contextvars — Context Variables

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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