The `contextvars` library is a backport of the standard library `contextvars` module (introduced in Python 3.7 via PEP 567), providing APIs to manage, store, and access context-local state. It enables task-local variables in asynchronous code, ensuring data isolation across different coroutines or threads without explicit argument passing. The current version is 2.4, and it typically releases on demand for bug fixes or dependency updates.
pip install contextvarsVerified import paths — ran on the pinned version, not inferred.
This example demonstrates how `ContextVar` isolates state in asynchronous tasks. Each `handle_request` coroutine sets a `current_user` value, which remains local to its execution context, preventing state bleeding between concurrent tasks. The `token` returned by `set()` is crucial for `reset()`ing the variable to its previous state, often done in a `try-finally` block for proper cleanup.
Always declare `ContextVar` instances at the module level or as class attributes to ensure a single instance is created per application lifetime.
Ensure `reset(token)` is called in the same `Context` where its corresponding `set()` operation occurred and that each token is used only once. Using `try-finally` blocks or `contextlib.contextmanager` is a common pattern to manage `set`/`reset` pairs safely.
Always pair `ContextVar.set()` with `ContextVar.reset(token)` in a `try-finally` block to guarantee the context is restored, even if exceptions occur. Python 3.14+ `Token` objects support the context manager protocol for automatic reset.
Always provide a `default` value when creating a `ContextVar` if a value isn't guaranteed to be set, or wrap `ContextVar.get()` calls in `try-except LookupError` blocks. Alternatively, `ContextVar.get(default_value_for_this_call)` can be used.
Avoid using `contextvars` for sharing state directly between `ProcessPoolExecutor` processes. Instead, use multiprocessing primitives (e.g., `Queue`, `Pipe`, `shared_memory`) for inter-process communication if state needs to be truly shared or synchronized.
If using Python < 3.7, install the backport: `pip install contextvars`. If on Python 3.7+ and still encountering the error, ensure your Python installation or virtual environment is not corrupted, or check for issues with the underlying `_contextvars` C module.
Provide a default value when creating the `ContextVar` (e.g., `my_var = ContextVar('my_var', default='default_value')`) or when calling `get()` (e.g., `my_var.get('fallback_value')`). Alternatively, ensure `ContextVar.set()` is called before `get()` in the relevant execution context.Ensure that `ContextVar.set()` and `ContextVar.reset(token)` calls are made within the same execution context. If using frameworks that run parts of your code in different threads/tasks (e.g., FastAPI's sync dependencies), be mindful that contextvars do not automatically propagate changes back across thread boundaries. Re-evaluate if `contextvars` are the appropriate mechanism for cross-thread communication in such scenarios.
A `Token` returned by `ContextVar.set()` can only be used once with `ContextVar.reset()`. Ensure that you are not attempting to reset the variable multiple times with the same token. If you need to manage context changes, use `try...finally` blocks with `set()` and `reset(token)` to ensure proper cleanup, or consider using tokens as context managers (Python 3.14+).
Pass a function or other callable object as the first argument to `Context.run()`, followed by any arguments the callable needs. For example: `ctx.run(my_function, arg1, arg2)`.