Error: "Invalid property type" while checking TextInput - kivy-language

Question

I'm trying to check if user's TextInput is empty but getting error "Invalid property type". I'm programming in kivy.
My kv code:
<Label>
font_size: self.height * .15
background_color: (59/255,191/255,174/255,1)
canvas.before:
Color:
rgba: self.background_color
Rectangle:
size: self.size
pos: self.pos
color: (0/255,0/255,0/255,1)
<TextInput>
multiline: False
input_filter: "int"
font_size: self.height * .15
background_normal: ""
background_color: (192/255,251/255,243/255,1)
color: (0/255,0/255,0/255,1)
<Button>
font_size: self.height * .25
color: (0/255,0/255,0/255,1)
pos_hint: {'center_x': .5}
size_hint: (1, .3)
background_normal: ""
background_color: (8/255, 122/255, 111/255, 1)
<MyLayout>
BoxLayout:
orientation: 'vertical'
size: root.width, root.height
padding: 1
spacing: 1
TextInput:
id: calc_output
text: "waiting for data..."
halign: 'center'
valign: 'center'
font_size: self.height * .20
color: (0/255,0/255,0/255,1)
background_normal: ""
background_color: (1,1,1,1)
size_hint: (1, .3)
GridLayout:
cols: 1
size: root.width, root.height
GridLayout:
cols: 2
padding: 1
spacing: 1
Label:
text: "Broj rođenih"
TextInput:
id: N
Label:
text: "Broj umrlih"
TextInput:
id: M
Label:
text: "Broj stanovnika prve godine"
TextInput:
id: bspg
Label:
text: "Broj stanovnika druge godine"
TextInput:
id: bsdg
Button:
text: "Definiraj tip općeg kretanja stanovništva"
on_press: root.tipopceg()
I tried to check it with:
if - else
while not
try - except
if len()...
Everything I tried I would put in this part of code:
<TextInput>
multiline: False
input_filter: "int"
while not id:
print("You forgot...")
break
or inside every TextInput
Label:
text: "Broj rođenih"
TextInput:
id: N
while not N:
print("You forgot...")
break
but every time I get same error. Since I'm new in this world of programming, I guess I made some minor mistake why program doesn't recognize "id" of every user's TextInput and return error.
Any thoughts?

Answer 1

I don't know if I'm missing something here as well, but ain't kivy a library ? I think the whole purpose of kivy is to make a interface in a more practical way and separated from your logic statements, so the kivy language is made only for interface creation. In that case, I think you are supposed to write your logic part using python, and then call your widgets if you want to make any operation with their properties.
I suggest you take the user input through python, and then use it to define a property of the widget:
self.userinput = TextInput(multiline=False)
Now you can take the property TextInput.text as the actual text inputed by the user. You can check if it's blank with:
if len(self.userinput.text) == 0:
#do something