[dimitri-yatsenko]

Review follow-up to #1617, from @MilagrosMarin's note. test_alter_explicit_context said Parent resolves only in the explicit context, but the test module imports Parent, so it resolves in the caller's namespace too. The test still told the two apart, but its comment did not hold. The reference is now Upstream, which nothing else defines, so it can resolve only through the explicit context. The alter tests pass (8).

[MilagrosMarin]

LGTM. Upstream is defined neither in test_alter.py nor in the generated pipeline module, so it can only resolve through the explicit context, and the comment now holds.

Two tiny, non-blocking notes:

1. "Defined nowhere else" is slightly broad: other test modules (test_cascade_delete.py, test_dependencies.py, test_trace.py, …) define an Upstream. None of them is in scope here, so "defined in neither this module nor the pipeline module" would be exact.
2. The docstring says the explicit context "takes precedence over the declaration context". With a name only the explicit context defines, the test shows that the context is used, not that it wins a conflict. Mapping a name the pipeline module also defines (e.g. Item) to a different table would test precedence itself, maybe as a follow-up; the previous version didn't test it either.