`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, thread_inheritable])¶

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.

The optional keyword-only thread_inheritable parameter controls whether the variable’s current binding is automatically inherited by a threading.Thread when no explicit Context is supplied. If None (the default), sys.flags.thread_inherit_context determines whether the binding is inherited. If true, the binding is inherited regardless of that flag; if false, it is not inherited regardless of the flag.

This parameter only affects implicit context inheritance by threading.Thread.start(). The bindings are captured from the caller of start(), not the creator of the Thread object. It does not affect threads started directly through _thread or by extension modules.

An explicitly supplied thread context is not filtered, and neither are contexts copied for other purposes, including asynchronous tasks and asyncio.to_thread(). An inherited binding is an ordinary binding in the new thread’s context: it is visible to copy_context(), may be changed independently, and may in turn be inherited by threads started from that thread. Only the binding is copied; the bound object itself is not copied. A mutable bound object can therefore be accessed concurrently by the starting and new threads.

For a thread pool, inheritance occurs when each worker thread starts, not each time work is submitted. Consequently, ThreadPoolExecutor does not propagate the submitter’s current bindings to an existing worker merely because a variable is thread-inheritable.

Changed in version 3.16: Added the thread_inheritable parameter.

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.

ContextVars are generic over the type of their contained value.

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

Added in version 3.7.1.

thread_inheritable¶: The value of the thread_inheritable constructor parameter: None, True, or False. This is a read-only property.

Added in version 3.16.

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.

For convenience, the token object can be used as a context manager to avoid calling ContextVar.reset() manually:

var = ContextVar('var', default='default value')

with var.set('new value'):
    assert var.get() == 'new value'

assert var.get() == 'default value'

It is a shorthand for:

var = ContextVar('var', default='default value')

token = var.set('new value')
try:
    assert var.get() == 'new value'
finally:
    var.reset(token)

assert var.get() == 'default value'

Added in version 3.14: Added support for using tokens as context managers.

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.

Tokens support the context manager protocol to automatically reset context variables. See ContextVar.set().

Tokens are generic over the same type as the ContextVar which created them.

Added in version 3.14: Added support for usage as a context manager.

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, it is now possible to get the
    # 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

`contextvars` — Context Variables¶

Context Variables¶

Manual Context Management¶

asyncio support¶

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contextvars — Context Variables¶

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`contextvars` — Context Variables¶