data_manager.py

import os
import psycopg2
import psycopg2.extras


def establish_connection(connection_data=None):
    """
    Create a database connection based on the :connection_data: parameter
    :connection_data: Connection string attributes
    :returns: psycopg2.connection
    """
    if connection_data is None:
        connection_data = get_connection_data()
    try:
        connect_str = os.environ.get('DATABASE_URL')
        conn = psycopg2.connect(connect_str)
        conn.autocommit = True
    except psycopg2.DatabaseError as e:
        print("Cannot connect to database.")
        print(e)
    else:
        return conn


def get_connection_data(db_name=None):
    """
    Give back a properly formatted dictionary based on the environment variables values which are started
    with :MY__PSQL_: prefix
    :db_name: optional parameter. By default it uses the environment variable value.
    """
    if db_name is None:
        db_name = os.environ.get('MY_PSQL_DBNAME')

    return {
        'dbname': db_name,
        'user': os.environ.get('MY_PSQL_USER'),
        'host': os.environ.get('MY_PSQL_HOST'),
        'password': os.environ.get('MY_PSQL_PASSWORD')
    }


def execute_select(statement, variables=None, fetchall=True):
    """
    Execute SELECT statement optionally parameterized.
    Use fetchall=False to get back one value (fetchone)

    Example:
    > execute_select('SELECT %(title)s; FROM shows', variables={'title': 'Codecool'})
    statement: SELECT statement
    variables:  optional parameter dict, optional parameter fetchall"""
    result_set = []
    with establish_connection() as conn:
        with conn.cursor(cursor_factory=psycopg2.extras.RealDictCursor) as cursor:
            cursor.execute(statement, variables)
            result_set = cursor.fetchall() if fetchall else cursor.fetchone()
    return result_set


def get_connection_string():

    if os.environ.get('DATABASE_URL'):
        return os.environ.get('DATABASE_URL')
    else:
        raise KeyError('Some necessary environment variable(s) are not defined')


def open_database():
    try:
        connection_string = get_connection_string()
        connection = psycopg2.connect(connection_string)
        connection.autocommit = True
    except psycopg2.DatabaseError as exception:
        print('Database connection problem')
        raise exception
    return connection


def connection_handler(function):
    def wrapper(*args, **kwargs):
        connection = open_database()
        dict_cur = connection.cursor(cursor_factory=psycopg2.extras.RealDictCursor)
        ret_value = function(dict_cur, *args, **kwargs)
        dict_cur.close()
        connection.close()
        return ret_value

    return wrapper