Dynamically add method to instance

[ttmetro]

How do I add a function dynamically to a MicroPython object?

In standard python "MethodType" is used for this, but it seems missing from MicroPython. Is there a workaround?

Thanks,
Bernhard

[jickster]

Obj.thing = function

?


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[ttmetro]

I don't think that binds it to the instance. Hence no reference to "self".

I used a global as a workaround. Not pretty, but works.

[jickster]

I don't think that binds it to the instance. Hence no reference to "self".

I used a global as a workaround. Not pretty, but works.

Have you actually tried it?



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[ttmetro]

yes!

Also what I found with a web search is that this is exactly what

Code: Select all

MethodType

is for.

[stijn]

Not 100% sure what you mean, can you give an example maybe?

Possibly you mean something like

Code: Select all

class Foo:
  pass

def Stuff(obj):
  print('Stuff', obj)

f = Foo()
print(f)
f.stuff = lambda: Stuff(f)
f.stuff()

?

[mattyt]

I believe Bernhard is talking about adding a member function to an instance of a class. MethodType is the standard way to do this. It appears to be implemented in the micropython-lib types module.

[ttmetro]

Yes, that's what I want:

Code: Select all

from types import MethodType

class DynamicTest:

    def __init__(self):
        self.x = 5

    def a(self):
        return self.x

dt = DynamicTest()

def b(self):
    return self.x

dt.b = b

def c(self):
    return self.x

setattr(dt, 'c', MethodType(c, dt))

print('a', dt.a())
# print('b', dt.b())
print('c', dt.c())

Result:

• a works as expected.
• b fails: TypeError: function takes 1 positional arguments but 0 were given (also expected)
• c fails: TypeError: cannot create 'bound_method' instances

a and c work with cpython, b fails (as it should).

So seems like a feature that is missing from MicroPython. Fortunately there is a workaround, at least for my case.

Thanks for all the suggestions!