site.py

from flask import render_template
from random import randrange
#create the app object
app = Flask(__name__)

#get the database
from tables import engine, temperature_table

@app.before_request
def before_request():
    g.con = engine.connect()

@app.teardown_request
def teardown_request(exception):
    g.con.close()

@app.route("/add_reading")
def add_data():
    data = {'temperature': randrange(-40,140)}
    stmt = temperature_table.insert(data)
    res = g.con.execute(stmt)
    return render_template('added_temp.html', results=res)

tables.py

from sqlalchemy import *
from datetime import datetime

engine = create_engine('postgres://blahblahblah')
metadata = MetaData(bind=engine)

temperature_table = Table(
    'tf_temperatures', metadata,
    Column('id', Integer, primary_key=True),
    Column('temperature', Float, nullable=False),
    Column('datetime',DateTime, default=datetime.now))
metadata.create_all()