How to correctly hang an event in Kivy?

Question

Python

I'm trying to make an application in Kivy like this:

from kivy.app import App
from kivy.lang import Builder
from kivy.core.window import Window


class Program(App):
    def build(self):
        Window.clearcolor = (184/255.0, 228/255.0, 107/255.0, 255/255.0)
        Activity = f'''
FloatLayout:
    
    Button:
        text: "Check product!"
        pos: 300, 250
        size_hint: .2, .2
        background_normal: ''
        background_color: 228/255.0, 202/255.0, 31/255.0, 1
        on_press: {self}.test()

        '''
        return Builder.load_string(Activity)
    def test(self):
        return("test is work!")
   
if __name__ in ('__main__', '__android__'):
    Program().run()

But unfortunately it gives an error:

[INFO   ] [Logger      ] Record log in C:\Users\olegp\.kivy\logs\kivy_20-12-07_13.txt
[INFO   ] [deps        ] Successfully imported "kivy_deps.angle" 0.3.0
[INFO   ] [deps        ] Successfully imported "kivy_deps.glew" 0.3.0
[INFO   ] [deps        ] Successfully imported "kivy_deps.sdl2" 0.3.1
[INFO   ] [Kivy        ] v2.0.0rc4, git-b714003, 20201206
[INFO   ] [Kivy        ] Installed at "C:\Python380\lib\site-packages\kivy\__init__.py"
[INFO   ] [Python      ] v3.8.1 (tags/v3.8.1:1b293b6, Dec 18 2019, 22:39:24) [MSC v.1916 32 bit (Intel)]
[INFO   ] [Python      ] Interpreter at "C:\Python380\python.exe"
[INFO   ] [Factory     ] 186 symbols loaded
[INFO   ] [Image       ] Providers: img_tex, img_dds, img_sdl2, img_pil (img_ffpyplayer ignored)
[INFO   ] [Window      ] Provider: sdl2
[INFO   ] [GL          ] Using the "OpenGL" graphics system
[INFO   ] [GL          ] GLEW initialization succeeded
[INFO   ] [GL          ] Backend used <glew>
[INFO   ] [GL          ] OpenGL version <b'4.6.0 - Build 26.20.100.7870'>
[INFO   ] [GL          ] OpenGL vendor <b'Intel'>
[INFO   ] [GL          ] OpenGL renderer <b'Intel(R) UHD Graphics 620'>
[INFO   ] [GL          ] OpenGL parsed version: 4, 6
[INFO   ] [GL          ] Shading version <b'4.60 - Build 26.20.100.7870'>
[INFO   ] [GL          ] Texture max size <16384>
[INFO   ] [GL          ] Texture max units <32>
[INFO   ] [Window      ] auto add sdl2 input provider
[INFO   ] [Window      ] virtual keyboard not allowed, single mode, not docked
 Traceback (most recent call last):
   File "c:/Users/olegp/OneDrive/Рабочий стол/ki.py", line 26, in <module>
     Program().run()
   File "C:\Python380\lib\site-packages\kivy\app.py", line 949, in run
     self._run_prepare()
   File "C:\Python380\lib\site-packages\kivy\app.py", line 919, in _run_prepare
     root = self.build()
   File "c:/Users/olegp/OneDrive/Рабочий стол/ki.py", line 21, in build
     return Builder.load_string(Activity)
   File "C:\Python380\lib\site-packages\kivy\lang\builder.py", line 373, in load_string
     parser = Parser(content=string, filename=fn)
   File "C:\Python380\lib\site-packages\kivy\lang\parser.py", line 402, in __init__
     self.parse(content)
   File "C:\Python380\lib\site-packages\kivy\lang\parser.py", line 515, in parse
     rule.precompile()
   File "C:\Python380\lib\site-packages\kivy\lang\parser.py", line 264, in precompile
     x.precompile()
   File "C:\Python380\lib\site-packages\kivy\lang\parser.py", line 262, in precompile
     x.precompile()
   File "C:\Python380\lib\site-packages\kivy\lang\parser.py", line 187, in precompile
     self.co_value = compile(value, self.ctx.filename or '<string>', mode)
   File "<string>", line 10
     <__main__.Program object at 0x0326F8B0>.test()
     ^
 SyntaxError: invalid syntax

I need to attach the test method to the button, so that when you click on it, it works, please tell me how to do it right?