🐛 fix(config): set_env cross-section substitution loses values - #3737
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gaborbernat merged 1 commit intoFeb 17, 2026
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When using cross-section substitution like {[testenv]set_env} in a
child environment, expanded values now correctly override default
environment variables like PYTHONHASHSEED. Previously the defaults
were materialized before the substitution was expanded.
Fixes tox-dev#2872.
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When a child environment inherits
set_envvia cross-section substitution (e.g.,set_env = {[testenv]set_env}), environment variables likePYTHONHASHSEEDwere silently replaced with their auto-generated defaults. This meant explicitly settingPYTHONHASHSEED=0in the base environment had no effect in child environments that referenced it — the child would always get a random hash seed. 🐛The root cause is a timing issue: tox applies default environment variables (including
PYTHONHASHSEED) during post-processing, before the{[testenv]...}substitution is expanded. Since the unexpanded reference is not yet in the raw config dict, the defaults are materialized first. When the substitution later expands and populates the raw dict, the already-materialized default takes precedence during lookup.The fix ensures that when a deferred substitution is expanded, any previously materialized defaults for the same keys are evicted, allowing the explicitly configured values to take priority as intended.
Fixes #2872.