Kivy Screen Manager: Invalid property name

[Shoumik Das]

Hi!I am new to Kivy and I am just learning to build a simple with a couple of screens. I saw the Kivy Crash Course videos and came up with the following code:

The Python file looks like this (main.py):

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.tabbedpanel import TabbedPanel
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.graphics.context_instructions import Color
from kivy.uix.label import Label
from kivy.lang.builder import Builder


class sivaLoginScreen(Screen):
    def __init__(self,**kwargs):
        super(sivaLoginScreen,self).__init__(**kwargs)
        with self.canvas.before:
            Color(253,253,152,1)

class sivaTabbedScreen(Screen):
    pass


class sivaScreenManager(ScreenManager):
    pass

# Tell Kivy to directly load a file. If this file defines a root widget, it will be returned by the method.
root_widget = Builder.load_file('siva.kv')

class sivaApp(App):
    def build(self):
        # Initialize root widget
                return root_widget


if __name__ == '__main__':
    # Run application
    sivaApp().run()



The kv file looks something like this (siva.kv):


sivaScreenManager:
    sivaLoginScreen:


<sivaLoginScreen>:
    name='login_screen'
    FloatLayout:
        Label:
            id: login_label
            pos: 100, 100
            text: 'S.I.V.A'


When I run the app with python3 main.py I get the following error:

Traceback (most recent call last):
   File "main.py", line 30, in <module>
     root_widget = Builder.load_file('siva.kv')
   File "/usr/lib/python3/dist-packages/kivy/lang/builder.py", line 301, in load_file
     return self.load_string(data, **kwargs)
   File "/usr/lib/python3/dist-packages/kivy/lang/builder.py", line 368, in load_string
     parser = Parser(content=string, filename=fn)
   File "/usr/lib/python3/dist-packages/kivy/lang/parser.py", line 401, in __init__
     self.parse(content)
   File "/usr/lib/python3/dist-packages/kivy/lang/parser.py", line 510, in parse
     objects, remaining_lines = self.parse_level(0, lines)
   File "/usr/lib/python3/dist-packages/kivy/lang/parser.py", line 624, in parse_level
     'Invalid property name')
 kivy.lang.parser.ParserException: Parser: File "/home/shoumik/code/siva/siva.kv", line 6:
 ...
       4:
       5:<sivaLoginScreen>:
 >>    6:   name='login_screen'
       7:   FloatLayout:
       8:       Label:
 ...
 Invalid property name


Can you please help me understand where I am going wrong? In the 
official crash course videos, the name attribute was not defined as a 
property in the main.py file.

Thanks in advance!!

[Elliot Garbus]

Replace: name='login_screen'

 

With: name: ‘login_screen’

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[Shoumik Das]

Thank you very much, Elliot. So silly of me.  :D  That fixed the error. However, I get a black blank screen when I run the program. I was expecting a colored screen with a label on top of it. Could you guide me where I might be going wrong?

Thanks in advance!!

Shoumik

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[Elliot Garbus]

I combined the kv and the python file into one to make things a little easier.

You had set the color, but did not provide any direction to draw anything.  I move the canvas instructions to kv, and added the Rectangle command, setting the size and position to match the size and pos of the screen.

 

I moved the name of the screen to the instance, rather than the Screen definition.  You can put them in either place, but I find it is easier to manage when all of the names are together under the screen manager.

 

I changed the names of your classes to use CamelCase or CapWords case.  This is a convention for Python, and occasionally Kivy requires things this way. 

See: https://pep8.org/#class-names

 

I also put a black outline on your text, the white text on a yellow background was difficult to read.

 

 

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.tabbedpanel import TabbedPanel
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.screenmanager import ScreenManager, Screen

# from kivy.graphics.context_instructions import Color   # removed, put your color commands in kv

from kivy.uix.label import Label
from kivy.lang.builder import 

Builder

kv = """
SivaScreenManager:  # Use CamelCase for Classes
    SivaLoginScreen:
        name: 'login_screen'
 
<SivaLoginScreen>:
    canvas:
        Color: 
            rgba: 253/255, 253/255, 152/255, 1
        Rectangle:
            size: self.size
            pos: self.pos

    FloatLayout:
        Label:
            id: login_label
            pos: 100, 100
            text: 'S.I.V.A'

            font_size: 40
            outline_color: 0, 0, 0  # added a black outline to the chars, white is hard to see on the yellow background
            outline_width: 1
"""

class SivaLoginScreen(Screen):
    # def __init__(self, **kwargs):
    #     super(sivaLoginScreen, self).__init__(**kwargs)
    #     with self.canvas.before:
    #         Color(253, 253, 152, 1)
    pass


class SivaTabbedScreen(Screen):
    pass


class SivaScreenManager(ScreenManager):
    pass

# Tell Kivy to directly load a file. If this file defines a root widget, it will be returned by the method.

#root_widget = Builder.load_file('siva.kv')


class SivaApp(App):
    def build(self):
        # Initialize root widget
        return Builder.load_string(kv)


if __name__ == '__main__':
    # Run application
    SivaApp().run()

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[Shoumik Das]

That really helped. Thank you very much. Kivy is really fun and your guidance makes it encouraging for new-comers like us. Thanks again.

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[Elliot Garbus]

Glad to hear it helped you!  Enjoy the journey.

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