Summary

Fixes #3574 — custom install_command was silently ignored when specified in tox.toml or pyproject.toml, always falling back to the default pip install command.

Root cause

Two bugs conspired to produce this behavior:

1. get_base_sections placed base sections under the env namespace. It used test_env(b) which wraps the section name under env.*, but env_run_base and env_pkg_base are top-level sections — [env_run_base] in tox.toml and [tool.tox.env_run_base] in pyproject.toml. This caused the loader to navigate to env.env_run_base (which doesn't exist) and return None, so no settings from env_run_base were applied.

2. The TOML replacement engine raised on unknown {...} placeholders. When Unroll processed strings like {packages} inside an install_command array, TomlReplaceLoader tried to resolve packages as a config key, failed with KeyError, and raised — causing the config loader to fall through to the default. The INI loader's equivalent (ReplaceReferenceIni) returns None for unresolvable references (keeping the original text), matching the documented contract of ReplaceReference.__call__.

Changes

• toml_pyproject.py: get_base_sections now constructs sections with the core prefix (or None), correctly placing env_run_base/env_pkg_base at the top level rather than under env.
• _replace.py: TomlReplaceLoader.__call__ returns None (instead of raising) when a KeyError occurs with no default, consistent with the INI loader and the ReplaceReference protocol.
• Test: Parameterized test verifying install_command is respected from both tox.toml and pyproject.toml.
• Changelog: Added docs/changelog/3574.bugfix.rst.

Test plan

• New parameterized test (tox.toml and pyproject.toml variants)
• All 27 show_config tests pass
• All 143 config/loader/source tests pass
• All 49 pip install tests pass