Use INI interpolation for a derived local path

By Batu · English technical notes

Also published in our Blogger archive.

INI interpolation can express a derived local path without repeating its base component. ConfigParser uses basic interpolation by default, so %(root)s/cache refers to the root option in the same section. The example reads a paths section where root is the relative local directory local-data; retrieving cache expands the reference to local-data/cache.

The assertions deliberately inspect both forms. get(..., raw=True) returns the stored template, while an ordinary get() returns the interpolated value. This distinction matters for diagnostics and for tools that need to preserve a template rather than consume its resolved setting. Interpolation is resolved when the value is retrieved, so referenced options do not need to appear earlier in the INI text.

A derived string is not automatically a usable filesystem location. This example does not create a directory, normalize separators, reject traversal components, or confirm that the path exists. It only demonstrates configuration substitution. A literal percent sign in basic interpolation must be written as %%, and malformed or missing references raise interpolation-related errors when the value is read. read_string() and this Python 3 example require Python 3.2+; no newer API is used. The official interpolation documentation describes the default syntax and on-demand resolution.

AI assistance disclosure: This article was drafted with AI assistance and should be adapted to the target platform's path rules.

import configparser

ini_text = "[paths]\nroot = local-data\ncache = %(root)s/cache\n"
parser = configparser.ConfigParser()
parser.read_string(ini_text)

raw_cache = parser.get("paths", "cache", raw=True)
cache_path = parser.get("paths", "cache")
assert raw_cache == "%(root)s/cache"
assert cache_path == "local-data/cache"

print("cache={0}".format(cache_path))

cache=local-data/cache