Question

Trying to have a value that gets set in a method done in a multiprocess process get used by the ‘main’ process after it finishes.

It currently gives me

TypeError: 'str' object does not support item assignment
DID WE? ['']

Which, uh, isn’t really what I want given the following code:

def load_dat_site(d):
global seek_site
browser.execute_script("window.location = '"+seek_site+"';")
print "SITE LOADED"
d[0] = 'YEAH!'


manager = Manager()
d = manager.list([''])
site_load_process = multiprocessing.Process(target=load_dat_site, args=(d))
site_load_process.start()
site_load_process.join(15)
print "DID WE?", d

Is there any reason why example scripts seem to be able to set their lists’ values but this can’t?

Answer 1

From the multiprocessing.Process documentation (emphasis added):

“args is the argument tuple for the target invocation.”

Change your multiprocessing.Process instantiation to what is shown below and all will work as expected.

site_load_process = multiprocessing.Process(target=load_dat_site, 
                                            args=(d,)) # the comma is essential!

There is a self-contained, working example here: http://ideone.com/VeTFYk

---

Remember that it is NOT the brackets/parens which make a tuple, it is the comma.

>>> not_a_tuple = (0)
>>> type(not_a_tuple)
<type 'int'>

>>> t = 0,
>>> type(t)
<type 'tuple'>