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Q: Is there a way to alter a method of an existing object in Python (3.6)? (By "method" I mean a function that is passed self as an argument.)

Example

Let's say I have a class Person having some very useful method SayHi():

class Person(object):
    Cash = 100
    def HasGoodMood(self):
        return self.Cash > 10

    def SayHi(self):
        if self.HasGoodMood():
            print('Hello!')
        else:
            print('Hmpf.')

>>> joe = Person()
>>> joe.SayHi()
Hello!

As you can see, the response of the person depends on their current mood computed by the method HasGoodMood(). A default person has good mood whenever they have more than 10$ cash on them.

I can easily create a person who does not care about the money and is happy all the time:

>>> joe.HasGoodMood = lambda: True
>>> joe.SayHi()
Hello!
>>> joe.Cash = 0
>>> joe.SayHi()
Hello!

Cool. Notice how Python knows that when using the original implementation of HasGoodMood, it passes silently self as the first argument, but if I change it to lambda: True, it calls the function with no arguments. The problem is: What if I want to change the default HasGoodMood for another function which would also accept self as a parameter?

Let's continue our example: what if I want to create a greedy Person who is only happy if they have more than 100$ on them? I would like to do something like:

>>> greedy_jack = Person()
>>> greedy_jack.HasGoodMood = lambda self: self.Cash > 100
TypeError: <lambda>() missing 1 required positional argument: 'self'

Unfortunately, this does not work. Is there some other way to change a method?

Disclaimer: The above example is just for demonstration purposes. I know that I could use inheritance or keep a cash threshold as a property of the Person. But that is not the point of the question.

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6
  • @Arne that dupe is pretty out of date, it refers to Python 2 only. Commented Sep 12, 2018 at 10:12
  • @DanielRoseman Does it not work in 3 any more? Commented Sep 12, 2018 at 10:13
  • @Arne There is some info relevant to Python 3 there, but it's not obvious. In fact, it's not easy to tell when most of those answers are talking about old-style vs new-style classes. Commented Sep 12, 2018 at 10:17
  • As far as I'm aware, the top 3 answers in Arne's dupe do apply to python 3. I don't see anything wrong with that dupe target. Commented Sep 12, 2018 at 10:52
  • @Aran-Fey Fair enough. And Aaron's answer was written with Python 3 in mind. Commented Sep 12, 2018 at 13:22

3 Answers 3

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35

Using some tips from:

Is it possible to change an instance's method implementation without changing all other instances of the same class?

you can do the following, by using the types module to assign a method to the object created without affecting the class. You need to do this because a function does not automatically receive the self object as the first variable, but a method does.

import types

joe = Person()
bob = Person()

joe.SayHi()
>>> Hello!

def greedy_has_good_mood(self):
    return self.Cash > 100

joe.HasGoodMood = types.MethodType(greedy_has_good_mood, joe)


joe.SayHi()
>>> Hmpf.
bob.SayHi()

>>> Hello!
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Comments

12

When you write a def in a class, and then call it on an instance, that's a method, and the mechanics of method-calling will fill in the self argument when you call it.

By assigning to HasGoodMood in your instance, you are not putting a new method there, but putting a function into the attribute. You can read the attribute to get the function, and call it, and though that looks like a method call, it's just calling a function that happens to be stored in an attribute. You won't get the self parameter supplied automatically.

But you already know what self is going to be, since you're assigning this function into one particular object.

greedy_jack.HasGoodMood = (lambda self=greedy_jack: self.Cash > 100)

This associates the function argument self with the current value of the variable greedy_jack.

Lambdas in Python can only be one line. If you needed a longer function, you could use a def instead.

def greedy_jack_HasGoodMood(self=greedy_jack):
    return self.Cash > 100

greedy_jack.HasGoodMood = greedy_jack_HasGoodMood

For a less hacky solution, see Andrew McDowell's answer.

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3 Comments

It would be good if you said something about descriptors and method binding, to explain what the OP's actual problem is. However, while I'm not a big fan of your band-aid solution, I don't think it deserves a downvote.
Why make the function take an argument at all? Something like greedy_jack_HasGoodMood(temperate_sarah) would not make sense, for example.
@HelloGoodbye The argument is needed to "save" the greedy_jack object reference together with the function in the form of a default argument. Otherwise, the function would look for any object called "greedy_jack" in its execution scope, and fail if none exists. This is a frequent catch with using lambdas...
1

Inheritance is the way to go.

It can be something as simple as:

class Person(object):
    Cash = 100
    def HasGoodMood(self):
        return self.Cash > 10

    def SayHi(self):
        if self.HasGoodMood():
            print('Hello!')
        else:
            print('Hmpf.')


class newPersonObject(Person):
    def HasGoodMood(self):
        return self.Cash > 100

>>> greedy = newClassPerson()
>>> greedy.SayHi()
hmpf

When you do greedy_jack.HasGoodMood = lambda self: self.Cash > 100 you're somewhat doing the same thing. You're only overriding greedy_jacks attributes. Using the way mentioned about, you can create greedy people, happy people, forever unhappy people, hippies etc.

A better option in my opinion would be to accept cash a parameter while defining the object. Hence, you can dynamically make people greedy or normal. (not tested)

class Person(object):
    def __init__(self, cash_to_be_happy):
        self.cash = cash_to_be_happy

    def HasGoodMood(self, has_money):
        return has_money > self.cash

    def SayHi(self, has_money):
        if self.HasGoodMood(has_money):
            print('Hello!')
        else:
            print('Hmpf.')

>>> joe = Person(100)
>>> joe.SayHi(150)
Hello!
>>> greedy_joe = Person(200)
>>> greedy_joe.SayHi(150)
Hmpf
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1 Comment

See the disclaimer: These are correct and definitely preferable solutions for a real scenario, however, the question is specifically not asking for them.

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