How to share a string amongst multiple processes using Managers() in Python?
Question: I need to read strings written by multiprocessing.Process instances from the
main process. I already use Managers and queues to pass arguments to
processes, so using the Managers seems obvious, [but Managers do not support
strings](http://docs.python.org/2/library/multiprocessing.html#sharing-state-
between-processes):

> A manager returned by Manager() will support types list, dict, Namespace,
> Lock, RLock, Semaphore, BoundedSemaphore, Condition, Event, Queue, Value and
> Array.

How do I share state represented by a string using Managers from the
multiprocessing module?


Answer: multiprocessing's Managers can hold
[Values](http://docs.python.org/2/library/multiprocessing.html#multiprocessing.Value)
which in turn can hold instances of the type
[c_char_p](http://docs.python.org/2/library/ctypes.html#fundamental-data-
types) from the ctypes module:

    
    
    >>> import multiprocessing
    >>> import ctypes
    >>> v = multiprocessing.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
    >>> cstring = multiprocessing.Value(ctypes.c_char_p, "Hello, World!")
    >>> cstring
    <Synchronized wrapper for c_char_p(166841564)>
    >>> cstring.value
    'Hello, World!'
    

See also: [Post with the original
solution](http://stackoverflow.com/a/16485061/906658) that I had a hard time
finding.

So a Manager can be used to share a string beneath multiple processes in
Python like this:

    
    
    >>> from multiprocessing import Process, Manager, Value
    >>> from ctypes import c_char_p
    >>> 
    >>> def greet(string):
    >>>     string.value = string.value + ", World!"
    >>> 
    >>> if __name__ == '__main__':
    >>>     manager = Manager()
    >>>     string = manager.Value(c_char_p, "Hello")
    >>>     process = Process(target=greet, args=(string,))
    >>>     process.start()
    >>>     process.join()    
    >>>     print string.value
    'Hello, World!'
    

