How to change screen transition in different screens

Put the transitions in on_release events.

Button:
    text: 'next screen'
    on_release:
        app.root.transition = SlideTransition(direction='right')
        app.root.current = '2ndScreen'
...

For anyone else coming here looking for the answer to use entirely different transitions from one screen to another, this is what worked for me:

1. Load the desired transitions at the top of the KV file with lines like this (hash included):

#: import FadeTransition kivy.uix.screenmanager.FadeTransition
#: import SlideTransition kivy.uix.screenmanager.SlideTransition

2. Play around with this format changing the transition type to the desired type:

app.root.transition = FadeTransition(duration=1.2)

Keep it linked with your triggering event. I tried

on_release

for my login and sign off buttons but found that it was glitchy and not exactly the behavior I was going for. I ended up adding a line for

on_press

in order to create the desired transitions without the unexpected behavior. I kept the actual line that changes the screen under the

on_release

line.