On 9/4/17, Koos Zevenhoven <k7hoven@gmail.com> wrote:
Core concept ''''''''''''
A context-local variable is represented by a single instance of ``contextvars.Var``, say ``cvar``. Any code that has access to the ``cvar`` object can ask for its value with respect to the current context. In the high-level API, this value is given by the ``cvar.value`` property::
cvar = contextvars.Var(default="the default value", description="example context variable")
assert cvar.value == "the default value" # default still applies
# In code examples, all ``assert`` statements should # succeed according to the proposed semantics.
No assignments to ``cvar`` have been applied for this context, so ``cvar.value`` gives the default value. Assigning new values to contextvars is done in a highly scope-aware manner::
with cvar.assign(new_value): assert cvar.value is new_value # Any code here, or down the call chain from here, sees: # cvar.value is new_value # unless another value has been assigned in a # nested context assert cvar.value is new_value # the assignment of ``cvar`` to ``new_value`` is no longer visible assert cvar.value == "the default value"
I feel that of "is" and "==" in assert statements in this PEP has to be used (or described) more precisely. What if new_value above is 123456789? maybe using something like could be better? -> def equals(a, b): return a is b or a == b Doesn't PEP need to think about something like "context level overflow" ? Or members like: cvar.level ?