Python 虚拟环境符号链接如何工作？

Question

If I create a Python virtual environment like so:

$ python3 -m venv my_venv

... and then look at the Python binary in the bin directory like so:

$ ls -l my_env/bin/python*
lrwxrwxrwx 1 fred fred  7 Sep 12 15:57 my_env/bin/python -> python3
lrwxrwxrwx 1 fred fred 16 Sep 12 15:57 my_env/bin/python3 -> /usr/bin/python3

I see that the python is symlinked to the main global python . Therefore, what mechanism ensures that the Python packages we install after activating the virtual environment, that the packages are installed to site-packages ?

Answer 1

I was interested so I did a little searching and performed a couple experiments. A reference: How do I find the location of my Python site-packages directory?

I created a virtual environment my_venv in /tmp and did not activate it. On Linux activating a virtual environment ( my_venv/bin/activate ) adds the virtual environment's bin directory to the path; I found I could simulate that by using the full path; ie, executing ./my_venv/bin/python and ./my_venv/bin/pip directly. Results of such experimentation:

> ################### system executables
> python3 -m site       # system's python
sys.path = [
    <current directory>
    '/usr/lib/python310.zip',
    '/usr/lib/python3.10',
    '/usr/lib/python3.10/lib-dynload',
    '/usr/local/lib/python3.10/dist-packages',
    '/usr/lib/python3/dist-packages',
]
> pip3 show <a package>    # system's pip
...
Location: /usr/lib/python3/dist-packages
...
> pip3 show <package installed in the venv>
WARNING: Package(s) not found

> ################### venv executables
> /tmp/my_venv/bin/python3 -m site
sys.path = [
    <current directory>
    '/usr/lib/python310.zip',
    '/usr/lib/python3.10',
    '/usr/lib/python3.10/lib-dynload',
    '/tmp/my_venv/lib/python3.10/site-packages
]
> /tmp/my_venv/bin/pip3 show <package installed in the venv>
...
Location: /tmp/my_venv/lib/python3.10/site-packages
...
> /tmp/my_venv/bin/pip3 show <package installed on the system>
WARNING: Package(s) not found

I took a peek at the activation script, /tmp/my_venv/bin/activate . It set a couple environment variables, notably one named VIRTUAL_ENV . I set this environment variable to the path to my virtual environment, as it would have been set if I'd activated the virtual environment. Then I checked paths using the system's python -m site and pip show <package> , and got the same responses I'd gotten without the environment variable set. From this information I concluded that python and pip probably don't pay attention to the environment variable.

I then symlinked the system's python3 and pip3 executables to a local directory, and got the same responses from commands using these ( ./python3 and ./pip3 ) that I'd gotten from the system executables.

I then created the following directory structure:

bin/python3    # symlink to system python3
bin/pip3       # symlink to system pip3
lib/python3.10/site-packages/

These still gave me the system paths. However, as soon as I copied the file pyvenv.cfg from a virtual environment into my directory structure, it started to use my local paths!

Note: I copied pyvenv.cfg straight from the real virtual environment without changing it. That file contained the line "home = /usr/bin/".

I concluded that in order to determine the path to site-packages, Python uses the path of the executed file by preference (even if that's a symlink), and knows to look for a virtual environment configuration file one directory up from the executable. If the path is a symlink and doesn't yield a valid path, looks like it is able to back to the path of the actual executed file.

(this agrees with sinoroc 's summary of PEP 405 – Python Virtual Environments , written while I was fooling around)

This is deductive, of course. The source code would also provide the definitive answer.