instance
     """
+    import aurweb.config
+
     if not dbname:
         dbname = name()
 
@@ -254,6 +260,7 @@ def get_engine(dbname: str = None, echo: bool = False) -> Engine:
             "echo": echo,
             "connect_args": connect_args
         }
+        from sqlalchemy import create_engine
         _engines[dbname] = create_engine(get_sqlalchemy_url(), **kwargs)
 
         if is_sqlite:  # pragma: no cover
@@ -301,7 +308,10 @@ class ConnectionExecutor:
     _conn = None
     _paramstyle = None
 
-    def __init__(self, conn, backend=aurweb.config.get("database", "backend")):
+    def __init__(self, conn, backend=None):
+        import aurweb.config
+
+        backend = backend or aurweb.config.get("database", "backend")
         self._conn = conn
         if backend == "mysql":
             self._paramstyle = "format"
@@ -339,6 +349,7 @@ class Connection:
     _conn = None
 
     def __init__(self):
+        import aurweb.config
         aur_db_backend = aurweb.config.get('database', 'backend')
 
         if aur_db_backend == 'mysql':
@@ -357,7 +368,9 @@ class Connection:
         elif aur_db_backend == 'sqlite':  # pragma: no cover
             # TODO: SQLite support has been removed in FastAPI. It remains
             # here to fund its support for PHP until it is removed.
+            import math
             import sqlite3
+
             aur_db_name = aurweb.config.get('database', 'name')
             self._conn = sqlite3.connect(aur_db_name)
             self._conn.create_function("POWER", 2, math.pow)