bpo-32436: Add docs for contextvars (#5685)

2 files changed, 280 insertions, 0 deletions

diff --git a/Doc/library/contextvars.rst b/Doc/library/contextvars.rst
new file mode 100644
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--- /dev/null
+++ b/Doc/library/contextvars.rst
@@ -0,0 +1,279 @@
+:mod:`contextvars` --- Context Variables
+========================================
+
+.. module:: contextvars
+   :synopsis: Context Variables
+
+.. sectionauthor:: Yury Selivanov <yury@magic.io>
+
+--------------
+
+This module provides APIs to manage, store, and access non-local
+state.  The :class:`~contextvars.ContextVar` class is used to declare
+and work with *Context Variables*.  The :func:`~contextvars.copy_context`
+function and the :class:`~contextvars.Context` class should be used to
+manage the current context in asynchronous frameworks.
+
+Context managers that have state should use Context Variables
+instead of :func:`threading.local()` to prevent their state from
+bleeding to other code unexpectedly, when used in concurrent code.
+
+See also :pep:`567` for additional details.
+
+.. versionadded:: 3.7
+
+
+Context Variables
+-----------------
+
+.. class:: 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
+   :meth:`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.  :class:`Context` objects hold strong
+   references to context variables which prevents context variables
+   from being properly garbage collected.
+
+   .. attribute:: ContextVar.name
+
+      The name of the variable.  This is a read-only property.
+
+   .. method:: 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 :exc:`LookupError`.
+
+   .. method:: 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 :class:`~contextvars.Token` object that can be used
+      to restore the variable to its previous value via the
+      :meth:`ContextVar.reset` method.
+
+   .. method:: reset(token)
+
+      Reset the context variable to the value it had before the
+      :meth:`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 :meth:`ContextVar.set` method.
+   They can be passed to the :meth:`ContextVar.reset` method to revert
+   the value of the variable to what it was before the corresponding
+   *set*.
+
+   .. attribute:: Token.var
+
+      A read-only property.  Points to the :class:`ContextVar` object
+      that created the token.
+
+   .. attribute:: Token.old_value
+
+      A read-only property.  Set to the value the variable had before
+      the :meth:`ContextVar.set` method call that created the token.
+      It points to :attr:`Token.MISSING` is the variable was not set
+      before the call.
+
+   .. attribute:: Token.MISSING
+
+      A marker object used by :attr:`Token.old_value`.
+
+
+Manual Context Management
+-------------------------
+
+.. function:: copy_context()
+
+   Returns a copy of the current :class:`~contextvars.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:: Context()
+
+   A mapping of :class:`ContextVars <ContextVar>` to their values.
+
+   ``Context()`` creates an empty context with no values in it.
+   To get a copy of the current context use the
+   :func:`~contextvars.copy_context` function.
+
+   Context implements the :class:`collections.abc.Mapping` interface.
+
+   .. method:: 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 :exc:`RuntimeError` when called on the same
+      context object from more than one OS thread, or when called
+      recursively.
+
+   .. method:: copy()
+
+      Return a shallow copy of the context object.
+
+   .. describe:: var in context
+
+      Return ``True`` if the *context* has a value for *var* set;
+      return ``False`` otherwise.
+
+   .. describe:: context[var]
+
+      Return the value of the *var* :class:`ContextVar` variable.
+      If the variable is not set in the context object, a
+      :exc:`KeyError` is raised.
+
+   .. method:: 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``.
+
+   .. describe:: iter(context)
+
+      Return an iterator over the variables stored in the context
+      object.
+
+   .. describe:: len(proxy)
+
+      Return the number of variables set in the context object.
+
+   .. method:: keys()
+
+      Return a list of all variables in the context object.
+
+   .. method:: values()
+
+      Return a list of all variables' values in the context object.
+
+
+   .. method:: 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 :mod:`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}\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 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(line)
+
+        writer.write(render_goodbye())
+        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:
+    #     telnet 127.0.0.1 8081
diff --git a/Doc/library/index.rst b/Doc/library/index.rst
index a925e10..da6a460 100644
--- a/Doc/library/index.rst
+++ b/Doc/library/index.rst
@@ -55,6 +55,7 @@ the `Python Package Index <https://pypi.python.org/pypi>`_.
    crypto.rst
    allos.rst
    concurrency.rst
+   contextvars.rst
    ipc.rst
    netdata.rst
    markup.rst