Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>Here's a simple example: you need to try a few alternative URLs and return the contents of the first one to respond.</p> <pre><code>import Queue import threading import urllib2 # called by each thread def get_url(q, url): q.put(urllib2.urlopen(url).read()) theurls = ["http://google.com", "http://yahoo.com"] q = Queue.Queue() for u in theurls: t = threading.Thread(target=get_url, args = (q,u)) t.daemon = True t.start() s = q.get() print s </code></pre> <p>This is a case where threading is used as a simple optimization: each subthread is waiting for a URL to resolve and respond, in order to put its contents on the queue; each thread is a daemon (won't keep the process up if main thread ends -- that's more common than not); the main thread starts all subthreads, does a <code>get</code> on the queue to wait until one of them has done a <code>put</code>, then emits the results and terminates (which takes down any subthreads that might still be running, since they're daemon threads).</p> <p>Proper use of threads in Python is invariably connected to I/O operations (since CPython doesn't use multiple cores to run CPU-bound tasks anyway, the only reason for threading is not blocking the process while there's a wait for some I/O). Queues are almost invariably the best way to farm out work to threads and/or collect the work's results, by the way, and they're intrinsically threadsafe so they save you from worrying about locks, conditions, events, semaphores, and other inter-thread coordination/communication concepts.</p>
    singulars
    1. This table or related slice is empty.
    1. POHow to use threading in Python?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USOthers
    1. USAlex Martelli
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COThanks again, MartelliBot. I've updated the example to wait for all to urls to respond: import Queue, threading, urllib2 q = Queue.Queue() urls = '''http://www.a.com http://www.b.com http://www.c.com'''.split() urls_received = 0 def get_url(q, url): req = urllib2.Request(url) resp = urllib2.urlopen(req) q.put(resp.read()) global urls_received urls_received +=1 print urls_received for u in urls: t = threading.Thread(target=get_url, args = (q,u)) t.daemon = True t.start() while q.empty() and urls_received < len(urls): s = q.get() print s
      singulars
      1. PO
      1. UShtmldrum
    2. CO@JRM: if you look at the next answer below, I think that a better way to wait until the threads are finished would be to use the `join()` method, since that would make the main thread wait until they're done without consuming processor by constantly checking the value. @Alex: thanks, this is exactly what I needed to understand how to use threads.
      singulars
      1. PO
      1. USkrs013
    3. COFor python3, replace 'import urllib2' with 'import urllib.request as urllib2'. and put parentheses in the print statement.
      singulars
      1. PO
      1. USHarvey
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload