multiprocessing.pool.map and function with two arguments

Question

I am using multiprocessing.Pool()

here is what i want to Pool:

def insert_and_process(file_to_process,db):
    db = DAL("path_to_mysql" + db)
    #Table Definations
    db.table.insert(**parse_file(file_to_process))
    return True

if __name__=="__main__":
    file_list=os.listdir(".")
    P = Pool(processes=4)
    P.map(insert_and_process,file_list,db) # here having problem.

I want to pass 2 arguments What i want to do is to initialize only 4 DB connections (here will try to create connection on every function call so possibly millions of them and cause IO Freezed to death) . if i can create 4 db connections and 1 for each processes it will be ok.

Is there any solution for Pool ? or should i abandon it ?

EDIT:

From help of both of you i got this by doing this:

args=zip(f,cycle(dbs))
Out[-]: 
[('f1', 'db1'),
 ('f2', 'db2'),
 ('f3', 'db3'),
 ('f4', 'db4'),
 ('f5', 'db1'),
 ('f6', 'db2'),
 ('f7', 'db3'),
 ('f8', 'db4'),
 ('f9', 'db1'),
 ('f10', 'db2'),
 ('f11', 'db3'),
 ('f12', 'db4')]

So here it how it gonna work , i gonna move DB connection code out to the main level and do this:

def process_and_insert(args):

    #Table Definations
    args[1].table.insert(**parse_file(args[0]))
    return True

if __name__=="__main__":
    file_list=os.listdir(".")
    P = Pool(processes=4)

    dbs = [DAL("path_to_mysql/database") for i in range(0,3)]
    args=zip(file_list,cycle(dbs))
    P.map(insert_and_process,args) # here having problem.

Yeah , i going to test it out and let you guys know.

Answer 1

The Pool documentation does not say of a way of passing more than one parameter to the target function - I've tried just passing a sequence, but does not get unfolded (one item of the sequence for each parameter).

However, you can write your target function to expect the first (and only) parameter to be a tuple, in which each element is one of the parameters you are expecting:

from itertools import repeat

def insert_and_process((file_to_process,db)):
    db = DAL("path_to_mysql" + db)
    #Table Definations
    db.table.insert(**parse_file(file_to_process))
    return True

if __name__=="__main__":
    file_list=os.listdir(".")
    P = Pool(processes=4)
    P.map(insert_and_process,zip(file_list,repeat(db))) 

(note the extra parentheses in the definition of insert_and_process - python treat that as a single parameter that should be a 2-item sequence. The first element of the sequence is attributed to the first variable, and the other to the second)

Answer 2

Your pool will spawn four processes, each run by it's own instance of the Python interpreter. You can use a global variable to hold your database connection object, so that exactly one connection is created per process:

global_db = None

def insert_and_process(file_to_process, db):
    global global_db
    if global_db is None:
        # If this is the first time this function is called within this
        # process, create a new connection.  Otherwise, the global variable
        # already holds a connection established by a former call.
        global_db = DAL("path_to_mysql" + db)
    global_db.table.insert(**parse_file(file_to_process))
    return True

Since Pool.map() and friends only support one-argument worker functions, you need to create a wrapper that forwards the work:

def insert_and_process_helper(args):
    return insert_and_process(*args)

if __name__ == "__main__":
    file_list=os.listdir(".")
    db = "wherever you get your db"
    # Create argument tuples for each function call:
    jobs = [(file, db) for file in file_list]
    P = Pool(processes=4)
    P.map(insert_and_process_helper, jobs)