[Python-Dev] PEP 550 v4 (original) (raw)

Greg Ewing greg.ewing at canterbury.ac.nz
Wed Aug 30 08:55:43 EDT 2017

Yury Selivanov wrote:

BTW we already have mechanisms to always propagate context to the caller -- just use threading.local() or a global variable.

But then you don't have a way to not propagate the context change when you don't want to.

Here's my suggestion: Make an explicit distinction between creating a new binding for a context var and updating an existing one.

So instead of two API calls there would be three:

contextvar.new(value) # Creates a new binding only
                      # visible to this frame and
                      # its callees

contextvar.set(value) # Updates existing binding in
                      # context inherited from caller

contextvar.get()      # Retrieves the current binding

If we assume an extension to the decimal module so that decimal.localcontext is a context var, we can now do this:

async def foo():
   # Establish a new context for this task
   decimal.localcontext.new(decimal.Context())
   # Delegate changing the context
   await bar()
   # Do some calculations
   yield 17 * math.pi + 42

async def bar():
   # Change context for caller
   decimal.localcontext.prec = 5

-- Greg

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