import sys
import time
import pymysql

conn = pymysql.connect(host='localhost',
                       user='user',
                       password='password',
                       db='database',
                       charset='utf8')

def get_user():
    cursor = conn.cursor()
    query = """SELECT username FROM users WHERE username=testuser"""
    cursor.execute(query)
    result = cursor.fetchone()[0]
    cursor.close()
    return result


def worker():
    try:
        while True:
            time.sleep(1)
            print(get_user())
    except KeyboardInterrupt:
        print("Stopeed")


if __name__ == '__main__':
    worker()

>>> testuser = "me"
>>> query = 'SELECT username FROM users WHERE username='testuser''

>>> query = 'SELECT username FROM users WHERE username='testuser''
  File "<stdin>", line 1
    query = 'SELECT username FROM users WHERE username='testuser''
                                                               ^
SyntaxError: invalid syntax

query = """SELECT username FROM users WHERE username='%s'""", (testuser)

>>> testuser = "me"
>>> query = """SELECT username FROM users WHERE username='%s'""", (testuser)
>>> query
("SELECT username FROM users WHERE username='%s'", 'me')