Venv should check if directory exists while creating a virtual environment #106151

Hi, I have created an issue venv should check if directory exists while activating a virtual environment · Issue #106151 · python/cpython · GitHub and am advised to put it on discourse to decide if this really is an issue.

I recently came across a potential issue with the behavior of the venv module in Python. It seems that there is a missing check when activating a virtual environment. When the virtual environment is activated, the venv script adds the virtual environment’s bin directory to the PATH environment variable. However, there is no verification step to ensure that the virtual environment directory actually exists before modifying the PATH.

During my experimentation, I encountered a situation where the virtual environment was not properly activated, but the shell prompt displayed (venv) as if it were active. This led to confusion, as it gave the impression that the environment was activated when it was not.

To address this, I believe it would be beneficial to add a check in the venv script to verify the existence of the virtual environment directory before modifying the PATH. If the directory doesn’t exist, the script should throw an error to indicate that the virtual environment was not created successfully or that there was a failure during activation.

I understand that sharing a virtual environment folder with a different machine can have its own challenges, such as differences in processor architecture or dependencies on specific symlinks. However, I believe that adding this check and error handling would help prevent misleading scenarios where the environment appears to be active when it is not.

1 Like

How did that occur? The activation script shouldn’t be there unless the virtual environment was created, so how did you end up in a situation where you could activate your environment but have it not function? The only time that has ever happened to me was when I deleted the environment after activating it (which would not have been solved by this proposed change).

I created it on other machine and activated it on another machine. I have edited the discription now.

How did you share it between machines? Did you copy the directory? Use a shared file system? Something else?

Please provide step by step details, including the operating system(s) involved.

I hosted the directory(venv) on my local wifi network and my friend downloaded the venv, I created the venv in ubuntu and my friend activated it on pop-os.
My architecture: Linux-5.14.0-1059-oem-x86_64-with-glibc2.31, I guess he also as Linux-5.14

I created the env on 3.11 and my friend has only 3.10

below screenshots are from my friends machine

As you can see in zsh as well as in bash both it is not activating the environment but it is tricking the user that the environment is activated by adding venv in bash and adding an extra proshan on the right side panel on zsh , it should throw an error that virtual env is not activated and there is a failure.

the documentation says that we should do the sys.prefix != sys.base_prefix . check but we should throw an error if somehow the virtual environment doesn’t get activated

I suggest changing


if [ -d "$VIRTUAL_ENV" ]; then
    echo "Error: Path '$VIRTUAL_ENV' does not exist."
    exit 1

I am not able to edit the title, the title is misleading
Venv should check if directory exists while creating activating a virtual environment

Regardless of everything else, this is almost ertainly NOT going to succeed. I would strongly recommend creating each venv independently and not sharing them; venvs don’t generally work well across Python versions.

Yes, it’s not a big deal but we should add a check as he told me that venv is like a container and you can share it on any machine after creating it if you both have the same python version, and it actually misleads by adding (venv)

It’s possible for the directory to exist and the venv still won’t work, but I suppose a test like this can’t hurt. Someone more familiar with the activation scripts will need to chime in.

1 Like

That’s an oversimplification. I’ve also heard people say that a container is like a virtual computer, and various other parallels; they’re useful for explaining concepts, but can’t be used as “so this is what should be able to happen”.

That’s actually not true. Virtual environments are an isolation mechanism to make sure you don’t install stuff on your machine in unwanted places. There isn’t really any concept of sharing like you do with a container. Virtual environments are specifically designed to be lightweight enough you can just delete them and recreate them instead of move/share them. See the warning at venv — Creation of virtual environments — Python 3.11.4 documentation where it explicitly says, “environments are inherently non-portable”.

1 Like

got it, I will close the issue, but I want to understand one thing from your experience, The script should add (venv) only if the directory exists right? it should be smart enough to handle these kinds of scenarios. I want to hear your views on correctness of things in computer science as the documentation clearly says that If for any reason you need to move the environment to a new location, you should recreate it at the desired location and delete the one at the old location
I am not experienced enough to frame the question correctly
the script is correct according to its use case but it should not mislead the user by adding (venv) in bash and adding a directory name where the venv is activated on the right-side panel of zsh I even don’t know if it’s misleading or I am just sticking to my assumptions.

It definitely could, but I don’t know if they should. I personally don’t even use the activation scripts since they aren’t necessary for the interpreter to work.

You could also argue you misled the script by using it inappropriately against its design. You can’t guard every piece of software for every single possible (mis)use of it. Trying to cover every single case is a losing battle. Python is a language of “consenting adults” where we have to make some assumptions in order to simply function. For instance, do the activation scripts also have to verify the symlinks still work? Or that all the original information in pyvenv.cfg is accurate?

Since the activation scripts are just a nice-to-have and supporting multiple shells isn’t exactly fun, we have purposefully kept them simple for the common case and asked users to use them as they are designed to be used. Obviously if you want to come up with your own variant of the scripts that do the checks you want, then please go for it! Python’s code is licensed so you can do that without much issue.

1 Like

I agree with Brett. The scripts can’t be bulletproof, and it’s a slippery slope to try and cover new error cases. I suggest we don’t do anything here.

Got your points, I have closed the issue.

Actually, I’ve also ran into this issue a couple of times. A much more compelling example (imho) is simply moving/renaming the directory. While it’s understandable that copying the venv or trying to activate it on a different machine is an unsupported use case, the current state of things is kind of unfortunate.

mkdir -p /long/path/to/some/project/dir
cd /long/path/to/some/project/dir
python -m venv .venv

# A month later (re-organizing stuff)
mv /long/path/to/some /long/path/to/moved

# Two months later (continue working on project)
cd /long/path/to/moved/project/dir
. .venv/bin/activate

At this point the venv appears to be activated, but it actually isn’t. Trying to run scripts or install packages with pip would just silently use the user site (even though the shell would display the appropriate venv activation badge).

To be clear, I am not suggesting that the moved venv should work, but rather that it should produce an error (or at least a warning). The fact that it silently does the wrong thing is bad UX.

It also should be noted that at no point did the developer intend to “move the venv” in this hypothetical. Rather, the venv location was changed accidentally during operations that were unrelated to the venv, possibly even unrelated to the project itself (e.g. mv ~/projects ~/personal/projects). It is a bit unreasonable to expect developers to always keep track of whether any directory happens to contain any venvs.


maybe we should add the check and rewrite pyvenv.cfg if the directory doesn’t exist.

I am pretty sure that the absolute path to the old venv is used all over the place inside the venv (and even inside the package files installed in the venv). So updating the venv is probably impossible (I am not 100% sure, but pretty sure).

But yes, a simple warning/error if the activation script detects that it’s being sourced from a different absolute path than the one that was originally used during the venv creation should be doable.

That’s still not a supported use-case. Think of a virtual environment like a cache. It is designed to be lightweight and disposable. I personally never expect a cache to survive a directory rename or move (which are effectively the same thing).

If venv isn’t doing what you need then I would suggest coding up something that does what you want or help us make virtual environments even more disposable. I have a blog post at How virtual environments work that covers how they work. But the stdlib doesn’t have to solve all problems for all people, and I think this is one of those situations where it’s better for the community to solve their own needs than for the stdlib to do it for them.

I want to reiterate that the problem here isn’t that the venv doesn’t work after the move, but that it appears to work while silently doing “the wrong thing™”. I also wouldn’t expect a cache to survive a move, but I would expect said cache to be invalidated instead of producing nonsense results.

Pointing to the documentation or suggesting that we ought to implement our own version of venv is not constructive. The current standard venv implementation has a silent foot gun and I don’t see any reasons against adding a trivial check+warning to the activation script.

Suggesting that “you shouldn’t move venvs” at this point is like suggesting that you shouldn’t dereference null pointers or forget to free memory. Sure, these are also “unsupported use cases”, but that doesn’t preclude us from adding guardrails in cases where we CAN automatically detect that the venv got broken by a move/rename/copy/etc.

I don’t think that the activation script is such a performance critical piece of code, that we can’t spare a single realpath call and a string equality check.

I also don’t think that this is feature creep or a “slippery slope”. This isn’t C++ we are talking about, it’s Python. I think, that when something “isn’t supported” in Python, we should (whenever possible) strive to detect such cases and provide readable error messages instead of invoking “nasal demons”.