install-poetry.py fails when using an empty PATH

[dmarinuswoodwing]

Description

We deploy poetry from cfn-init (which is a tool from AWS to run commands when an virtual machine is started). Apparently cfn-init starts processes with an (almost) empty environment.

Steps to reproduce

1. Start a shell with an empty environment:

   env -i bash

2. Execute the poetry installer:

   curl -sSL https://install.python-poetry.org/ | python3.7 - --version 1.2.0

Expected result

Either:

• Poetry properly installed (using a sane PATH) (preferred)
• A warning that you should configure your PATH
• A warning that some command could not be executed.

Actual result:

Retrieving Poetry metadata

# Welcome to Poetry!

This will download and install the latest version of Poetry,
a dependency and package manager for Python.

It will add the `poetry` command to Poetry's bin directory, located at:

/root/.local/bin

You can uninstall at any time by executing this script with the --uninstall option,
and these changes will be reverted.

Installing Poetry (1.2.0): An error occurred. Removing partial environment.
Traceback (most recent call last):
  File "<stdin>", line 935, in <module>
  File "<stdin>", line 914, in main
  File "<stdin>", line 545, in run
  File "<stdin>", line 566, in install
  File "/usr/lib64/python3.7/contextlib.py", line 112, in __enter__
    return next(self.gen)
  File "<stdin>", line 638, in make_env
  File "<stdin>", line 624, in make_env
  File "<stdin>", line 314, in make
  File "/usr/lib64/python3.7/venv/__init__.py", line 66, in create
    self.setup_python(context)
  File "/usr/lib64/python3.7/venv/__init__.py", line 233, in setup_python
    copier(context.executable, path)
  File "/usr/lib64/python3.7/venv/__init__.py", line 176, in symlink_or_copy
    shutil.copyfile(src, dst)
  File "/usr/lib64/python3.7/shutil.py", line 120, in copyfile
    with open(src, 'rb') as fsrc:
FileNotFoundError: [Errno 2] No such file or directory: ''

[neersighted]

How realistic a concern is an empty PATH? Poetry is merely invoking the venv module, where the actual non-handling of this condition occurs.

While it is possible for us to prevent the condition (by detecting an empty PATH and trying to guess what a sane default or the OS path-helper is), or to try and detect this error (by combining a FileNotFoundError and the condition of os.environ["PATH"]), I really think this is either out of scope for Poetry, or an unreasonable expectation by the end user.

Shouldn't this be solved in venv, if it should be solved at all?

[meeuw]

How realistic a concern is an empty PATH?

Like I reported, it happens with cfn-init and I ran into this problem.

I digged a bit futher and this is caused by venv trying to determine where the python executable is located by using sys._base_executable.
see: https://github.com/python/cpython/blob/16c33a9676e2f3ef330d09f2ab515c56636fa09f/Lib/venv/__init__.py#L130

The Poetry installer is recommended to be started without a full path so I presume Python uses PATH to determine which executable was started. If it cannot be found sys._base_executable is left empty and the installer fails.

This also means that setting PATH from the script doesn't help, I think the only solution would be to error out with a sane error message.

I will report a bug for the venv module but that wouldn't fix the Poetry installer.

Would it be acceptable to add:

if not sys._base_executable:
    print("Couldn't determine base executable, please provide a PATH or provide the full path to the base executable (ie: /usr/bin/python)")
    sys.exit(1)

[neersighted]

I'm not thrilled by the idea of introspecting an internal variable of the sys module that is undocumented -- is there any other good way to detect this error state?

[meeuw]

From my testing sys.executable has the same behavior as _base_executable.