Lib/configparser.py

Source:

cpython 3.14 @ ab2d84fe1023/Lib/configparser.py ↗

configparser reads and writes INI-format configuration files. The core class is RawConfigParser; ConfigParser adds value interpolation. The module uses OrderedDict-like section storage and a pluggable Interpolation class hierarchy.

Map

Lines	Symbol	Role
1-100	Constants, exceptions	DEFAULTSECT, error hierarchy
101-200	Interpolation, BasicInterpolation, ExtendedInterpolation	Value expansion strategies
201-600	RawConfigParser	Core read/write, sections, options, no interpolation
601-900	ConfigParser	Adds %(key)s interpolation; SafeConfigParser is an alias
901-1000	SectionProxy	Mapping view of a single section
1001-1280	ConverterMapping, helper regexes	Type converters, line parsing patterns

Reading

File format

INI files have sections ([section]) and key-value pairs (key = value or key: value). Keys are case-normalized to lowercase by default. Continuation lines start with whitespace. Comment lines start with # or ;.

RawConfigParser internals

Sections are stored in self._sections, a dict of OrderedDict. The DEFAULT section (self._defaults) provides fallback values for all sections. Reads are case-insensitive via optionxform = str.lower.

# CPython: Lib/configparser.py:584 RawConfigParser.read_file
def read_file(self, f, source=None):
    ...
    self._read(f, source)

_read is the line-by-line parser that handles section headers, key-value pairs, continuation lines, and comments using the compiled regex SECTCRE and OPTCRE.

BasicInterpolation

%(key)s syntax: when get(section, option) is called, BasicInterpolation.before_get recursively substitutes %(...)s references within the same section or DEFAULT.

# CPython: Lib/configparser.py:388 BasicInterpolation.before_get
def before_get(self, parser, section, option, value, vars):
    L = []
    self._interpolate_some(parser, option, L, value, section, vars, 1)
    return ''.join(L)

Recursion depth is capped at MAX_INTERPOLATION_DEPTH = 10 to prevent infinite loops.

ExtendedInterpolation

Uses ${section:key} syntax and allows cross-section references:

# CPython: Lib/configparser.py:416 ExtendedInterpolation.before_get
def before_get(self, parser, section, option, value, vars):
    L = []
    self._interpolate_some(parser, option, L, value, section, vars, 1)
    return ''.join(L)

Patterns like ${othersection:key} retrieve key from othersection.

Type converters

ConfigParser ships converters for int, float, and boolean. getboolean accepts yes/no, true/false, on/off, 1/0. Custom converters can be added via the converters constructor argument, which automatically generates get<type> methods.

# CPython: Lib/configparser.py:768 ConfigParser.getboolean
_boolean_states = {'1': True, 'yes': True, 'true': True, 'on': True,
                   '0': False, 'no': False, 'false': False, 'off': False}

Write

write(fileobject) serializes all sections and options back to INI format, preserving the section order.

gopy notes

Status: not yet ported. The Go port would store sections as map[string]map[string]string with a separate defaults map. Interpolation can be a pluggable interface. The line parser is straightforward regex-based Go code.