Related
I am at the end of a very long journey...
Here is the long story if you are interested.
https://github.com/hardbyte/python-can/issues/1336
Sorry for the incredibly long code snippet but I am not sure where I am going wrong so I thought more is more.
The code is as follows request_inst() requests instrumentation data using the dict request_info from an MCU, the MCU responds and this is picked up by the listener. obtain_message() creates a future with which to store all_data that is yielded from the listener with msg = await reader.get_message(). I attempt to structure this process with lock. store_data() is where I store the response data from the MCU, this is a dict called all_data. all_data when printed outside of the listener appears with zero values as shown below. The purpose of the code is to make all_data available outside of the event loop but currently even with this implementation I cannot get all_data to appear without zero values showing up in the dict.
import asyncio
import can
from can.notifier import MessageRecipient
from typing import List
freq = 0.0003
# this is the respond ids and the and the parameter ids of the data
# stored data is suppose to fill up the None with a value
all_data = {268439810: {16512: [None], 16513: [None], 16514: [None], 16515: [None]},
268444162: {16512: [None], 16513: [None], 16514: [None], 16515: [None]}}
request_info = {286326784: {16512, 16513, 16514, 16515},
287440896: {16512, 16513, 16514, 16515}}
# all the request ids in that have been configured
cm4_read_ids = [286326784, 287440896]
# all the response ids in that have been configured
mcu_respond_ids = [268439810, 268444162]
# finds arb id and pid and logs the data from the message
# async def store_data(arb_id, msg_data, msg_dlc):
async def store_data(msg: can.Message, lock):
pid = int.from_bytes(msg.data[0:2], 'little')
arb_id = msg.arbitration_id
if arb_id in mcu_respond_ids:
async with lock:
if msg.dlc == 5:
all_data[arb_id][pid][0] = int.from_bytes(msg.data[2:4], 'little', signed=False)
elif msg.dlc == 7:
all_data[arb_id][pid][0] = int.from_bytes(msg.data[2:6], 'little', signed=False)
return all_data
async def request_inst(bus: can.Bus):
print('Request inst active')
while True:
for key in request_info:
for val in request_info[key]:
pid = int(val)
pidbytes = pid.to_bytes(2, 'little')
msg = can.Message(arbitration_id=key, data=pidbytes)
bus.send(msg)
await asyncio.sleep(freq)
# await store_data(reader)
async def message_obtain(reader: can.AsyncBufferedReader, lock):
print('Started it the get message process')
while True:
await asyncio.sleep(0.01)
msg = await reader.get_message()
future = await store_data(msg, lock)
async with lock:
print('This is the future')
print(future)
async def main() -> None:
with can.Bus(
interface="socketcan", channel="can0", receive_own_messages=True
) as bus:
# Create Notifier with an explicit loop to use for scheduling of callbacks
loop = asyncio.get_running_loop()
reader = can.AsyncBufferedReader()
lock = asyncio.Lock()
listeners: List[MessageRecipient] = [reader]
notifier = can.Notifier(bus, listeners, loop=loop)
try:
task1 = asyncio.create_task(request_inst(bus))
task2 = asyncio.create_task(message_obtain(reader, lock))
await asyncio.gather(task1, task2)
except KeyboardInterrupt:
notifier.stop()
bus.shutdown()
if __name__ == "__main__":
asyncio.run(main())
The issue I am seeing is that even on this cut down one page wonder I am seeing what I believe to be concurrency issues.
As you can see I have tried locking but still I am seeing these zero values appear in all_data See below where pid 16514's balue becomes 0 when the messages being returned by the MCU are not zero.
n.b. the output below where the incorrect data is shown is the output from print(future)
The real value should never be 0 as it is a measured value.
b6 = (1016872961).to_bytes(4, 'little')
struct.unpack('<f', b6)
(0.01907348819077015,)
Am I doing anything very stupid? It feels like I am not accessing the data in the listener correctly despite using lock when all_data is being modified.
If I print from the listeners the data is always correct even when all_data is returning 0 values.
If anyone is able to help me it would be much appreciated.
It appears the problem was not software related and the zeros were real. Every day is a learning day!
This is a PCAN image of the highlighted send and a response shown at the top.
EDIT: Confirmed MCU response issue - looks like the firmware on the MCU. My Rigol wouldn't trigger on the ID so I had to 1/8 video and then screen shot that to catch it in the act. You can see the response is all 7 bytes of nothing.
Ok ok ok, it turns out I was doing something really silly. I was using nested dictionaries to store data about different ids but the keys were the same. After some investigation using id(id1[some_pid1]) and id(id2[some_pid1]) I discovered the keys had the same memory address.
data_dict = {id1: {some_pid1: value, some_pid2: value},
id2: {some_pid1: value, some_pid2: value}}
This appeared to all that it was a race condition but actually I was just writing zeros (which turned out to be forced from the MCU) to the wrong id because it shared a key with the other id.
Whoops
I want to monitor my servers after every few seconds. I have a generator inside an object method which will return overall metrics and after that it should wait for a while and check again:
def ping_servers(self):
while True:
all_server_metrics = {}
for server in self.servers:
response = requests.get(server, timeout=self.timeout_secs)
server_metrics = {
'response_time': response.elapsed.total_seconds(),
'response_code': response.status_code,
}
all_server_metrics[server] = server_metrics
yield json.dumps(all_server_metrics)
time.sleep(self.period_secs)
Here it will not sleep at all. But if I put time.sleep() at the beginning of the while loop it will sleep for a given period of time. What could be the reason, why it is not sleeping?
This is the code where I call above method:
monitoring = MonitoringService(period_secs=30, timeout_secs=5)
while True:
ping_results = next(monitoring.ping_servers())
print(ping_results)
I have written a sample Subscriber. I want to feed the data that I have obtained from the rospy.Subscriber into another variable, so that I can use it later in the program for processing. At the moment I could see that the Subscriber is functioning as I can see the subscribed values being printed when I use rospy.loginfo() function. Although I donot know how to store this data into another varible. I have tried assigning it directly to a variable by using assignment operator '=', but I get error.
I have tried writing a callback function with rospy.loginfo to print the position data from the subscribed object. I have subscribed JointState and it containes, header, position, velocity and effort arrays. using rospy.loginfo I can verify that the subscriber is subscribing. But when i tried to assign it directly to a variable, I get an error.
I am displaying loginfo from a call back function as follows
def callback(data):
rospy.loginfo(data.position)
global listen
listen = rospy.Subscriber("joint_states", JointState,
callback)
rospy.spin()
and this works fine. But when i slightly modify the code to assign the subscribed values, I get following error i.e.
listen1 = rospy.Subscriber("joint_states", JointState,
callback=None)
listen = listen1.position
#rospy.loginfo(listen)
print(listen)
rospy.spin()```
The error is as follows,
```listen = listen1.position
AttributeError: 'Subscriber' object has no attribute 'position'
EDIT:
Here is my node I have defined in my program,
#rospy.loginfo(msg.data)
global tactile_states
tactile_states = data.data
def joint_callback(data):
#rospy.loginfo(data.position)
global g_joint_states
global g_position
global g_pos1
g_joint_states = data
#for i in len(data.position):
#g_position[i] = data.position[i]
g_position = data.position
if len(data.position) > 0:
print("jointstate more than 0")
g_pos1 = data.position[0]
#print(g_position)
def joint_modifier(*args):
#choice describes what the node is supposed to do whether act as publisher or subscribe to joint states or tactile sensors
rospy.init_node('joint_listener_publisher', anonymous=True)
pub1 = rospy.Publisher('joint_states', JointState, queue_size = 10)
if(len(args)>1):
choice = args[0]
joint_name = args[1]
position = args[2]
else:
choice = args[0]
if (choice == 1):
rate = rospy.Rate(1)
robot_configuration = JointState()
robot_configuration.header = Header()
robot_configuration.name = [joint_name]
robot_configuration.position = [position]
robot_configuration.velocity = [10]
robot_configuration.effort = [100]
while not rospy.is_shutdown():
robot_configuration.header.stamp = rospy.Time.now()
rospy.loginfo(robot_configuration)
break
pub1.publish(robot_configuration)
rospy.sleep(2)
if (choice == 2):
#rospy.Timer(rospy.Duration(2), joint_modifier)
listen = rospy.Subscriber("joint_states", JointState, joint_callback)
rospy.spin()
if (choice == 3):
#rospy.Timer(rospy.Duration(2), joint_modifier)
tactile_sub = rospy.Subscriber("/sr_tactile/touch/ff", Float64, tactile_callback)
rospy.spin()
This is how I am calling the node inside the main body of the program,
joint_modifier(2)
print("printing g_position")
print(g_position)#to check the format of g_position
print("printed g _position")
leg_1 = Leg_attribute(g_position[0], g_position[1], g_position[2], velocity1 = 10, velocity2 = 10, velocity3 = 10, effort1 = 100, effort2 = 100, effort3 = 100, acceleration=1)
When calling this way, the program is stuck at joint_modifier(2) as that function has rospy.spin().
The style which you're using is not very standard. I assume you've seen the example on ROS wiki, I've modified it to demonstrate standard usage below.
Chiefly, addressing the code you posted, you needed to make listen have global scope outside of the callback. This is to store the data you want, not the Subscriber object. The rospy.spin() never goes in a callback, only the main node function/section. The subscriber object, listen1, which is used infrequently, doesn't return anything, and doesn't store the data it acquires. That is, you need Subscriber() to have a non-None callback.
It's more of a bind, giving the data to the callback instead of returning it from Subscriber. That's why listen1 (Subscriber) has no attribute position (JointState).
import rospy
from sensor_msgs.msg import JointState
# Subscribers
# joint_sub (sensor_msgs/JointState): "joint_states"
# This is where you store all your data you recieve
g_joint_states = None
g_positions = None
g_pos1 = None
def timer_callback(event): # Type rospy.TimerEvent
print('timer_cb (' + str(event.current_real) + '): g_positions is')
print(str(None) if g_positions is None else str(g_positions))
def joint_callback(data): # data of type JointState
# Each subscriber gets 1 callback, and the callback either
# stores information and/or computes something and/or publishes
# It _does not!_ return anything
global g_joint_states, g_positions, g_pos1
rospy.loginfo(data.position)
g_joint_states = data
g_positions = data.position
if len(data.position) > 0:
g_pos1 = data.position[0]
print(g_positions)
# In your main function, only! here do you subscribe to topics
def joint_logger_node():
# Init ROS
rospy.init_node('joint_logger_node', anonymous=True)
# Subscribers
# Each subscriber has the topic, topic type, AND the callback!
rospy.Subscriber('joint_states', JointState, joint_callback)
# Rarely need to hold onto the object with a variable:
# joint_sub = rospy.Subscriber(...)
rospy.Timer(rospy.Duration(2), timer_callback)
# spin() simply keeps python from exiting until this node is stopped
# This is an infinite loop, the only code that gets ran are callbacks
rospy.spin()
# NO CODE GOES AFTER THIS, NONE! USE TIMER CALLBACKS!
# unless you need to clean up resource allocation, close(), etc when program dies
if __name__ == '__main__':
joint_logger_node()
Edit 1:
There seems to be some confusion on what Subscriber(), spin(), and _callback(s) do.
It's a bit obscured in the Python, but there is a master program that manages all nodes, and sending nodes between them. In each node, we register with that master program that the node exists, and what publishers and subscribers it has. By register, it means we tell the master program, "Hey, I want that topic!"; in your case, for your (undeclared) joint_sub Subscriber, "Hey, I want all the JointState msgs from the joint_states topic!" The master program will, every time it gets (from some publisher somewhere) a new joint_states JointState msg, send it to that subscriber.
The subscriber handles, deals with, and processes the msg (data) with a callback: when(!) I receive a message, run the callback.
So the master program receives a new joint_states JointState msg from some publisher. Then it, because we registered a subscriber to it, sends it to this node. rospy.spin() is an infinite loop waiting for that data. This is what it does (kinda-mostly):
def rospy.spin():
while rospy.ok():
for new_msg in get_new_messages from master():
if I have a subscriber to new_msg:
my_subscriber.callback(new_msg)
rospy.spin() is where your callback, joint_callback (and/or timer_callback, etc) actually get called, and executed. It only runs when there is data for it.
More fundamentally, I think because of this confusion, your program structure is flawed; your functions don't do what you think they do. This is how you should make your node.
Make your math-portion (all the real non-ros code), the one doing the NN, into a separate module, and make a function to run it.
If you only want to run it when you receive data, run it in the callback. If you want to publish the result, publish in the callback.
Don't call the main function! The if __name__ == '__main__': my_main_function() should be the only place it gets called, and this will call your code. I repeat: the main function, declaring subscribers/publishers/init/timers/parameters, is only run in if __name__ ..., and this function runs your code. To have it run your code, place your code in a callback. Timer callbacks are handy for this.
I hope this code sample clarifies:
import rospy
from std_msgs.msg import Header
from sensor_msgs.msg import JointState
import my_nn as nn # nn.run(data)
# Subscribers
# joint_sub (sensor_msgs/JointState): "joint_states"
# Publishers
# joint_pub (sensor_msgs/JointState): "target_joint_states"
joint_pub = None
def joint_callback(data): # data of type JointState
pub_msg = JointState() # Make a new msg to publish results
pub_msg.header = Header()
pub_msg.name = data.name
pub_msg.velocity = [10] * len(data.name)
pub_msg.effort = [100] * len(data.name)
# This next line might not be quite right for what you want to do,
# But basically, run the "real code" on the data, and get the
# result to publish back out
pub_msg.position = nn.run(data.position) # Run NN on data, store results
joint_pub.publish(pub_msg) # Send it when ready!
if __name__ == '__main__':
# Init ROS
rospy.init_node('joint_logger_node', anonymous=True)
# Subscribers
rospy.Subscriber('joint_states', JointState, joint_callback)
# Publishers
joint_pub = rospy.Publisher('target_joint_states', JointState, queue_size = 10)
# Spin
rospy.spin()
# No more code! This is not a function to call, but its
# own program! This is an executable! Run your code in
# a callback!
Notice that a python module we design to be a ros node, has no functions to be called. It has a defined structure of callbacks and global data shared between them, all initialized and registered in the main function / if __name__ == '__main__'.
I'm working on a Raspberry Pi (3 B+) making a data collection device and I'm
trying to spawn a process to record the data coming in and write it to a file. I have a function for the writing that works fine when I call it directly.
When I call it using the multiprocess approach however, nothing seems to happen. I can see in task monitors in Linux that the process does in fact get spawned but no file gets written, and when I try to pass a flag to it to shut down it doesn't work, meaning I end up terminating the process and nothing seems to have happened.
I've been over this every which way and can't see what I'm doing wrong; does anyone else? In case it's relevant, these are functions inside a parent class, and one of the functions is meant to spawn another as a thread.
Code I'm using:
from datetime import datetime, timedelta
import csv
from drivers.IMU_SEN0 import IMU_SEN0
import multiprocessing, os
class IMU_data_logger:
_output_filename = ''
_csv_headers = []
_accelerometer_headers = ['Accelerometer X','Accelerometer Y','Accelerometer Z']
_gyroscope_headers = ['Gyroscope X','Gyroscope Y','Gyroscope Z']
_magnetometer_headers = ['Bearing']
_log_accelerometer = False
_log_gyroscope= False
_log_magnetometer = False
IMU = None
_writer=[]
_run_underway = False
_process=[]
_stop_value = 0
def __init__(self,output_filename='/home/pi/blah.csv',log_accelerometer = True,log_gyroscope= True,log_magnetometer = True):
"""data logging device
NOTE! Multiple instances of this class should not use the same IMU devices simultaneously!"""
self._output_filename = output_filename
self._log_accelerometer = log_accelerometer
self._log_gyroscope = log_gyroscope
self._log_magnetometer = log_magnetometer
def __del__(self):
# TODO Update this
if self._run_underway: # If there's still a run underway, end it first
self.end_recording()
def _set_up(self):
self.IMU = IMU_SEN0(self._log_accelerometer,self._log_gyroscope,self._log_magnetometer)
self._set_up_headers()
def _set_up_headers(self):
"""Set up the headers of the CSV file based on the header substrings at top and the input flags on what will be measured"""
self._csv_headers = []
if self._log_accelerometer is not None:
self._csv_headers+= self._accelerometer_headers
if self._log_gyroscope is not None:
self._csv_headers+= self._gyroscope_headers
if self._log_magnetometer is not None:
self._csv_headers+= self._magnetometer_headers
def _record_data(self,frequency,stop_value):
self._set_up() #Run setup in thread
"""Record data function, which takes a recording frequency, in herz, as an input"""
previous_read_time=datetime.now()-timedelta(1,0,0)
self._run_underway = True # Note that a run is now going
Period = 1/frequency # Period, in seconds, of a recording based on the input frequency
print("Writing output data to",self._output_filename)
with open(self._output_filename,'w',newline='') as outcsv:
self._writer = csv.writer(outcsv)
self._writer.writerow(self._csv_headers) # Write headers to file
while stop_value.value==0: # While a run continues
if datetime.now()-previous_read_time>=timedelta(0,1,0): # If we've waited a period, collect the data; otherwise keep looping
print("run underway value",self._run_underway)
if datetime.now()-previous_read_time>=timedelta(0,Period,0): # If we've waited a period, collect the data; otherwise keep looping
previous_read_time = datetime.now() # Update previous readtime
next_row = []
if self._log_accelerometer:
# Get values in m/s^2
axes = self.IMU.read_accelerometer_values()
next_row += [axes['x'],axes['y'],axes['z']]
if self._log_gyroscope:
# Read gyro values
gyro = self.IMU.read_gyroscope_values()
next_row += [gyro['x'],gyro['y'],gyro['z']]
if self._log_magnetometer:
# Read magnetometer value
b= self.IMU.read_magnetometer_bearing()
next_row += b
self._writer.writerow(next_row)
# Close the csv when done
outcsv.close()
def start_recording(self,frequency_in_hz):
# Create recording process
self._stop_value = multiprocessing.Value('i',0)
self._process = multiprocessing.Process(target=self._record_data,args=(frequency_in_hz,self._stop_value))
# Start recording process
self._process.start()
print(datetime.now().strftime("%H:%M:%S.%f"),"Data logging process spawned")
print("Logging Accelerometer:",self._log_accelerometer)
print("Logging Gyroscope:",self._log_gyroscope)
print("Logging Magnetometer:",self._log_magnetometer)
print("ID of data logging process: {}".format(self._process.pid))
def end_recording(self,terminate_wait = 2):
"""Function to end the recording multithread that's been spawned.
Args: terminate_wait: This is the time, in seconds, to wait after attempting to shut down the process before terminating it."""
# Get process id
id = self._process.pid
# Set stop event for process
self._stop_value.value = 1
self._process.join(terminate_wait) # Wait two seconds for the process to terminate
if self._process.is_alive(): # If it's still alive after waiting
self._process.terminate()
print(datetime.now().strftime("%H:%M:%S.%f"),"Process",id,"needed to be terminated.")
else:
print(datetime.now().strftime("%H:%M:%S.%f"),"Process",id,"successfully ended itself.")
====================================================================
ANSWER: For anyone following up here, it turns out the problem was my use of the VS Code debugger which apparently doesn't work with multiprocessing and was somehow preventing the success of the spawned process. Many thanks to Tomasz Swider below for helping me work through issues and, eventually, find my idiocy. The help was very deeply appreciated!!
I can see few thing wrong in your code:
First thing
stop_value == 0 will not work as the multiprocess.Value('i', 0) != 0, change that line to
while stop_value.value == 0
Second, you never update previous_read_time so it will write the readings as fast as it can, you will run out of disk quick
Third, try use time.sleep() the thing you are doing is called busy looping and it is bad, it is wasting CPU cycles needlessly.
Four, terminating with self._stop_value = 1 probably will not work there must be other way to set that value maybe self._stop_value.value = 1.
Well here is a pice of example code based on the code that you have provided that is working just fine:
import csv
import multiprocessing
import time
from datetime import datetime, timedelta
from random import randint
class IMU(object):
#staticmethod
def read_accelerometer_values():
return dict(x=randint(0, 100), y=randint(0, 100), z=randint(0, 10))
class Foo(object):
def __init__(self, output_filename):
self._output_filename = output_filename
self._csv_headers = ['xxxx','y','z']
self._log_accelerometer = True
self.IMU = IMU()
def _record_data(self, frequency, stop_value):
#self._set_up() # Run setup functions for the data collection device and store it in the self.IMU variable
"""Record data function, which takes a recording frequency, in herz, as an input"""
previous_read_time = datetime.now() - timedelta(1, 0, 0)
self._run_underway = True # Note that a run is now going
Period = 1 / frequency # Period, in seconds, of a recording based on the input frequency
print("Writing output data to", self._output_filename)
with open(self._output_filename, 'w', newline='') as outcsv:
self._writer = csv.writer(outcsv)
self._writer.writerow(self._csv_headers) # Write headers to file
while stop_value.value == 0: # While a run continues
if datetime.now() - previous_read_time >= timedelta(0, 1,
0): # If we've waited a period, collect the data; otherwise keep looping
print("run underway value", self._run_underway)
if datetime.now() - previous_read_time >= timedelta(0, Period,
0): # If we've waited a period, collect the data; otherwise keep looping
next_row = []
if self._log_accelerometer:
# Get values in m/s^2
axes = self.IMU.read_accelerometer_values()
next_row += [axes['x'], axes['y'], axes['z']]
previous_read_time = datetime.now()
self._writer.writerow(next_row)
# Close the csv when done
outcsv.close()
def start_recording(self, frequency_in_hz):
# Create recording process
self._stop_value = multiprocessing.Value('i', 0)
self._process = multiprocessing.Process(target=self._record_data, args=(frequency_in_hz, self._stop_value))
# Start recording process
self._process.start()
print(datetime.now().strftime("%H:%M:%S.%f"), "Data logging process spawned")
print("ID of data logging process: {}".format(self._process.pid))
def end_recording(self, terminate_wait=2):
"""Function to end the recording multithread that's been spawned.
Args: terminate_wait: This is the time, in seconds, to wait after attempting to shut down the process before terminating it."""
# Get process id
id = self._process.pid
# Set stop event for process
self._stop_value.value = 1
self._process.join(terminate_wait) # Wait two seconds for the process to terminate
if self._process.is_alive(): # If it's still alive after waiting
self._process.terminate()
print(datetime.now().strftime("%H:%M:%S.%f"), "Process", id, "needed to be terminated.")
else:
print(datetime.now().strftime("%H:%M:%S.%f"), "Process", id, "successfully ended itself.")
if __name__ == '__main__':
foo = Foo('/tmp/foometer.csv')
foo.start_recording(20)
time.sleep(5)
print('Ending recording')
foo.end_recording()
After some lookup into google and posts of stackoverflow and other sites, i'm still confused on how i can apply a queue and threading on my code:
import psycopg2
import sys
import re
# for threading and queue
import multiprocessing
from multiprocessing import Queue
# for threading and queue
import time
from datetime import datetime
class Database_connection():
def db_call(self,query,dbHost,dbName,dbUser,dbPass):
try:
con = None
con = psycopg2.connect(host=dbHost,database=dbName,
user=dbUser,password=dbPass)
cur = con.cursor()
cur.execute(query)
data = cur.fetchall()
resultList = []
for data_out in data:
resultList.append(data_out)
return resultList
except psycopg2.DatabaseError, e:
print 'Error %s' % e
sys.exit(1)
finally:
if con:
con.close()
w = Database_connection()
sql = "select stars from galaxy"
startTime = datetime.now()
for result in w.db_call(sql, "x", "x", "x", "x"):
print result[0]
print "Runtime: " + str(datetime.now()-startTime)
lets supose the result will be 100+ values. How can i, put those 100+ results on queue and execute ( print, for example ) then 5 at time using queue and multiprocessing module?
What do you want this code to do?
You get no output from this code because get() returns the next item from the queue (doc). You are putting the letters from the sql response into the queue one letter at a time. the i in for i... is looping over the list returned by w.db_call. Those items are (I assume) strings, which you are then iterating over and adding one at a time to the queue. The next thing you do is to remove the element you just added to the queue from the queue, which leaves the queue unchanged over each pass through the loop. If you put a print statement in the loop it prints out the letter it just got from the queue.
Queues are used to pass information between processes. I think you are trying to set-up a producer/consumer pattern where you have one process add things to the queue and multiple other processes which consume things from the queue. See working example of multiprocessing.Queue and links contained there in (example, main documentation).
Probably the simplest way to get this working, as long as you don't need it to run in an interactive shell, is to use Pool (lifted almost verbatim from the documentation of multiprocess)
from multiprocessing import Pool
p = Pool(5) # sets the number of worker threads you want
def f(res):
# put what ever you want to do with each of the query results in here
return res
result_lst = w.db_call(sql, "x", "x", "x", "x")
proced_results = p.map(f, result_lst)
which apply what ever you want to do to each result (written into the function f) and returns the results of that manipulation as a list. The number of sub-processes to use is set by the argument to Pool.
This is my suggestion...
import Queue
from threading import Thread
class Database_connection:
def db_call(self,query,dbHost,dbName,dbUser,dbPass):
# your code here
return
# in this example each thread will execute this function
def processFtpAddrMt(queue):
# loop will continue until queue containing FTP addresses is empty
while True:
# get an ftp address, a exception will be called when the
# queue is empty and the loop will break
try: ftp_addr = queue.get()
except: break
# put code to process the ftp address here
# let queue know this task is done
queue.task_done()
w = Database_connection()
sql = "select stars from galaxy"
ftp_addresses = w.db_call(sql, "x", "x", "x", "x")
# put each result of the SQL call in a Queue class
ftp_addr_queue = Queue.Queue()
for addr in ftp_addresses:
ftp_addr_queue.put(addr)
# create five threads where each one will run analyzeFtpResult
# pass the queue to the analyzeFtpResult function
for x in range(0,5):
t = Thread(target=processFtpAddrMt,args=(ftp_addr_queue,))
t.setDaemon(True)
t.start()
# blocks further execution of the script until all queue items have been processed
ftp_addr_queue.join()
It uses the Queue class to store your SQL results and then the Thread class to process the queue. Five thread classes are created and each one uses a processFtpAddrMt function which take ftp addresses from the queue until the queue is empty. All you have to do is add the code for processing the ftp address. Hope this helps.
I was able to solve the problem with the following:
def worker():
w = Database_connection()
sql = "select stars from galaxy"
for result in w.db_call(sql, "x", "x", "x", "x"):
if result:
jobs = []
startTime = datetime.now()
for i in range(1):
p = multiprocessing.Process(target=worker)
jobs.append(p)
p.start()
print "Runtime: " + str(datetime.now()-startTime)
I belive it is not the best way to do it, but for now solved my problem :)