———
This module provides APIs to manage, store, and access context-local state. The
class is used to declare and work with Context Variables. The
function and the
class should be used to manage the current context in asynchronous frameworks.
Context managers that have state should use Context Variables instead of
to prevent their state from bleeding to other code unexpectedly, when used in concurrent code.
See also
for additional details.
Added in version 3.7.
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 required name parameter is used for introspection and debug purposes.
The optional keyword-only default parameter is returned by
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.
objects hold strong references to context variables which prevents context variables from being properly garbage collected.
ContextVars are
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.
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
.
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
object that can be used to restore the variable to its previous value via the
method.
For convenience, the token object can be used as a context manager to avoid calling
manually:
var=ContextVar('var',default='default value')withvar.set('new value'):assertvar.get()=='new value'assertvar.get()=='default value'It is a shorthand for:
var=ContextVar('var',default='default value')token=var.set('new value')try:assertvar.get()=='new value'finally:var.reset(token)assertvar.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
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.
classcontextvars.Token
Token objects are returned by the
method. They can be passed to the
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
to automatically reset context variables. See
.
Tokens are
over the same type as the
which created them.
Added in version 3.14: Added support for usage as a context manager.
var
A read-only property. Points to the
object that created the token.
old_value
A read-only property. Set to the value the variable had before the
method call that created the token. It points to
if the variable was not set before the call.
MISSING
A marker object used by
.
Manual Context Management
contextvars.copy_context()
Returns a copy of the current
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.
classcontextvars.Context
A mapping of
to their values.
Context() creates an empty context with no values in it. To get a copy of the current context use the
function.
Each thread has its own effective stack of Context objects. The
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
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
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,
objects behave in a similar fashion to
when values are assigned in different threads.
Attempting to enter an already entered context, including contexts entered in other threads, raises a
.
After exiting a context, it can later be re-entered (from any thread).
Any changes to
values via the
method are recorded in the current context. The
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
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:
importcontextvarsvar=contextvars.ContextVar('var')var.set('spam')print(var.get())# 'spam'ctx=contextvars.copy_context()defmain():# '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.
varincontextReturn True if the context has a value for var set; return False otherwise.
context[var]Return the value of the var
variable. If the variable is not set in the context object, a
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
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:
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}\r\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(b'HTTP/1.1 200 OK\r\n')# status linewriter.write(b'\r\n')# headerswriter.write(render_goodbye())# bodywriter.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 or curl:# telnet 127.0.0.1 8081# curl 127.0.0.1:8081