Document what happens when venv is run using existing directory

Documentation

Please consider specifying the behavior of venv module when the ENV_DIR already exists, and/or is not empty and/or contains an existing virtual environment. Currently the docs mentions only: “running this command creates the target directory”, and “creates” implies the command should be executed when ENV_DIR is not present [1].

However, I am able to run python -m venv .venv twice, and there is no error. There is no mention in the docs what happens when the directory is already there. Does venv skips creation of virtual environment? Does it try to create it anyways? What if ENV_DIR content is already a virtual environment? What if it is somewhat broken, does venv try to “fix” it?

[1] On the other hand, there are options --clear, --update, --update-deps, and those imply the ENV_DIR should be present. But it contradicts the previous assumption.

Linked PRs

• gh-151419

venv モジュールのドキュメントとリンク先の PR gh-151419 を確認し、その記述を、既存の空でない有効な ENV_DIR ディレクトリおよび壊れた ENV_DIR ディレクトリに対して python -m venv を実行した場合と比較します。完了条件は、ドキュメントでそれらの動作と、--clear、--update、--update-deps がそれらに与える影響を説明していることです。