class_docstr: A Database. Encapsulates a url and an SQLAlchemy engine with a pool of
  connections.
class_docstr_tok: ["A","Database",".","Encapsulates","a","url","and","an","SQLAlchemy","engine","with","a","pool","of","connections","."]
code: def __init__(self, db_url=None, **kwargs):
      # If no db_url was provided, fallback to $DATABASE_URL.
      self.db_url = db_url or os.environ.get("DATABASE_URL")
  
      if not self.db_url:
          raise ValueError("You must provide a db_url.")
  
      # Create an engine.
      self._engine = create_engine(self.db_url, **kwargs)
      self.open = True
  
imports: import os
  from sys import stdout
  from collections import OrderedDict
  from contextlib import contextmanager
  from inspect import isclass
  import tablib
  from docopt import docopt
  from sqlalchemy import create_engine, exc, inspect, text
label_desc: Use image node_id 1 to create a new Database object from inherited base classes: object with example: obj = Database(db_url)