Read a fallback value from DEFAULT in ConfigParser

By Batu · English technical notes

Also published in our Blogger archive.

ConfigParser has two similarly named mechanisms that behave differently. The special [DEFAULT] section supplies inherited option values to ordinary sections. The fallback= keyword of ConfigParser.get() is used only when the requested option is unavailable after normal lookup. A value in [DEFAULT] therefore takes precedence over a fallback= value.

This example reads an INI string with color = teal in [DEFAULT] and a separate [ui] section. parser.get("ui", "color", fallback="gray") returns teal: the section does not define color, so lookup inherits it from DEFAULT. The missing font option is absent from both places, so parser.get("ui", "font", fallback="sans") returns sans. The tuple assertion checks both concrete results, and the two printed lines make that precedence visible.

This inheritance can make configuration concise, but it also means a section may appear to contain an option that was not written under its own header. Do not use fallback= to override a value deliberately supplied by DEFAULT; select a different configuration design if per-call override semantics are needed. Values returned by get() are strings and may be interpolated by the default parser, so use getint(), getboolean(), a converter, or explicit validation when the application needs another type. The example uses read_string() solely for an in-memory fixture and does not test file-loading precedence.

This uses long-standing ConfigParser functionality available in supported Python 3 releases; no newer API is required. AI assistance disclosure: this article was drafted with AI assistance.

See the official configparser documentation.

import configparser

parser = configparser.ConfigParser()
parser.read_string(
    "[DEFAULT]\n"
    "color = teal\n"
    "\n"
    "[ui]\n"
    "theme = dark\n"
)

color = parser.get("ui", "color", fallback="gray")
font = parser.get("ui", "font", fallback="sans")

assert (color, font) == ("teal", "sans")
print(f"color={color}")
print(f"font={font}")

color=teal
font=sans