ConfigParser and String interpolation with env variable

If you want to expand some environment variables, you can do so using os.path.expandvars before parsing a StringIO stream:

import ConfigParser
import os
import StringIO

with open('config.ini', 'r') as cfg_file:
    cfg_txt = os.path.expandvars(cfg_file.read())

config = ConfigParser.ConfigParser()
config.readfp(StringIO.StringIO(cfg_txt))

the trick for proper variable substitution from environment is to use the ${} syntax for the environment variables:

[DEFAULT]
test_home=${HOME}

[test]
test_1=%(test_home)s/foo.csv
test_2=%(test_home)s/bar.csv

You can write custom interpolation in case of Python 3:

import configparser
import os


class EnvInterpolation(configparser.BasicInterpolation):
    """Interpolation which expands environment variables in values."""

    def before_get(self, parser, section, option, value, defaults):
        value = super().before_get(parser, section, option, value, defaults)
        return os.path.expandvars(value)


cfg = """
[section1]
key = value
my_path = $PATH
"""

config = configparser.ConfigParser(interpolation=EnvInterpolation())
config.read_string(cfg)
print(config['section1']['my_path'])

First of all according to the documentation you should use %(test_home)s to interpolate test_home. Moreover the key are case insensitive and you can't use both HOME and home keys. Finally you can use SafeConfigParser(os.environ) to take in account of you environment.

from ConfigParser import SafeConfigParser
import os


parser = SafeConfigParser(os.environ)
parser.read('config.ini')

Where config.ini is

[DEFAULT]
test_home=%(HOME)s

[test]
test_1=%(test_home)s/foo.csv
test_2=%(test_home)s/bar.csv