Can't use transitions in kv lang

Question

App code:

from kivy.app import App
from kivy.lang import Builder
from kivy.factory import Factory
from kivy.uix.screenmanager import Screen
from kivy.uix.screenmanager import ScreenManager
from kivy.uix.screenmanager import FadeTransition<br/>
kv = '''
ScreenManagement:
    transition: FadeTransition()
    screen1:
    screen2:
<screen1>:
    name: 'one'
    Button:
        text: 'Hello!'
        on_release: app.root.current = 'two'
<screen2>:
    name: 'two'
    Button:
        text: 'Hello2'
        on_release: app.root.current = 'one'
'''<br/>
class screen1(Screen):
    pass
class screen2(Screen):
    pass
class ScreenManagement(ScreenManager):
    pass
class test23(App):
    def build(self):
        return Builder.load_string(kv)
        <br/>
if __name__ == '__main__':
    test23().run()

Every time I execute it I have the following traceback. What's wrong with it?

 runfile('/home/pc/python and kivy/test23.py', wdir='/home/pc/python and kivy')<br/>
 Traceback (most recent call last):<br/>
   File "<stdin>", line 1, in <module><br/>
   File "/usr/lib/python2.7/dist-packages/spyderlib/widgets/externalshell/sitecustomize.py", line 699, in runfile<br/>
     execfile(filename, namespace)<br/>
   File "/usr/lib/python2.7/dist-packages/spyderlib/widgets/externalshell/sitecustomize.py", line 81, in execfile<br/>
     builtins.execfile(filename, *where)<br/>
   File "/home/pc/python and kivy/test23.py", line 42, in <module><br/>
     test23().run()<br/>
   File "/usr/lib/python2.7/dist-packages/kivy/app.py", line 802, in run
     root = self.build()<br/>
   File "/home/pc/python and kivy/test23.py", line 39, in build
     return Builder.load_string(kv)<br/>
   File "/usr/lib/python2.7/dist-packages/kivy/lang.py", line 1921, in load_string<br/>
     self._apply_rule(widget, parser.root, parser.root)<br/>
   File "/usr/lib/python2.7/dist-packages/kivy/lang.py", line 2130, in _apply_rule
     e), cause=tb)<br/>
 kivy.lang.BuilderException: Parser: File "<inline>", line 3:
 ..<br/>
       1:<br/>
       2:ScreenManagement:<br/>
 >>    3:    transition: FadeTransition()<br/>
       4:    screen1:<br/>
       5:    screen2:<br/>
 ..<br/>
 BuilderException: Parser: File "<inline>", line 3:
 ..<br/>
       1:<br/>
       2:ScreenManagement:<br/>
 >>    3:    transition: FadeTransition()<br/>
       4:    screen1:<br/>
       5:    screen2:<br/>
 ..<br/>
 NameError: name 'FadeTransition' is not defined<br/>
   File "/usr/lib/python2.7/dist-packages/kivy/lang.py", line 1742, in create_handler<br/>
     return eval(value, idmap), bound_list<br/>
   File "<string>", line 3, in <module><br/><br/>

   File "/usr/lib/python2.7/dist-packages/kivy/lang.py", line 2115, in _apply_rule
     rctx['ids'])<br/>
   File "/usr/lib/python2.7/dist-packages/kivy/lang.py", line 1747, in create_handler
     cause=tb)<br/>

This question is very unclear at present and not focussed on a specific issue. Try and explain what you have tried so far to fix the various errors thrown up by the compiler and where exactly you are struggling. — Marcus, Oct 29 '16 at 16:19
@Sparky The question isn't unclear, the author is just confused, because this is specifically framework-related way of handling imports in framework's own language. Error is clear enough though = `NameError: name is not defined`. — Peter Badida, Oct 29 '16 at 16:49

Peter Badida · Accepted Answer · 2016-10-29T17:13:18.237

10

Transitions aren't widgets, nor properties, therefore they aren't imported by default while creating the enviroment for parsing the .kv file or kv string, so you'll need to import it at the top of the .kv file / string.

Example:

#:import FadeTransition kivy.uix.screenmanager.FadeTransition

edited Oct 29 '16 at 17:13

answered Oct 29 '16 at 16:43

Peter Badida

11,310
10
44
90

Keeweusr: sorry about not being specific. I'm script kiddie and a big time noob. So thank you for ur response – Poorna Chandra Oct 30 '16 at 04:19
@PoornaChandra I guess they downvoted you because of the formatting now, but the better made question, the less downvotes :) – Peter Badida Oct 30 '16 at 07:48

Robert Holmes · Answer 2 · 2017-08-16T03:51:45.923

2

At the top of the kv file you need an import. To get the transition it will be

#: import sm kivy.uix.screenmanager

ScreenManager: transition: sm.FadeTransition() screen1: screen2: ....

This is how I do it, and this works for all transitions a full documentation can be found at https://kivy.org/docs/api-kivy.uix.screenmanager.html

I am using python 2.7 and kivy 1.10.0

edited Aug 16 '17 at 03:51

answered Aug 16 '17 at 03:44

Robert Holmes

95
1
5

Can't use transitions in kv lang

2 Answers2