References to member functions in Python

Question

I have a class similar to the below, which works well for my simple usage.

However calling the functions in this way means they don't have access to self. so I can't use any of the other member functions or variables. Note that some items share the same callback.

Is there a better way to reference or call the functions so that I have access to other items relating to self?

class Foo:
    def dostuff(self, bar):
        # ...
        # Do stuff with input data, occasionally invoking the right callback
        # function like so:
        self.options[i]['func'](mydata)

    def callback1(data):
        pass

    def callback2(data):
        pass

    options = [
        0x10: { 'name': 'cat', 'func': callback1 },
        0x20: { 'name': 'dog', 'func': callback2 },
        0x40: { 'name': 'emu', 'func': callback2 },
        0x80: { 'name': 'bat', 'func': callback2 }
    ]

Ned Batchelder · Accepted Answer · 2014-11-30 02:04:59Z

1

Since you don't have access to self, you will need to defer looking up the method. You can look it up by name inside dostuff:

class Foo:
    def dostuff(self, bar):
        # ...
        # Do stuff with input data, occasionally invoking the right callback
        # function like so:
        method = getattr(self, self.options[i]['func'])
        method(mydata)

    def callback1(self, data):
        pass

    def callback2(self, data):
        pass

    options = [
        0x10: { 'name': 'cat', 'func': 'callback1' },
        0x20: { 'name': 'dog', 'func': 'callback2' },
        0x40: { 'name': 'emu', 'func': 'callback2' },
        0x80: { 'name': 'bat', 'func': 'callback2' }
    ]

edited Nov 30, 2014 at 2:04

answered Nov 29, 2014 at 22:55

Ned Batchelder

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2 Comments

user2357112 Over a year ago

Don't forget to add self arguments to the callbacks.

David Over a year ago

Thanks for this, makes total sense.

Jim Dennis · Accepted Answer · 2014-11-29 23:05:55Z

1

Your member functions need to include a "self" parameter even if they are intended only for self dispatched calls.

In other words you need something like:

#!/bin/python
# ...
class ...
   def callback(self, data):
       # ...

... and it needs to be called with self.callback(some_data)

Also note that you've defined Too.options as class data rather than a member attribute. It's accessible without any instantiation, but you should keep your intentions clear to those reading your code by consistent referring to these as Foo.options and never as some_instance.options (the latter would work, but obscures the nature of .options by being easily mistaken for an instance member (attribute).

edited Nov 29, 2014 at 23:05

answered Nov 29, 2014 at 22:59

Jim Dennis

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1 Comment

David Over a year ago

Thank you for the second point, which I now understand. However the thung I was missing was explained by Ned, specifically he deferred lookup.

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