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\         


        

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)

Answer 3

No need to use zip. If for example you have 2 parameters, x and y, and each of them can get several values, like:

X=range(1,6)
Y=range(10)

The function should get only one parameter, and unpack it inside:

def func(params):
    (x,y)=params
    ...

And you call it like that:

params = [(x,y) for x in X for y in Y]
pool.map(func, params)

Answer 4

You can use

from functools import partial 

library for this purpose

like

func = partial(rdc, lat, lng)
r = pool.map(func, range(8))

and

def rdc(lat,lng,x):
    pass 

Answer 5

Using

params=[(x,y) for x in X for y in Y]

you create a full copy of x and y, and that may be slower than using

from itertools import repeat
P.map(insert_and_process,zip(file_list,repeat(db)))