How to call a function inside python's match pattern?

June 13, 2022

I have code

join_prefix = lambda text: config.get("CM_PREFIX") + text

match shlex.split(message.message):
    case [join_prefix("setmode"), mode] if mode.isdigit():
        await self._set_mode_handler(message, int(mode))
        return
    case [join_prefix("getmode")]:
        await self._get_mode_handler(message)
        return

mode is not defined, so it is used for list unpacking.
But python throws an error:

    case [join_prefix("getmode")]:
TypeError: called match pattern must be a type

How do i call a function inside match pattern?

>Solution :

The pattern is not an expression, so you cannot build up a pattern dynamically with other expressions at runtime. In fact, [join_prefix("setmode"), mode] is not a list; it’s a sequence pattern consisting of a class pattern join_prefix("setmode") and a capture pattern mode.

Instead, you need to match against a namespace you define before the match statement with a value pattern (which, roughly speaking, is a dotted name, not a simple name). For example,

from types import SimpleNamespace


modes = SimpleNamespace(
    set=join_prefix("setmode"),
    get=join_prefix("getmode")
)


match shlex.split(message.message):
    case [modes.set, mode] if mode.isdigit():
        ...
    case [modes.get]:
        ...