Read a config file in Python (JSON and INI, with defaults)

A config file keeps settings such as a port number or a debug switch outside your code, so you can change them without editing the program. This reader starts from default values and lets the file replace only the settings it lists.

The program

python

import json

DEFAULTS = {"host": "localhost", "port": 8000, "debug": False, "log_level": "INFO"}

# create a sample config file so this example has something to read
with open("config.json", "w", encoding="utf-8") as f:
    json.dump({"port": 8080, "debug": True}, f)


def load_config(path, defaults):
    config = dict(defaults)
    try:
        with open(path, encoding="utf-8") as f:
            config.update(json.load(f))
    except FileNotFoundError:
        print(f"{path} not found, using the defaults")
    return config


config = load_config("config.json", DEFAULTS)
for key, value in config.items():
    print(f"{key} = {value!r}")

print(load_config("missing.json", DEFAULTS)["port"])

output

host = 'localhost'
port = 8080
debug = True
log_level = 'INFO'
missing.json not found, using the defaults
8000

How it works

dict(defaults) makes a copy, so loading a file never changes DEFAULTS itself.
config.update(...) copies every setting from the file over the copy. Settings the file does not mention, here host and log_level, keep their default values.
JSON keeps types: 8080 comes back as an int and true as the Python value True, so you do not need to convert them.
A missing file raises FileNotFoundError. The program catches it and uses the defaults. A file with broken JSON raises json.JSONDecodeError, which is not caught, because a broken config file is something you want to see and fix.

Another way: an INI file

INI files are plain text with [sections] and key = value lines. The standard library reads them with configparser:

python

import configparser

with open("settings.ini", "w", encoding="utf-8") as f:
    f.write("[server]\nhost = example.com\nport = 8080\n\n[app]\ndebug = yes\n")

parser = configparser.ConfigParser()
parser.read("settings.ini", encoding="utf-8")

host = parser.get("server", "host")
port = parser.getint("server", "port")
debug = parser.getboolean("app", "debug")
timeout = parser.getint("server", "timeout", fallback=30)
print(host, port, debug, timeout)

output

example.com 8080 True 30

INI values are always read as text, so use getint() and getboolean() to convert them. getboolean() accepts yes/no, on/off, true/false and 1/0. fallback=30 is the value to use when the setting is missing.