expandvars

Expand system variables Unix style

Inspiration

This module is inspired by GNU bash's variable expansion features. It can be used as an alternative to Python's os.path.expandvars function.

A good use case is reading config files with the flexibility of reading values from environment variables using advanced features like returning a default value if some variable is not defined. For example:

[default]
my_secret_access_code = "${ACCESS_CODE:-default_access_code}"
my_important_variable = "${IMPORTANT_VARIABLE:?}"
my_updated_path = "$PATH:$HOME/.bin"
my_process_id = "$$"
my_nested_variable = "${!NESTED}

NOTE: Although this module copies most of the common behaviours of bash, it doesn't follow bash strictly. For example, it doesn't work with arrays.

Usage

from expandvars import expandvars

print(expandvars("$PATH:${HOME:?}/bin:${SOME_UNDEFINED_PATH:-/default/path}"))
# /bin:/sbin:/usr/bin:/usr/sbin:/home/you/bin:/default/path

Examples

For now, refer to the test cases to see how it behaves.

TIPs

nounset=True

If you want to enable strict parsing by default, (similar to set -u / set -o nounset in bash), pass nounset=True.

# All the variables must be defined.
expandvars("$VAR1:${VAR2}:$VAR3", nounset=True)

# Raises UnboundVariable error.

NOTE: Another way is to use the ${VAR?} or ${VAR:?} syntax. See the examples in tests.

EXPANDVARS_RECOVER_NULL="foo"

If you want to temporarily disable strict parsing both for nounset=True and the ${VAR:?} syntax, set environment variable EXPANDVARS_RECOVER_NULL=somevalue. This helps with certain use cases where you need to temporarily disable strict parsing of critical env vars, e.g. in testing environment, without modifying the code.

e.g.

EXPANDVARS_RECOVER_NULL=foo myapp --config production.ini && echo "All fine."

WARNING: Try to avoid export EXPANDVARS_RECOVER_NULL because that will disable strict parsing permanently until you log out.

Contributing

To contribute, setup environment following way:

First you need to install poetry.

Then

# Clone repo
git clone https://github.com/sayanarijit/expandvars && cd expandvars

# Install poetry dependencies
poetry install

• Follow general git guidelines.
• Keep it simple. Run poetry run black to auto format the code.
• Test your changes locally by running poetry run pytest (pass --cov --cov-report html for browsable coverage report).
• If you are familiar with tox, you may want to use it for testing in different python versions.