Cumbersome to pass curly brackets to field converters

[vytas7]

At the time of this writing, it is cumbersome to instantiate converters with any parameter containing curly brackets in its expression.

For instance, the following attempt to validate color to be one of "red", "green" or "blue"

import falcon
import falcon.routing


class In(falcon.routing.BaseConverter):
    def __init__(self, values):
        self._values = frozenset(values)

    def convert(self, value):
        if value not in self._values:
            return None
        return value


class ColorResource:
    def on_get(self, req, resp, color):
        resp.media = {'color': color, 'path': req.path}


app = falcon.App()
app.router_options.converters['in'] = In

app.add_route('/colors/{color:in({"red", "green", "blue"})}', ColorResource())

... fails with the following ValueError: Field names must be valid identifiers (""red", "green", "blue"" is not valid).

One could easily work around the problem in this contrived example by using a list or tuple instead of the set literal, but what if we need curly brackets in a regex, or need to pass a dict?

[vytas7]

Perhaps we could support escaping curly brackets by doubling them ({{ & }}), just as the Python format string syntax does?

[CaselIT]

Perhaps we could support escaping curly brackets by doubling them ({{ & }}), just as the Python format string syntax does?

I like that idea, since it would be somewhat natural to python users to escape them that way. Especially since f-string has become a thing

[vytas7]

(It's the same {{ & }} even in str.format(), which dates back to early 3.x versions and 2.7.)