Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

The assertion failed, but saying the process failed is confusing. Maybe just drop failed. (Unless this last fragment refers to the tuple return value.) I would rewrite as something like

If the exit status is zero, the function raises an :exc:`AssertionError`, which includes the *stdout* and *stderr* from the interpreter. Otherwise, the function returns an (*exit status*, *stdout*, *stderr*) tuple.