"contextvars" — Context Variables
*********************************

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

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.

See also **PEP 567** for additional details.

Added in version 3.7.


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.

      Added in version 3.7.1.

   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.

      For example:

         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.

   Each thread has its own effective stack of "Context" objects.  The
   *current context* is the "Context" object at the top of the current
   thread’s stack.  All "Context" objects in the stacks are considered
   to be *entered*.

   *Entering* a context, which can be done by calling its "run()"
   method, makes the context the current context by pushing it onto
   the top of the current thread’s context stack.

   *Exiting* from the current context, which can be done by returning
   from the callback passed to the "run()" method, restores the
   current context to what it was before the context was entered by
   popping the context off the top of the context stack.

   Since each thread has its own context stack, "ContextVar" objects
   behave in a similar fashion to "threading.local()" when values are
   assigned in different threads.

   Attempting to enter an already entered context, including contexts
   entered in other threads, raises a "RuntimeError".

   After exiting a context, it can later be re-entered (from any
   thread).

   Any changes to "ContextVar" values via the "ContextVar.set()"
   method are recorded in the current context.  The "ContextVar.get()"
   method returns the value associated with the current context.
   Exiting a context effectively reverts any changes made to context
   variables while the context was entered (if needed, the values can
   be restored by re-entering the context).

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

   run(callable, *args, **kwargs)

      Enters the Context, executes "callable(*args, **kwargs)", then
      exits the Context.  Returns *callable*’s return value, or
      propagates an exception if one occurred.

      Example:

         import contextvars

         var = contextvars.ContextVar('var')
         var.set('spam')
         print(var.get())  # 'spam'

         ctx = contextvars.copy_context()

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

             var.set('ham')

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

         # 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:
         print(ctx[var])  # 'ham'

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

   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
