#3

[HananRosenthal]

What is the purpose of "_all.build()" ?

[ynonp]

_all.build() starts the process:

Each widget depends on other widgets to build. Building a widget should call build on all of its dependencies.

The widget named _all depends on all other widgets, so when building it is equivalent to calling build on each individual widget. That's what's causing the print in the next line.