proxyvars

Callback-based object proxies in Python.

Example Usage

The library might have many use cases.

Asynchronous Apps

Imagine a web app that processes many requests asynchronously. A fun design we can learn from contextvars is that we can run request handlers in proper contexts with all the request-related data held in context variables instead of requiring endpoint functions to accept parameters like request. This approach heavily used in Flask, where one can access request data using a global proxy flask.request. With bare contextvars, we could achieve this:

from contextvars import ContextVar
from proxyvars import proxy
from asgi_framework import App  # some random ASGI web framework

class Request:
    args: dict[str, str]
    headers: dict[str, str]
    data: str
    # etc...

request_context: ContextVar[Request] = ContextVar("request_context")
app = App()

# the app will manage to set the appropriate Request object in the context
# before actually triggering the index() coroutine
@app.get("/")
async def echo():
    request: Request = request_context.get()
    # do something with request...
    return request.data

With proxyvars, we can skip the assignment and simply create a dynamic-lookup proxy:

# below the Request class
request = lookup_proxy(request_context)

@app.get("/")
async def echo():
    return request.data  # request delegated attribute access to request_context.get()

Flexible "Global" State

One of the crucial caveats of using global variables is reduction of modularity and flexibility. Typically we want to test our code in an easily-parametrizable environment, which becomes unobvious in case of some global state that other parts of your program might depend on. Problems arise in multi-threaded or asynchronous applications, where some global data typically should be thread- or task-local. Python offers threading.local and contextvars (respectively) for achieving these goals.

But sometimes global state is just the most convenient solution. Imagine running an app with a core App class with a config read from a file. We don't want to populate the configuration into attributes of the App instance and rather store configuration data properly in a class named Config.

We can store Config instance as an attribute config of our main class App that manages the whole application life cycle and then, everytime we need a config value, we can request the app's config attribute. With a lot of codebase, you can find this solution more and more tedious.

Creating a global app_config variable, not bound to an app, is not a good direction either though. What if you want to 2 apps with distinct configurations? Will you modify the global app_config to the proper Config object everytime?

Get That "Global" Experience With Proxyvars!

Simply create a global context variable.

app_config_var: ContextVar[Config] = ContextVar("app_config_var")
app_config: Config = lookup_proxy(app_config_var)

Now just run your tests in properly set-up contexts with app_config_var holding objects local to every thread, task or just a custom copy of the current context.

Mutable Immutables

Ever dreamt of thread-safe, mutable integers in Python? I don't think anybody did, but it's possible with lookup proxies & beloved contextvars just in case.

from contextvars import ContextVar
from proxyvars import proxy

my_iq_var = ContextVar("my_iq_var", 100)
my_iq = lookup_proxy(my_iq_var)

print(my_iq)  # 100

# Users of proxyvars typically have 200 IQ. Patch our current variable.
my_iq_var.set(200)

print(my_iq)  # 200

# Alright, let's be real. Assuming we have 200 IQ is a non-200 IQ behavior.
# Subtract 60 IQ points. We are entitled to 140 as programmers.
my_iq -= 60

print(my_iq_var.get())  # 140
print(my_iq)  # 60

This way, we got a mutable immutable. Or, more correctly, we got a proxy object that can change its state which is represented by an immutable object.

Have fun with proxyvars!

For Enterprise

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Installation

You might simply install it with pip:

pip install proxyvars

If you use Poetry, then you might want to run:

poetry add proxyvars

For Contributors

1. Fork the proxyvars repository on GitHub.

2. Install Poetry.
   Poetry is an amazing tool for managing dependencies & virtual environments, building packages and publishing them. You might use pipx to install it globally (recommended):

   pipx install poetry
   

   _{If you encounter any problems, refer to the official documentation for the most up-to-date installation instructions.}

   Be sure to have Python 3.8 installed—if you use pyenv, simply run:

   pyenv install 3.8
   

3. Clone your fork locally and install dependencies.

   git clone https://github.com/your-username/proxyvars path/to/proxyvars
   cd path/to/proxyvars
   poetry env use $(cat .python-version)
   poetry install
   

   Next up, simply activate the virtual environment and install pre-commit hooks:

   poetry shell
   pre-commit install
   

For more information on how to contribute, check out CONTRIBUTING.md.
Always happy to accept contributions! ❤️

Legal Info

© Copyright by Bartosz Sławecki (@bswck).
This software is licensed under the terms of MIT License.