contextvars — Context Variables — Python 3.7.4 documentation

# `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.

New 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.

    New 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.

  `Token.` `var`

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

  `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` is the variable was not set before the call.

  `Token.` `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.

  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}\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(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