pyenv activate <env> fails due to unbound variable when set -u is in effect

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Too many issues will kill our team's development velocity, drastically.
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Description

• Platform information (e.g. Ubuntu Linux 20.04): Void Linux (rolling release)
• OS architecture (e.g. amd64): x86_64
• pyenv version: pyenv 2.2.4-1
• pyenv-virtualenv version: pyenv-virtualenv 1.1.5 (python -m venv)
• Python version: Python 3.10.2
• virtualenv version (if installed): n/a
• Please attach the debug log of a faulty Pyenv invocation as a gist
  • If the problem happens in a Pyenv invocation, you can turn on debug logging by setting PYENV_DEBUG=1, e.g. env PYENV_DEBUG=1 pyenv install -v 3.6.4
  • If the problem happens outside of a Pyenv invocation, get the debug log like this:

    export PS4='+(${BASH_SOURCE:-}:${LINENO}): ${FUNCNAME[0]:+${FUNCNAME[0]}(): }'
    set -x
    <reproduce the problem>
    set +x
    

Using Bash, when set -u is in effect, pyenv activate <env> fails with unbound variable errors. This happens both in interactive shells and shell scripts in which eval "$(pyenv init -)" is used.

EDIT: Cleaned-up gists are in the next comment.

[ackalker]

Gist reproducing the problem in an interactive shell: here
Gist reproducing the problem in an interactive shell, with PYENV_DEBUG=1 set: here

[ackalker]

Similar issues with set -u and unbound variables when activating virtualenvs were found and fixed in pypa/virtualenv#1029 and python/cpython#3804 .

[ackalker]

@native-api
Please also look at my other PR #423, this fixes the case where a script with set -u but with PYENV_VIRTUALENV_DISABLE_PROMPT not set tries to activate a virtualenv. Sorry for splitting this up, I forgot to test this case on my side. I have now also fixed the Bats tests which were failing due to my change.