multiprocessing.value clear syntax?

Question

I want to use multiprocessing.Value to use a variable in multiple processes, but the syntax is not clear on Python\'s documentation. Can anyone tell me what should I use as

Answer 1

I think the original issue you were having (causing the TypeError) was because the lock argument to the multiprocessing.Value constructor is a keyword-only argument. You need to call multiprocessing.Value("c", temp, lock=False) in order for it to do what you were wanting to do.

However, I don't think you need to use a Value object at all. You're passing the keycode as an argument to your other process, and the Value isn't being used at all. I'd get rid of it completely:

def key(event): 
   instance = multiprocessing.Process(target=player, args=(event.char,)) 
   instance.start()

Answer 2

There is no special syntax for multiprocessing.Value, it's just a class like any other. The signature of the Value constructor is perfectly well described:

multiprocessing.Value(typecode_or_type, *args[, lock])

Return a ctypes object allocated from shared memory. By default the return value is actually a synchronized wrapper for the object.

typecode_or_type determines the type of the returned object: it is either a ctypes type or a one character typecode of the kind used by the array module. *args is passed on to the constructor for the type.

If lock is True (the default) then a new lock object is created to synchronize access to the value. If lock is a Lock or RLock object then that will be used to synchronize access to the value. If lock is False then access to the returned object will not be automatically protected by a lock, so it will not necessarily be “process-safe”.

You even have some examples of its use afterwards. In particolar the typecode_or_type can be one of the typecodes that are listed in the documentation for the array module(e.g. 'i' for signed integer, 'f' for float etc.) or a ctypes type, like ctypes.c_int etc.

If you want to have a Value containing a single letter you can do:

>>> import multiprocessing as mp
>>> letter = mp.Value('c', 'A')
>>> letter
<Synchronized wrapper for c_char('A')>
>>> letter.value
'A'

Update

The problem with your code is that the typecode 'c' means character not string. If you want to hold a string you can use the type ctypes.c_char_p:

>>> import multiprocessing as mp
>>> import ctypes
>>> v = mp.Value('c', "Hello, World!")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/multiprocessing/__init__.py", line 253, in Value
    return Value(typecode_or_type, *args, **kwds)
  File "/usr/lib/python2.7/multiprocessing/sharedctypes.py", line 99, in Value
    obj = RawValue(typecode_or_type, *args)
  File "/usr/lib/python2.7/multiprocessing/sharedctypes.py", line 73, in RawValue
    obj.__init__(*args)
TypeError: one character string expected
>>> v = mp.Value(ctypes.c_char_p, "Hello, World!")
>>> v
<Synchronized wrapper for c_char_p(166841564)>
>>> v.value
'Hello, World!'

For Python 3, use c_wchar_p instead of c_char_p