Context Variables¶

classcontextvars.ContextVar(name[,*,default])¶

This class is used to declare a new Context Variable, e.g.:

var:ContextVar[int]=ContextVar('var',default=42)

The requiredname parameter is used for introspection and debugpurposes.

The optional keyword-onlydefault parameter is returned byContextVar.get() when no value for the variable is foundin the current context.

Important: Context Variables should be created at the top modulelevel and never in closures.Context objects hold strongreferences to context variables which prevents context variablesfrom 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 thedefault argument of the method,if provided; or

• return the default value for the context variable,if it was created with one; or

• raise aLookupError.

set(value)¶

Call to set a new value for the context variable in the currentcontext.

The requiredvalue argument is the new value for the contextvariable.

Returns aToken object that can be usedto restore the variable to its previous value via theContextVar.reset() method.

reset(token)¶

Reset the context variable to the value it had before theContextVar.set() that created thetoken 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.

classcontextvars.Token¶

Token objects are returned by theContextVar.set() method.They can be passed to theContextVar.reset() method to revertthe value of the variable to what it was before the correspondingset.

var¶

A read-only property. Points to theContextVar objectthat created the token.

old_value¶

A read-only property. Set to the value the variable had beforetheContextVar.set() method call that created the token.It points toToken.MISSING is the variable was not setbefore the call.

MISSING¶

A marker object used byToken.old_value.

Manual Context Management¶

contextvars.copy_context()¶

Returns a copy of the currentContext object.

The following snippet gets a copy of the current context and printsall 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 forcontexts with a few context variables and for contexts that havea lot of them.

classcontextvars.Context¶

A mapping ofContextVars to their values.

Context() creates an empty context with no values in it.To get a copy of the current context use thecopy_context() function.

Every thread will have a different top-levelContextobject. This means that aContextVar object behaves in a similarfashion tothreading.local() when values are assigned in differentthreads.

Context implements thecollections.abc.Mapping interface.

run(callable,*args,**kwargs)¶

Executecallable(*args,**kwargs) code in the context objecttherun method is called on. Return the result of the executionor propagate an exception if one occurred.

Any changes to any context variables thatcallable makes willbe contained in the context object:

var=ContextVar('var')var.set('spam')defmain():# '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 aRuntimeError when called on the samecontext object from more than one OS thread, or when calledrecursively.

copy()¶

Return a shallow copy of the context object.

var in context

ReturnTrue if thecontext has a value forvar set;returnFalse otherwise.

context[var]

Return the value of thevarContextVar variable.If the variable is not set in the context object, aKeyError is raised.

get(var[,default])¶

Return the value forvar ifvar has the value in the contextobject. Returndefault otherwise. Ifdefault is not given,returnNone.

iter(context)

Return an iterator over the variables stored in the contextobject.

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 theirvalues in the context object.

asyncio support¶

Context variables are natively supported inasyncio and areready to be used without any extra configuration. For example, hereis a simple echo server, that uses a context variable to make theaddress of a remote client available in the Task that handles thatclient:

importasyncioimportcontextvarsclient_addr_var=contextvars.ContextVar('client_addr')defrender_goodbye():# The address of the currently handled client can be accessed# without passing it explicitly to this function.client_addr=client_addr_var.get()returnf'Good bye, client @{client_addr}\n'.encode()asyncdefhandle_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()'.whileTrue:line=awaitreader.readline()print(line)ifnotline.strip():breakwriter.write(line)writer.write(render_goodbye())writer.close()asyncdefmain():srv=awaitasyncio.start_server(handle_request,'127.0.0.1',8081)asyncwithsrv:awaitsrv.serve_forever()asyncio.run(main())# To test it you can use telnet:#     telnet 127.0.0.1 8081