# `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.

    The same token cannot be used twice.

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. A single token cannot reset a context variable more than once.

  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