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.

Customization

You can customize the variable symbol and data used for the expansion by using the more general expand function.

from expandvars import expand

print(expand("%PATH:$HOME/bin:%{SOME_UNDEFINED_PATH:-/default/path}", environ={"PATH": "/example"}, var_symbol="%"))
# /example:$HOME/bin:/default/path

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.