User defined python variable

From a user voice request

provide some kind of global variable definition

Often I’d like to have a “global variable”, so that I can change a value at only one position, which is referenced by several filters, including Python programmable filters/sources.
The smartest thing would be, to have a slider widget, with which I could interactively change the value of this variable and so could examine different stages/points of view/slices… (depends on how I used the variable) of the dataset.