pyvenv-auto automatically activates a Python venv with pyvenv.
When you open a file in python-mode, it searches for the venv directory near the file, and activates it.
You can install the package from Melpa:
M-x package-install RET pyvenv-auto
You can enable pyvenv-auto as below.
(use-package pyvenv-auto
:hook ((python-mode . pyvenv-auto-run)))
When you open a Python file, pyvenv-auto searches for a venv directory with a name in pyvenv-auto-venv-dirnames.
The default value of the variable is (list "venv" ".venv").
The search behavior is similar to that of locate-dominating-file.
The directory name with a smaller index has higher priority than that with a greater index.