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Python
INI file
read and write
configuration file
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How to read and write INI file with Python3?

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Python

Introduction

Python's built-in configparser module reads and writes INI files without any external dependencies. To read: create a ConfigParser instance, call config.read('file.ini'), then access values with config['section']['key']. To write: set values on the ConfigParser object and call config.write(open('file.ini', 'w')). The module handles sections, key-value pairs, defaults, interpolation, and type conversion out of the box.

INI File Structure

ini
1; config.ini
2[DEFAULT]
3debug = false
4log_level = INFO
5
6[database]
7host = localhost
8port = 5432
9name = myapp
10user = admin
11password = secret
12
13[server]
14host = 0.0.0.0
15port = 8080
16workers = 4

Sections are enclosed in [], keys use = or : as delimiters, and comments start with ; or #.

Reading an INI File

python
1import configparser
2
3config = configparser.ConfigParser()
4config.read('config.ini')
5
6# List all sections
7print(config.sections())  # ['database', 'server']
8
9# Access values
10db_host = config['database']['host']
11db_port = config['database']['port']
12print(f"Database: {db_host}:{db_port}")  # Database: localhost:5432
13
14# Check if a section or key exists
15print(config.has_section('database'))     # True
16print(config.has_option('database', 'host'))  # True
17
18# Iterate over a section
19for key, value in config['database'].items():
20    print(f"  {key} = {value}")
21# Includes DEFAULT values: debug, log_level, host, port, name, user, password

Type Conversion

All values are stored as strings. Use typed getters for conversion:

python
1config = configparser.ConfigParser()
2config.read('config.ini')
3
4# String (default)
5host = config.get('database', 'host')  # 'localhost'
6
7# Integer
8port = config.getint('database', 'port')  # 5432
9
10# Float
11timeout = config.getfloat('server', 'timeout', fallback=30.0)  # 30.0
12
13# Boolean — recognizes yes/no, on/off, true/false, 1/0
14debug = config.getboolean('DEFAULT', 'debug')  # False
15
16# With fallback for missing keys
17max_conn = config.getint('database', 'max_connections', fallback=10)

Writing an INI File

python
1import configparser
2
3config = configparser.ConfigParser()
4
5# Set values
6config['DEFAULT'] = {'debug': 'false', 'log_level': 'INFO'}
7
8config['database'] = {
9    'host': 'localhost',
10    'port': '5432',
11    'name': 'myapp',
12    'user': 'admin',
13    'password': 'secret'
14}
15
16config['server'] = {
17    'host': '0.0.0.0',
18    'port': '8080',
19    'workers': '4'
20}
21
22# Write to file
23with open('config.ini', 'w') as f:
24    config.write(f)

Modifying an Existing File

python
1config = configparser.ConfigParser()
2config.read('config.ini')
3
4# Update a value
5config['database']['port'] = '3306'
6
7# Add a new section
8config['redis'] = {
9    'host': 'localhost',
10    'port': '6379',
11    'db': '0'
12}
13
14# Remove a key
15config.remove_option('database', 'password')
16
17# Remove a section
18config.remove_section('server')
19
20# Write changes back
21with open('config.ini', 'w') as f:
22    config.write(f)

Interpolation (Variable Substitution)

configparser supports referencing other values within the same section or DEFAULT:

python
1# config_with_interpolation.ini
2# [paths]
3# home = /home/user
4# data = %(home)s/data
5# logs = %(home)s/logs
6
7config = configparser.ConfigParser()
8config.read('config_with_interpolation.ini')
9
10print(config['paths']['data'])  # /home/user/data
11print(config['paths']['logs'])  # /home/user/logs
12
13# Extended interpolation uses ${section:key} syntax
14config = configparser.ConfigParser(
15    interpolation=configparser.ExtendedInterpolation()
16)
17# [database]
18# host = localhost
19# [connection]
20# url = postgresql://${database:host}:5432/mydb

Reading from a String

python
1config = configparser.ConfigParser()
2
3ini_string = """
4[app]
5name = MyApp
6version = 1.0.0
7"""
8
9config.read_string(ini_string)
10print(config['app']['name'])  # MyApp
11
12# Reading from a dictionary
13config.read_dict({
14    'app': {'name': 'MyApp', 'version': '2.0.0'},
15    'features': {'dark_mode': 'true'}
16})

Real-World Example

python
1import configparser
2from pathlib import Path
3
4def load_config(path='config.ini'):
5    config = configparser.ConfigParser()
6
7    # Read default config, then override with local
8    config.read(['config.defaults.ini', path])
9
10    return {
11        'db': {
12            'host': config.get('database', 'host'),
13            'port': config.getint('database', 'port'),
14            'name': config.get('database', 'name'),
15        },
16        'server': {
17            'host': config.get('server', 'host', fallback='127.0.0.1'),
18            'port': config.getint('server', 'port', fallback=8000),
19            'debug': config.getboolean('server', 'debug', fallback=False),
20        }
21    }
22
23settings = load_config()
24print(f"Server running at {settings['server']['host']}:{settings['server']['port']}")

Common Pitfalls

  • All values are strings: config['server']['port'] returns '8080' (a string), not 8080. Use getint(), getfloat(), or getboolean() for typed values. Passing the string directly to code expecting an integer causes TypeError.
  • DEFAULT section bleeds into all sections: Keys in [DEFAULT] appear in every section when iterating with config['section'].items(). This is by design — DEFAULT provides fallback values. If you want section-specific keys only, check config.options('section') and filter out DEFAULT keys.
  • Case-insensitive keys by default: configparser lowercases all keys. config['DB']['Host'] and config['DB']['host'] are the same. To preserve case, set config.optionxform = str before reading.
  • read() silently ignores missing files: config.read('nonexistent.ini') returns an empty list and does not raise an error. Check the return value or use config.read_file(open('file.ini')) which raises FileNotFoundError if the file does not exist.
  • Multi-line values need indentation: To store multi-line values, indent continuation lines with whitespace. Unindented lines are treated as new keys, causing MissingSectionHeaderError or corrupted data.

Summary

  • Use configparser.ConfigParser() with read(), get(), and write() for INI file operations
  • Use getint(), getfloat(), getboolean() for type-safe value retrieval with optional fallback defaults
  • The [DEFAULT] section provides fallback values inherited by all sections
  • Use %(key)s for basic interpolation or ExtendedInterpolation for cross-section references
  • Always open the file with 'w' mode when writing — config.write() outputs the entire configuration
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