event loop vs threads

[Terry Reedy]

On 9/27/2016 12:01 AM, srinivas devaki wrote:
> how does Python switch execution and maintain context i.e function stack
> etc,.. for co-routines and why is it less costly than switching threads
> which almost do the same, and both are handled by Python Interpreter
> itself(event loop for co-routines and GIL scheduling for threading), so
> where does the extra overhead for threads come from ?

Thread switching is done by the interpreter, either when a thread calls 
a blocking i/o function or when a time slice expires.  You can read the 
asyncio.base_event code and other submodules for details on how asyncio 
works.

-- 
Terry Jan Reedy