UPPER_CONSTRAINTS_FILE is deprecated and TOX_CONSTRAINTS_FILE is the new environment variable name that replaces it [1]. This allows to use upper-constraints file as more readable way instead of UPPER_CONSTRAINTS_FILE=<lower-constraints file>. [1] https://zuul-ci.org/docs/zuul-jobs/python-roles.html#rolevar-tox.tox_constraints_file [2] https://review.opendev.org/#/c/722814/ Change-Id: I56b20e563262e22969ef86df21f5c62fb1355193changes/85/761385/4