FR: Flask config field about flask-marshmallow

[sloria]

See comment in #222

Thanks for the PR. I think I'm going to have to pass on this since the use case here is too specific and makes too many assumptions to justify the added API surface.

If you need marshmallow validation for your configuration and you're using environment variables per the 12 factor methodology, I recommend taking a look at environs.

[mjpieters]

I think you misunderstood the use case here.

This is used to expose configuration stored in the standard Flask config to other consumers such as an SPA.

In the specific case were I created this field implementation, the front-end is rather TypeScript heavy but is configured using JSON embedded in a <script> tag, and we used Marshmallow schemas to define their structure.

The support to set Flask configuration with a FlaskConfig field was only implemented to keep the field symmetrical, is disabled by default and was kept primitive on purpose (hence the warnings in the documentation).

On a meta level: I would have appreciated discussion on this much, much earlier before I had put in the time to create a PR for this. I posted the FR 2 months ago. It would have saved us both some time.

[sloria]

I understand the use case. I still think it's too specific to justify the added code and API surface. The use case can be met without too much trouble using existing API:

from flask import Flask
from flask_marshmallow import Marshmallow

app = Flask(__name__)
app.config["SOME_ACCESS_TOKEN"] = "public-token"
ma = Marshmallow(app)


class MyConfig(ma.Schema):
    my_field = ma.Str()
    some_access_token = ma.Constant(app.config["SOME_ACCESS_TOKEN"], dump_only=True)


print(MyConfig().dump(dict(my_field="foo")))
# {'some_access_token': 'public-token', 'my_field': 'foo'}

You could easily write a helper to improve the ergonomics a bit:

def app_config_field(config_key, **kwargs):
    kwargs.setdefault("dump_only", True)
    return ma.Constant(app.config[config_key], **kwargs)


class MyConfig(ma.Schema):
    my_field = ma.Str()
    some_access_token = app_config_field("SOME_ACCESS_TOKEN")

I would have appreciated discussion on this much, much earlier before I had put in the time to create a PR for this. I posted the FR 2 months ago. It would have saved us both some time.

That would have been ideal, yes. Unfortunately, I've not been able to spend a lot of time on this lib the past couple months due to personal and professional priorities.

[mjpieters]

class MyConfig(ma.Schema):
    my_field = ma.Str()
    some_access_token = ma.Constant(app.config["SOME_ACCESS_TOKEN"], dump_only=True)

Unfortunately, that doesn't work with an app factory, as there is no app object available at import time. This is the norm for most real-world Flask apps, you'd have to generate the schema from the app factory, directly or via a Blueprint.record() hook, if you have a Blueprint to associate the schema with).

It also precludes the possibility of sharing the schema between apps (as a supporting library). I'm currently building a set of related APIs, where there is overlap in models and schemas that we'll put into a shared library, and FlaskConfig() covers that case much better.

[sloria]

To support app factories and blueprints, you could use a Function field.

def app_config_field(config_key, **kwargs):
    kwargs.setdefault("dump_only", True)
    return ma.Function(lambda _: current_app.config[config_key], **kwargs)

class MyConfig(ma.Schema):
    my_field = ma.Str()
    some_access_token = app_config_field("SOME_ACCESS_TOKEN")

But if your FlaskConfig field works best for your use case, then go ahead and use it. At work, we have a shared utility library with marshmallow Field and Schema classes that are shared across our services; this is a common practice.