Warn when `venv` target directory already contains an environment

Feature or enhancement

Proposal:

When a user runs:

python -m venv myenv

and the directory already contains a virtual environment, Python silently performs no action. This behavior can be confusing, as the user receives no indication that the command had no effect.

I propose that venv should display a simple warning when it detects an existing virtual environment so the user knows the operation didn’t make any changes.

Example of the suggested output:

$ python -m venv myenv
Existing virtual environment found at 'myenv'.
No changes were made.
Use '--clear' to recreate this environment.

This provides clarity while keeping behavior backward-compatible. Users who want to recreate the environment can explicitly use --clear.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

python -m venv のエントリーポイントから始め、対象ディレクトリにすでに環境が存在する場合の動作を比較します。既存の --clear オプションによってその動作がどのように変わるかを確認します。既存の環境が提案された警告を出力し、変更が行われなかったことを報告しつつ、明示的なクリアは引き続き利用できれば完了です。