user4979733 user4979733 - 5 months ago 82x
Python Question

Python: Using sqlite3 with multiprocessing

I have a SQLite3 DB. I need to parse 10000 files. I read some data from each file, and then query the DB with this data to get a result. My code works fine in a single process environment. But I get an error when trying to use the mulitprocessing Pool.

My approach without multiprocessing (works OK):
1. Open DB connection object
2. for f in files:
foo(f, x1=x1, x2=x2, ..., db=DB)
3. Close DB

My approach with multiprocessing (does NOT work):
1. Open DB
2. pool = multiprocessing.Pool(processes=4)
3., x1=x1, x2=x2, ..., db=DB), [files])
4. pool.close()
5. Close DB

I get the following error: sqlite3.ProgrammingError: Base Cursor.__init__ not called.

My DB class is implemented as follows:

def open_db(sqlite_file):
"""Open SQLite database connection.

sqlite_file -- File path

"""'Open SQLite database %s', sqlite_file)
conn = sqlite3.connect(sqlite_file)
except sqlite3.Error, e:
log.error('Unable to open SQLite database %s', e.args[0])

return conn

def close_db(conn, sqlite_file):
"""Close SQLite database connection.

conn -- Connection

if conn:'Close SQLite database %s', sqlite_file)

class MapDB:

def __init__(self, sqlite_file):

sqlite_file -- File path

# 1. Open database.
# 2. Setup to receive data as dict().
# 3. Get cursor to execute queries.
self._sqlite_file = sqlite_file
self._conn = open_db(sqlite_file)
self._conn.row_factory = sqlite3.Row
self._cursor = self._conn.cursor()

def close(self):
"""Close DB connection."""

if self._cursor:
close_db(self._conn, self._sqlite_file)

def check(self):

def get_driver_net(self, net):

def get_cell_id(self, net):

Function foo() looks like this:

def foo(f, x1, x2, db):

extract some data from file f
r1 = db.get_driver_net(...)
r2 = db.get_cell_id(...)

The overall not working implementation is as follows:

mapdb = MapDB(sqlite_file)'Create NetInfo objects')
pool = multiprocessing.Pool(processes=4)
files = [get list of files to process], x1=x1, x2=x2, db=mapdb), files)

To fix this, I think I need to create the MapDB() object inside each pool worker (so have 4 parallel/independent connections). But I'm not sure how to do this. Can someone show me an example of how to accomplish this with Pool?


What about defining foo like this:

def foo(f, x1, x2, db_path):
    mapdb = MapDB(db_path)
    ... open mapdb
    ... process data ...
    ... close mapdb

and then change your call to:, x1=x1, x2=x2, db_path="path-to-sqlite3-db"), files)    


Another option is to handle the worker threads yourself and distribute work via a Queue.

from Queue import Queue
from threading import Thread

q = Queue()

def worker():
  mapdb = the sqlite database
  while True:
    item = q.get()
    if item[0] == "file":
      file = item[1]
      ... process file ...
  ...close sqlite connection...

# Start up the workers

nworkers = 4

for i in range(nworkers):
  worker = Thread(target=worker)
  worker.daemon = True

# Place work on the Queue

for x in ...list of files...:

# Place termination tokens onto the Queue

for i in range(nworkers):

# Wait for all work to be done.


The termination tokens are used to ensure that the sqlite connections are closed - in case that matters.