"contextvars" --- 情境變數
**************************

======================================================================

This module provides APIs to manage, store, and access context-local
state.  The "ContextVar" class is used to declare and work with
*Context Variables*.  The "copy_context()" function and the "Context"
class should be used to manage the current context in asynchronous
frameworks.

Context managers that have state should use Context Variables instead
of "threading.local()" to prevent their state from bleeding to other
code unexpectedly, when used in concurrent code.

額外資訊請見 **PEP 567**。

在 3.7 版被加入.


Context Variables
=================

class contextvars.ContextVar(name[, *, default])

   This class is used to declare a new Context Variable, e.g.:

      var: ContextVar[int] = ContextVar('var', default=42)

   The required *name* parameter is used for introspection and debug
   purposes.

   The optional keyword-only *default* parameter is returned by
   "ContextVar.get()" when no value for the variable is found in the
   current context.

   **Important:** Context Variables should be created at the top
   module level and never in closures.  "Context" objects hold strong
   references to context variables which prevents context variables
   from being properly garbage collected.

   name

      The name of the variable.  This is a read-only property.

      在 3.7.1 版被加入.

   get([default])

      Return a value for the context variable for the current context.

      If there is no value for the variable in the current context,
      the method will:

      * return the value of the *default* argument of the method, if
        provided; or

      * return the default value for the context variable, if it was
        created with one; or

      * raise a "LookupError".

   set(value)

      Call to set a new value for the context variable in the current
      context.

      The required *value* argument is the new value for the context
      variable.

      Returns a "Token" object that can be used to restore the
      variable to its previous value via the "ContextVar.reset()"
      method.

   reset(token)

      Reset the context variable to the value it had before the
      "ContextVar.set()" that created the *token* was used.

      舉例來說：

         var = ContextVar('var')

         token = var.set('new value')
         # code that uses 'var'; var.get() returns 'new value'.
         var.reset(token)

         # After the reset call the var has no value again, so
         # var.get() would raise a LookupError.

class contextvars.Token

   *Token* objects are returned by the "ContextVar.set()" method. They
   can be passed to the "ContextVar.reset()" method to revert the
   value of the variable to what it was before the corresponding
   *set*.

   var

      A read-only property.  Points to the "ContextVar" object that
      created the token.

   old_value

      A read-only property.  Set to the value the variable had before
      the "ContextVar.set()" method call that created the token. It
      points to "Token.MISSING" if the variable was not set before the
      call.

   MISSING

      A marker object used by "Token.old_value".


Manual Context Management
=========================

contextvars.copy_context()

   Returns a copy of the current "Context" object.

   The following snippet gets a copy of the current context and prints
   all variables and their values that are set in it:

      ctx: Context = copy_context()
      print(list(ctx.items()))

   The function has an *O*(1) complexity, i.e. works equally fast for
   contexts with a few context variables and for contexts that have a
   lot of them.

class contextvars.Context

   A mapping of "ContextVars" to their values.

   "Context()" creates an empty context with no values in it. To get a
   copy of the current context use the "copy_context()" function.

   Every thread will have a different top-level "Context" object. This
   means that a "ContextVar" object behaves in a similar fashion to
   "threading.local()" when values are assigned in different threads.

   Context implements the "collections.abc.Mapping" interface.

   run(callable, *args, **kwargs)

      Execute "callable(*args, **kwargs)" code in the context object
      the *run* method is called on.  Return the result of the
      execution or propagate an exception if one occurred.

      Any changes to any context variables that *callable* makes will
      be contained in the context object:

         var = ContextVar('var')
         var.set('spam')

         def main():
             # 'var' was set to 'spam' before
             # calling 'copy_context()' and 'ctx.run(main)', so:
             # var.get() == ctx[var] == 'spam'

             var.set('ham')

             # Now, after setting 'var' to 'ham':
             # var.get() == ctx[var] == 'ham'

         ctx = copy_context()

         # Any changes that the 'main' function makes to 'var'
         # will be contained in 'ctx'.
         ctx.run(main)

         # The 'main()' function was run in the 'ctx' context,
         # so changes to 'var' are contained in it:
         # ctx[var] == 'ham'

         # However, outside of 'ctx', 'var' is still set to 'spam':
         # var.get() == 'spam'

      The method raises a "RuntimeError" when called on the same
      context object from more than one OS thread, or when called
      recursively.

   copy()

      Return a shallow copy of the context object.

   var in context

      Return "True" if the *context* has a value for *var* set; return
      "False" otherwise.

   context[var]

      Return the value of the *var* "ContextVar" variable. If the
      variable is not set in the context object, a "KeyError" is
      raised.

   get(var[, default])

      Return the value for *var* if *var* has the value in the context
      object.  Return *default* otherwise.  If *default* is not given,
      return "None".

   iter(context)

      Return an iterator over the variables stored in the context
      object.

   len(proxy)

      Return the number of variables set in the context object.

   keys()

      Return a list of all variables in the context object.

   values()

      Return a list of all variables' values in the context object.

   items()

      Return a list of 2-tuples containing all variables and their
      values in the context object.


asyncio support
===============

Context variables are natively supported in "asyncio" and are ready to
be used without any extra configuration.  For example, here is a
simple echo server, that uses a context variable to make the address
of a remote client available in the Task that handles that client:

   import asyncio
   import contextvars

   client_addr_var = contextvars.ContextVar('client_addr')

   def render_goodbye():
       # The address of the currently handled client can be accessed
       # without passing it explicitly to this function.

       client_addr = client_addr_var.get()
       return f'Good bye, client @ {client_addr}\r\n'.encode()

   async def handle_request(reader, writer):
       addr = writer.transport.get_extra_info('socket').getpeername()
       client_addr_var.set(addr)

       # In any code that we call is now possible to get
       # client's address by calling 'client_addr_var.get()'.

       while True:
           line = await reader.readline()
           print(line)
           if not line.strip():
               break

       writer.write(b'HTTP/1.1 200 OK\r\n')  # status line
       writer.write(b'\r\n')  # headers
       writer.write(render_goodbye())  # body
       writer.close()

   async def main():
       srv = await asyncio.start_server(
           handle_request, '127.0.0.1', 8081)

       async with srv:
           await srv.serve_forever()

   asyncio.run(main())

   # To test it you can use telnet or curl:
   #     telnet 127.0.0.1 8081
   #     curl 127.0.0.1:8081
