Dynamically assigning function implementation in Python

Question

I want to assign a function implementation dynamically.

Let's start with the following:

class Doer(object):

    def __init__(self):
        self.name = "Bob"

    def doSomething(self):
        print "%s got it done" % self.name

def doItBetter(self):
    print "Done better"

In other languages we would make doItBetter an anonymous function and assign it to the object. But no support for anonymous functions in Python. Instead, we'll try making a callable class instance, and assign that to the class:

class Doer(object):

    def __init__(self):
        self.name = "Bob"

class DoItBetter(object):

    def __call__(self):
        print "%s got it done better" % self.name

Doer.doSomething = DoItBetter()
doer = Doer()
doer.doSomething()

That gives me this:

Traceback (most recent call last): Line 13, in doer.doSomething() Line 9, in call print "%s got it done better" % self.name AttributeError: 'DoItBetter' object has no attribute 'name'

Finally, I tried assigning the callable to the object instance as an attribute and calling it:

class Doer(object):

    def __init__(self):
        self.name = "Bob"

class DoItBetter(object):

    def __call__(self):
        print "%s got it done better" % self.name


doer = Doer()
doer.doSomething = DoItBetter()
doer.doSomething()

This DOES work as long as I don't reference self in DoItBetter, but when I do it gives me an name error on self.name because it's referencing the callable's self, not the owning class self.

So I'm looking for a pythonic way to assign an anonymous function to a class function or instance method, where the method call can reference the object's self.

Nothing wrong with the very first approach that you rejected. Why are you making it so complex? — David Heffernan
– David Heffernan, Commented Apr 29, 2012 at 18:00
Your error is really odd - are you trying to run a Python script with a ruby interpreter? (Your file is called prog.rb, .py is the Python file extension). The error definitely isn't from Python - self isn't a keyword in Python. — Gareth Latty
– Gareth Latty, Commented Apr 29, 2012 at 18:02
My mistake- ran it on an online codepad with ruby settings- fixed — Yarin
– Yarin, Commented Apr 29, 2012 at 18:19

LeWoody · Accepted Answer · 2019-12-26 04:06:45Z

41

yak's answer works great if you want to change something for every instance of a class.

If you want to change the method only for a particular instance of the object, and not for the entire class, you'd need to use the MethodType type constructor to create a bound method:

from types import MethodType

doer.doSomething = MethodType(doItBetter, doer)

edited Dec 26, 2019 at 4:06

LeWoody

3,6494 gold badges28 silver badges31 bronze badges

answered Apr 29, 2012 at 18:11

Amber

531k89 gold badges643 silver badges558 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

yanlend Over a year ago

At least in Python 3, it should be doer.doSomething = MethodType(doItBetter, doer) . A good explanation of the different cases for methods and functions is given in stackoverflow.com/questions/972/…

Amber Over a year ago

@yanlend this question was asked for Python 2.

Yarin · Accepted Answer · 2012-04-29 18:23:13Z

40

Your first approach was OK, you just have to assign the function to the class:

class Doer(object):
    def __init__(self):
        self.name = "Bob"

    def doSomething(self):
        print "%s got it done" % self.name

def doItBetter(self):
    print "%s got it done better" % self.name

Doer.doSomething = doItBetter

Anonymous functions have nothing to do with this (by the way, Python supports simple anonymous functions consisting of single expressions, see lambda).

edited Apr 29, 2012 at 18:23

Yarin

186k156 gold badges414 silver badges527 bronze badges

answered Apr 29, 2012 at 18:03

yak

9,1012 gold badges34 silver badges25 bronze badges

1 Comment

Gareth Latty Over a year ago

+1 being anonymous doesn't matter here - all that matters is you can assign a function.

Collectives™ on Stack Overflow

Dynamically assigning function implementation in Python

2 Answers 2

2 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related