Storing Postgres Connection in Flask g

Question

I'm trying to use the flask g object to share a single open database connection and keep it open as long as a user is logged in. The problem I'm running into is it looks like the function get_db() isn't using the current open connection but rather opening a new one every time there's a new page request?

How can I not open up a new request each time I go to a new page?

Database File (qry is imported by the other modules):

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import os
import json
import psycopg2
import psycopg2.extras
from flask import g
from urllib import parse

from lingco import app


def load_row(row):
    loaded = {}
    for k, v in row.items():
        loaded[k] = json.loads(v) if type(v) == str else v
    return loaded

def connect_db():
    parse.uses_netloc.append("postgres")
    if "DATABASE_URL" in os.environ:
        url = parse.urlparse(os.environ["DATABASE_URL"])
    else:
        url = parse.urlparse(app.config["DATABASE_URL"])
    conn = psycopg2.connect(
        database=url.path[1:],
        user=url.username,
        password=url.password,
        host=url.hostname,
        port=url.port
        )
    print("***OPENING CONNECTION***")
    return conn


@app.teardown_appcontext
def close_db(error):
    """Closes the database again at the end of the request 
    if user is not logged in."""
    if hasattr(g, 'db_conn') and not hasattr(g, 'user'):
        print("***CLOSING CONNECTION***")
        g.db_conn.close()
        g.pop('db_conn', None)


def get_db():
    """Opens a new database connection if there is none yet for the
    current application context.
    """
    if not hasattr(g, 'db_conn'):
        g.db_conn = connect_db()
    return g.db_conn


def qry(query="",args=(),**kwargs):
    conn = get_db()
    cursor = conn.cursor(cursor_factory=psycopg2.extras.RealDictCursor)
    cursor.execute(query,args)
    if "commit" in kwargs and kwargs["commit"]==True:
        conn.commit()
    if "fetch" in kwargs and kwargs["fetch"]=="rowcount":
        return cursor.rowcount
    if "fetch" in kwargs and kwargs["fetch"]=="all":
        records = cursor.fetchall()
        if "load"in kwargs and kwargs["load"]==True:
            loaded_records = []
            for row in records:
                loaded_records.append(load_row(row))
            return loaded_records
        return records
    if "fetch" in kwargs and kwargs["fetch"]=="one":
        record = cursor.fetchone()
        if "load"in kwargs and kwargs["load"]==True:
            record = load_row(record)
        if "attr" in kwargs:
            return record[kwargs["attr"]]
        return record

Terminal Output:

host-162571:flask_try sethkillian$ python run.py
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 692-579-362
127.0.0.1 - - [25/Jan/2018 16:28:51] "GET /login HTTP/1.1" 200 -
***OPENING CONNECTION***
127.0.0.1 - - [25/Jan/2018 16:29:01] "POST /login HTTP/1.1" 302 -
***OPENING CONNECTION***
127.0.0.1 - - [25/Jan/2018 16:29:01] "GET /dashboard/ HTTP/1.1" 200 -
***OPENING CONNECTION***
127.0.0.1 - - [25/Jan/2018 16:29:07] "GET /dashboard/2/roster HTTP/1.1" 200 -
***OPENING CONNECTION***
127.0.0.1 - - [25/Jan/2018 16:29:13] "GET /dashboard/ HTTP/1.1" 200 -
***OPENING CONNECTION***
127.0.0.1 - - [25/Jan/2018 16:29:18] "GET /questions HTTP/1.1" 200 -
***OPENING CONNECTION***
127.0.0.1 - - [25/Jan/2018 16:29:20] "GET /static/js/question_database.js HTTP/1.1" 200 -
***OPENING CONNECTION***
127.0.0.1 - - [25/Jan/2018 16:29:34] "GET / HTTP/1.1" 200 -
***OPENING CONNECTION***
***CLOSING CONNECTION***
127.0.0.1 - - [25/Jan/2018 16:29:39] "GET /logout HTTP/1.1" 302 -
127.0.0.1 - - [25/Jan/2018 16:29:39] "GET / HTTP/1.1" 200 -
127.0.0.1 - - [25/Jan/2018 16:29:43] "GET /dashboard/ HTTP/1.1" 302 -
127.0.0.1 - - [25/Jan/2018 16:29:43] "GET /login HTTP/1.1" 200 -

dvnguyen · Accepted Answer · 2018-01-25 21:49:26Z

5

The g object is only global to the current request context. That means when the same user sends a new request, the new request has its own g object.

One solution to have a global connection object: just use normal global object and some bookkeeping if you want to make sure there's only one connection object.

conn = None

def get_db():
    global conn
    if conn is None:
        conn = connect_db()
    return conn

edited Jan 25, 2018 at 21:49

answered Jan 25, 2018 at 21:43

dvnguyen

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3 Comments

Seth Killian Over a year ago

So, if I store the connection in the conn variable in the database file, I don't need to worry about multiple connection instances from calling the qry() function in other files?

dvnguyen Over a year ago

No you don't need to worry about that

Siddhartha Over a year ago

Is there any disadvantages to storing a db connection like this? and when would one close the connection?

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