Reads a child item out of a container glib.Variant instance. This includes variants, maybes, arrays, tuples and dictionary entries. It is an error to call this function on any other type of glib.Variant
It is an error if index_ is greater than the number of child items in the container. See Variant.nChildren.
The returned value is never floating. You should free it with Variant.unref when you're done with it.
There may be implementation specific restrictions on deeply nested values, which would result in the unit tuple being returned as the child value, instead of further nested children. glib.Variant is guaranteed to handle nesting up to at least 64 levels.
This function is O(1).