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

检查 venv 模块文档和链接的 PR gh-151419，然后将其中的措辞与针对现有、非空、有效及损坏的 ENV_DIR 目录运行 python -m venv 时的行为进行比较。完成标准是文档解释这些行为，以及 --clear、--update 和 --update-deps 如何影响这些行为。