title: Get the number of rows in table using SQLAlchemy
body: <p>I am using SQLAlchemy in Python, and I want to know how to get the total number of rows in a column. I have variables defined:</p>
  
  <pre><code>engine = sqlalchemy.create_engine(url, ehco=False)
  Session = sqlalchemy.orm.sessionmaker(bind=engine)
  Session = session()
  metadata = sqlalchemy.MetaData(engine)
  Base = declarative_base(metadata=metadata)
  
  # A class representing the shape_congress_districts_2012 table
  class Congress(Base):
      __tablename__ = 'shape_congress_districts_2012'
      id = geoalchemy.Column(sqlalchemy.Integer, primary_key=True)
      name = geoalchemy.Column(sqlalchemy.Unicode)
      geom = geoalchemy.GeometryColumn(geoalchemy.Polygon(2))
      geom_simple = geoalchemy.GeometryColumn(geoalchemy.Polygon(2))
      area = geoalchemy.Column(sqlalchemy.Float)
      state_id = geoalchemy.Column(sqlalchemy.Integer)
      census_year = geoalchemy.Column(sqlalchemy.Date)
  
  geoalchemy.GeometryDDL(Congress.__table__)
  </code></pre>
  
  <p>I want to determine the total number of rows in the table without having to wait a whole bunch of time querying the database. Currently, I have a bit of code:</p>
  
  <pre><code>rows = session.query(Congress).all()
  </code></pre>
  
  <p>Then I can access them from list, but this requires me to load everything into memory at once.</p>