DEVELOPMENT: Add support for "direnv"

* On entering this directory: environment-vars are setup
* On leaving  this directory: original environment-vars are restored

OPTION:

* Activate virtualenv on entering this directory.
  HINT: Rename ".envrc.use_venv.disabled" to ".envrc.use_venv"
* Support use of PEP-0582 "__pypackages/${PYTHON_VERSION}/" subtree.

Author: jenisys

.envrc

@@ -0,0 +1,29 @@
+# ===========================================================================
+# PROJECT ENVIRONMENT SETUP: .envrc
+# ===========================================================================
+# SHELL: bash (or similiar)
+# REQUIRES: direnv >= 2.26.0  -- Needed for: source_env_if_exists, ...
+# USAGE:
+#   # -- BETTER: Use direnv (requires: Setup in bash -- $HOME/.bashrc)
+#   # BASH PROFILE NEEDS: eval "$(direnv hook bash)"
+#   direnv allow .
+#
+# SIMPLISTIC ALTERNATIVE (without cleanup when directory scope is left again):
+#   source .envrc
+#
+# SEE ALSO:
+#   * https://direnv.net/
+#   * https://peps.python.org/pep-0582/   Python local packages directory
+# ===========================================================================
+# MAYBE: HERE="${PWD}"
+
+# -- USE OPTIONAL PARTS (if exist/enabled):
+dotenv_if_exists .env
+source_env_if_exists .envrc.use_venv
+source_env_if_exists .envrc.use_pep0582
+
+# -- SETUP-PYTHON: Prepend ${HERE} to PYTHONPATH (as PRIMARY search path)
+# SIMILAR TO: export PYTHONPATH="${HERE}:${PYTHONPATH}"
+path_add PYTHONPATH .
+
+source_env_if_exists .envrc.override

.envrc.use_pep0582.disabled

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+# ===========================================================================
+# PROJECT ENVIRONMENT SETUP: tag-expressions/python/.envrc.use_pep0582
+# ===========================================================================
+# DESCRIPTION:
+#   Setup Python search path to use the PEP-0582 sub-directory tree.
+#
+# SEE ALSO:
+#   * https://direnv.net/
+#   * https://peps.python.org/pep-0582/   Python local packages directory
+# ===========================================================================
+
+if [ -z "${PYTHON_VERSION}" ]; then
+    # -- AUTO-DETECT: Default Python3 version
+    # EXAMPLE: export PYTHON_VERSION="3.9"
+    export PYTHON_VERSION=$(python3 -c "import sys; print('.'.join([str(x) for x in sys.version_info[:2]]))")
+fi
+echo "USE: PYTHON_VERSION=${PYTHON_VERSION}"
+
+# -- HINT: Support PEP-0582 Python local packages directory (supported by: pdm)
+path_add PATH __pypackages__/${PYTHON_VERSION}/bin
+path_add PYTHONPATH __pypackages__/${PYTHON_VERSION}/lib
+
+# -- SIMILAR-TO:
+# export PATH="${HERE}/__pypackages__/${PYTHON_VERSION}/bin:${PATH}"
+# export PYTHONPATH="${HERE}:${HERE}/__pypackages__/${PYTHON_VERSION}/lib:${PYTHONPATH}"

.envrc.use_venv.disabled

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+# ===========================================================================
+# PROJECT ENVIRONMENT SETUP: tag-expressions/python/.envrc.use_venv
+# ===========================================================================
+# DESCRIPTION:
+#   Setup and use a Python virtual environment (venv).
+#   On entering the directory: Creates and activates a venv for a python version.
+#   On leaving  the directory: Deactivates the venv (virtual environment).
+#
+# SEE ALSO:
+#   * https://direnv.net/
+#   * https://github.com/direnv/direnv/wiki/Python
+#   * https://direnv.net/man/direnv-stdlib.1.html#codelayout-python-ltpythonexegtcode
+# ===========================================================================
+
+# -- VIRTUAL ENVIRONMENT SUPPORT: layout python python3
+# VENV LOCATION: .direnv/python-$(PYTHON_VERSION)
+layout python python3