# Read the config file
config = eval(open('cfg.py').read())

# Use it like any other dict
config['Hello'] = 'Modified World'

# Write the changed config file back
lines = ['    {!r}: {!r},'.format(k, config[k]) for k in sorted(config)]
open('cfg.py', 'w').write('\n'.join(['# Modified config', '{']+lines+['}']))

# Sample cfg.py
{
    'Hello': 'World',
    'entry 1': 'This is a simple configuration file',
    'entry 2': 'loader and writer for python.',
    'entry 3': 'It can read and write any object that',
    'entry 4': 'can be reproduced with repr().',
    'sample dict': {'key': 'nested dict', 'purpose': 'test'},
    'sample list': [3.14, 'mixed list', 3000],
    'sample tuple': ('foo', 'bar', 123),
}