Cannot focus DynamicContainer

I would like to have the contents of a container update based on what is focused. I figured that using a DynamicContainer was the best way to achieve this. However, whenever I put a normally focusable widget in a DynamicContainer, it seems to be impossible to focus it. Here’s an example:

bindings = KeyBindings()
bindings.add("c-c")(lambda event: event.app.exit())
bindings.add("tab")(focus_next)
bindings.add("s-tab")(focus_previous)

regular_button = Button("Regular")
dynamic_button = DynamicContainer(lambda: Button("Dynamic"))

layout = Layout(
    HSplit([
        regular_button,
        dynamic_button,
    ]),
)

app = Application(layout=layout, key_bindings=bindings)
app.run()

In this example, I can focus regular_button but not dynamic_button. If I try to focus it using layout.focus(dynamic_button), I get the following stack trace:

Traceback (most recent call last):
  File "/home/garrett/.PyCharm2018.2/config/scratches/scratch_39.py", line 24, in <module>
    layout.focus(dynamic_button)
  File "/home/garrett/.local/share/virtualenvs/skiddie-x-macEj3/lib/python3.5/site-packages/prompt_toolkit/layout/layout.py", line 141, in focus
    raise ValueError('Invalid value. Container cannot be focused: %r' % (value, ))
ValueError: Invalid value. Container cannot be focused: <prompt_toolkit.layout.containers.DynamicContainer object at 0x7f725252e780>

If this is intended behavior, I would really appreciate it if anyone could let me know of another way to achieve what I’m trying to do.

Thanks, Garrett

[jonathanslenders]

Hi @lostatc,

I noticed the problem is on the following line:

dynamic_button = DynamicContainer(lambda: Button("Dynamic"))

This creates every a different button every single time. That doesn’t work because focus_next first tries to enumerate all widgets that are focusable, then checks in that list what the current widget is, and goes to the next one. By that time, this button is already gone. If we would set this specific instance as the focused button, the renderer also wouldn’t know where to find it.

Try to rewrite it as follows:

button = Button("Dynamic")
dynamic_button = DynamicContainer(lambda: button)

Maybe, that’s some thing that should be mentioned in the documentation of DynamicContainer.

[lostatc]

Thanks for all the help @jonathanslenders! That answers my question. I’ll go ahead and close this issue.