semaphores and Rlocks

["Martin v. Löwis"]

Jive Dadson wrote:
> Now I am even confuseder than I was.  I found threading.py, which looks like it 
> implements semaphores and condition variables using polling and sleeping.  Yes?  No?

Yes on condition variables, if they have a timeout. No on semaphores and 
condition variables without timeout.

> But I also found thread.c in the C sources, which appears to define semaphores
> and locks by means of the MS Windows API.  

Where did you find this? thread_nt.h is what is used on Windows, and it 
uses Win32 semaphores only in thread creation. thread.lock is 
implemented using 
InterlockedCompareExchange/InterlockedIncrement/WaitForsingleObject.

> If I had Python locks (like CRITICAL_SECTION in win32 land) 
> and true non-polling semaphores (like Win32 CreateSemaphore, WaitForSingleObject, 
> and ReleaseSemaphore), I could roll my own condition variables portably in Python.

In threading.py, the relationship is reverse: semaphores are implemented 
using condition variables, not vice versa. They are non-polling, as they 
don't give a timeout to the condition variable.

> But I look in the thread module per se and all I find is allocate_lock, 
 > no semaphores.

Correct.

> It takes a "LockType" parameter

Where did you find this? All allocate-lock functions I could find take 
no arguments; the thread.allocate_lock Python function returns an 
instance of thread.LockType.

> I also found a module called "mutex".  

See http://www.python.org/doc/current/lib/module-mutex.html. It's meant 
for use with sched.py, i.e. isn't directly related to threading.

> I don't want to do a C extension, in part because I don't currently 
> have access to UNIX or Mac development systems, but I would probably 
> do that rather than abandoning the idea of doing this project in Python.

The thread module is purposefully limited in the primitives it provides 
primarily to be able to provide it on all systems. On top of these 
primitives, more elaborate synchronization objects are implemented
in threading.py.

If you want threading objects implemented directly using the Win32 API, 
see http://python.org/sf/500447. If you find the time to revise this 
patch so that it transparently get those primitives instead of the 
pure-Python implementations on Windows, please do contribute.

Regards,
Martin