python 中的 SQLite WAL 模式。 一位作者和一位读者的并发

Question

I'm trying to share a sqlite3 database between one writer process, and one reader process. However, it does not work, and it seems to me that nothing is being written in example.db.

reader.py

import sqlite3
from time import sleep
conn = sqlite3.connect('example.db', isolation_level=None)

c = conn.cursor()
while True:
    c.execute("SELECT * FROM statistics")
    try:
        print '**'
        print c.fetchone()
    except:
        pass
    sleep(3)

writer.py

import sqlite3
from time import sleep
import os

if os.path.exists('example.db'):
    os.remove('example.db')

conn = sqlite3.connect('example.db', isolation_level=None)
c = conn.cursor()
c.execute('PRAGMA journal_mode=wal')
print c.fetchone()
c.execute("CREATE TABLE statistics (stat1 real, stat2 real)")

stat1 = 0
stat2 = 0.0
while True:
    stat1 += 1
    stat2 += 0.1
    cmd = "INSERT INTO statistics VALUES (%d,%f)" % (stat1, stat2)
    print cmd
    c.execute(cmd)
    #conn.commit()
    c.execute("PRAGMA wal_checkpoint=FULL")
    sleep(0.25)

I run writer.py in a background process, and then I run reader.py. This is what it shows:

**
(1.0, 0.1)
**
(1.0, 0.1)
**
(1.0, 0.1)

(...)

What is the correct (and best) way to set this framework with one reader and one writer?

Answer 1

What do you expect? You only select the first row of the table. And this is always the same row. The "PRAGMA"-statements have no visible effects here.