Python: re-defining a class method at runtime [duplicate]

Question

I've defined a class like this:

class MyClass: def GetValue(self): return 5 def PrintValue(self): print self.GetValue()

For some instances of MyClass I need to re-define the GetValue() dinamically, i.e. something like this:

def GetAGoodValue(self): return 7 oneObject=MyClass() oneObject.GetValue=GetAGoodValue oneObject.PrintValue()

After re-defining I get the errror:

TypeError: not enough arguments; expected 1, got 0

If inside the PrintValue method I code instead:

print self.GetValue(self)

then the above code works, but only for those MyClass instances where the GetValue method is re-defined. Instances where the GetValue method is not re-defined yeld the error:

TypeError: too many arguments; expected 1, got 2

Any suggestion?

It's more a duplicate of the older stackoverflow.com/questions/972/… , which has a nicer answer as well. — user707650
– user707650, Commented Jun 3, 2014 at 10:02
Exactly. The originally linked question is about changing methods of a class, not of an object, which is a slightly different topic. — glglgl
– glglgl, Commented Jun 3, 2014 at 10:03

glglgl · Accepted Answer · 2014-06-03 10:00:17Z

1

If you assign a method to a single object instead of changing the whole class, you have to implement the ususally-done binding yourself.

So you have two options:

Either you do

oneObject.GetValue = lambda: GetAGoodValue(oneObject)

or you "ask" the function object what it would do if called as a class attribute:

oneObject.GetValue = GetAGoodValue.__get__(oneObject, MyClass)

so that it works as intended.

answered Jun 3, 2014 at 10:00

glglgl

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4 Comments

user1737538 Over a year ago

Sorry for being back so late, not enough time! Thank you for your answer. Your first suggestion works. The second gives me the following error: AttributeError: __get__ Maybe it's because I'm working with a very old version of Python (1.5.2): unfortunately I have to!

glglgl Over a year ago

@user1737538 Yes, the descriptor protocol (the thing with __get__) is newer than 1.5.2...

user1737538 Over a year ago

One more question: I'm trying to use your suggestion to implement another "function substitution". This time the function has some parameters, something like GetScaledValue(self,min,max). What I'm trying to do is to assign

oneObject.GetScaledValue=lambda: GetAGoodScaledValue(oneObject,min,max)' but I get the error 'TypeError: no arguments expected' when I call

self.GetScaledValue(10,100)` from inside the class. Sorry, but I'm very new to this language (used to .NET).

user1737538 Over a year ago

I got it: it must be oneObject.GetScaledValue=lambda min,max: GetAGoodScaledValue(oneObject,min,max).

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