How to ConfigParse a file keeping multiple values for identical keys?

The accepted answer breaks config.sections(), it returns always an empty list (tested with Python 3.5.3). Replacing super(OrderedDict, self).__setitem__(key, value) by super().__setitem__(key, value) fixes this, but now config.get(section, key) returns a concatenated string, no longer a list of strings.

My solution is:

class ConfigParserMultiValues(collections.OrderedDict):

    def __setitem__(self, key, value):
        if key in self and isinstance(value, list):
            self[key].extend(value)
        else:
            super().__setitem__(key, value)

    @staticmethod
    def getlist(value):
        return value.split(os.linesep)

    config = configparser.ConfigParser(strict=False, empty_lines_in_values=False, dict_type=ConfigParserMultiValues, converters={"list": ConfigParserMultiValues.getlist})
    ...
    values = config.getlist("Section", "key") # => ["value1", "value2"]

The config INI file accepts duplicate keys:

[Section]
    key = value1
    key = value2

After a small modification, I was able to achieve what you want:

class MultiOrderedDict(OrderedDict):
    def __setitem__(self, key, value):
        if isinstance(value, list) and key in self:
            self[key].extend(value)
        else:
            super(MultiOrderedDict, self).__setitem__(key, value)
            # super().__setitem__(key, value) in Python 3

config = ConfigParser.RawConfigParser(dict_type=MultiOrderedDict)
config.read(['a.txt'])
print config.get("test",  "foo")
print config.get("test",  "xxx")

Outputs:

['value1', 'value2']
['yyy']

in python 3.8 you need to also add strict=False:

class MultiOrderedDict(OrderedDict):
    def __setitem__(self, key, value):
        if isinstance(value, list) and key in self:
            self[key].extend(value)
        else:
            super().__setitem__(key, value)

config = ConfigParser.RawConfigParser(dict_type=MultiOrderedDict, strict=False)
config.read(['a.txt'])
print config.get("test",  "foo")
print config.get("test",  "xxx")