Question

While I’m aware that you can’t reference self directly in a decorator, I was wondering if it’s bad practice to work around that by pulling it from args[0]. My hunch is that it is, but I want to be sure.

To be more specific, I’m working on an API to a web service. About half the commands require a token to be passed that can be later used to undo it. What I would like is to make that token an optional parameter and if none is supplied, to generate one. Generating a token requires making an authenticated call to the server, which needs data from the object.

While I know I could do it:

def some_command(self, ..., undo_token = None):
    if undo_token = None:
        undo_token = self.get_undo_token()
    ...
    return fnord

I feel like there could be a better way than to have the same code in a dozen or so methods. My thought was to write a decorator:

@decorator
def undoable(fn, *args, **kwargs):
    if 'undo_token' not in kwargs:
        kwargs['undo_token'] = args[0].get_undo_token()
    return (fn(*args, **kwargs), kwargs['undo_token'])

So I can more cleanly write

@undoable
def some_command(self, ...):
    ...
    return foo

@undoable
def some_other_command(self, ...):
    ...
    return bar

Am I setting myself up for trouble down the line?

Answer 1

I don’t understand what you’re coding for undoable — that’s not how decorators are normally coded and I don’t know where that @decorator is coming from (is there a from youforgottotelluswhence import decorator or something even more evil? see why I can’t stand the use of from to build “artificial barenames” instead of using nice decorated names?-).

With normal decorator coding, e.g….:

import functools

def undoable(f):
    @functools.wraps(f)
    def wrapper(self, *a, **k):
        tok = k.get('undo_token')
        if tok is None:
            tok = k['undo_token'] = self.get_undo_token()
        return f(self, *a, **k), tok
    return wrapper

there’s absolutely no problem naming the wrapper’s first, mandatory positional argument self, and much gain of clarity in using this rather than the less-readable args[0].