Can I replace an existing method of an object in Python?
Solution 1
Using some tips from:
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!
Solution 2
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.
Jan Kukacka
PhD student at Technical University Munich. Focus on computer vision, medical imaging, machine learning.
Updated on July 12, 2022Comments
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Jan Kukacka almost 2 years
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 methodSayHi()
: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 silentlyself
as the first argument, but if I change it tolambda: True
, it calls the function with no arguments. The problem is: What if I want to change the defaultHasGoodMood
for another function which would also acceptself
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. -
Jan Kukacka over 5 yearsSee 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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PM 2Ring over 5 yearsIt 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.