Dynamically calling nested functions based on arguments

Question

If I have the following Python class:

class Test(object):
    funcs = {
        "me"    : "action",
        "action": "action",
        "say"   : "say",
        "shout" : "say"
    }

    def dispatch(self, cmd):
        def say:
            print "Nested Say"

        def action:
            print "Nested Action"

        # The line below gets the function name as a string,
        # How can I call the nested function based on the string?
        Test.funcs.get(cmd, "say")

I would like to be able to do the following:

>>> Test().dispatch("me")
Nested Action
>>> Test().dispatch("say")
Nested Say

Any suggestions as to how I can go about this?

aaronasterling · Accepted Answer · 2010-11-25 06:54:19Z

4

I would probably do something like this:

def register(dict_, *names):
    def dec(f):
        m_name = f.__name__
        for name in names:
            dict_[name] = m_name
        return f
    return dec

class Test(object):

    commands = {}

    @register(commands, 'foo', 'fu', 'fOo')
    def _handle_foo(self):
        print 'foo'

    @register(commands, 'bar', 'BaR', 'bAR')
    def _do_bar(self):
        print 'bar'

    def dispatch(self, cmd):
        try:
            return getattr(self, self.commands[cmd])()
        except (KeyError, AttributeError):
            # Command doesn't exist. Handle it somehow if you want to
            # The AttributeError should actually never occur unless a method gets 
            # deleted from the class

Now, the class exposes a dict whose keys are commands for testing membership. All the methods and the dictionary are created only once.

t = Test()

if 'foo' in t.commands:
    t.dispatch('foo')

for cmd in t.commands:
    # Obviously this will call each method with multiple commands dispatched to it once
    # for each command
    t.dispatch(cmd)

Etc.

edited Nov 25, 2010 at 6:54

answered Nov 25, 2010 at 6:09

aaronasterling

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7 Comments

Noufal Ibrahim Over a year ago

+1. Looks like you beat me to it :). I was going to suggest something similar. I'd add one little thing which is a second parameter to register that will be a list of names the function can be called as using "dispatch". This would remove the hard handle_* naming requirement and allow multiple names for the same method as required by the original questioner.

mshsayem Over a year ago

'dict' should be 'dict_' in the first line of the 'dec' method, right?

aaronasterling Over a year ago

@Noufal Ibrahim. Excellent ideas. Implemented. @mshsayem. Thanks for pointing.

Noufal Ibrahim Over a year ago

Cool. This is a lot more DRY than the version into the post. :)

aaronasterling Over a year ago

@Noufal Ibrahim I don't know if it's more DRY since now I have to manually provide a command dispatch but it's definitely more flexible.

|

aaronasterling · Accepted Answer · 2010-11-25 05:38:47Z

2

class Test(object):

    def dispatch(self):
        def say():
            print "Nested Say"

        def action():
            print "Nested Action"

        funcs = {
            "me"    : action,
            "action": action,
            "say"   : say,
            "shout" : say
        }

        Test.funcs.get(cmd, say)()

Or, keeping your current structure:

class Test(object):

    funcs = {
        "me"    : "action",
        "action": "action",
        "say"   : "say",
        "shout" : "say"
        }

    def dispatch(self, cmd):
        def say():
            print "Nested Say"

        def action():
            print "Nested Action"

        locals()[Test.funcs.get(cmd, "say")]()

I find this design a bit odd, though. Why should the class-level dict know about dispatch's local functions?

edited Nov 25, 2010 at 5:38

aaronasterling

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answered Nov 25, 2010 at 5:28

Matthew Flaschen

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3 Comments

Mike Trpcic Over a year ago

I know I could do that, but I wanted to keep them as a "class level" attribute so I can access them in other locations in the code, and to avoid instantiating the dictionary every time I call that method.

aaronasterling Over a year ago

@Mike, the functions aren't class level methods as you have them defined. They're inner functions of the dispatch method. The dictionary will only be defined when the class is first created but the inner functions will be recreated each time the dispatch function runs.

Mike Trpcic Over a year ago

I want them exposed as class levels so other classes can check if any given command is available by doing Test.funcs.keys(). This way I don't need to instantiate a large hash on every call of the dispatch method. Eventually the methods will make use of instance variables, so I can't just make them instance methods, as the class level hash won't have reference to them. If you can recommend a better approach, I'd be glad to have my eyes opened.

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