
class ConfigError(Exception):
    """ABC for configuration exceptions"""
    def __init__(self, module, *args, **kwargs):
        message = "Module '{0}': {1}".format(module, self.format(*args, **kwargs))

        super().__init__(message)

class ConfigKeyError(ConfigError, KeyError):
    def format(self, key):
        return "invalid option '{0}'".format(key)

class ConfigMissingError(ConfigError):
    def format(self, missing):
        return "missing required options: {0}".format(missing)
        super().__init__(module)

class ConfigAmbigiousClassesError(ConfigError):
    def format(self, ambigious_classes):
        return "ambigious module specification, found multiple classes: {0}".format(ambigious_classes)

class ConfigInvalidModuleError(ConfigError):
    def format(self):
        return "no class found"


class ConfigError(Exception):
    """ABC for configuration exceptions"""
    def __init__(self, module, *args, **kwargs):
        message = "Module '{0}': {1}".format(module, self.format(*args, **kwargs))

        super().__init__(message)


class ConfigKeyError(ConfigError, KeyError):
    def format(self, key):
        return "invalid option '{0}'".format(key)


class ConfigMissingError(ConfigError):
    def format(self, missing):
        return "missing required options: {0}".format(missing)


class ConfigAmbigiousClassesError(ConfigError):
    def format(self, ambigious_classes):
        return "ambigious module specification, found multiple classes: {0}".format(ambigious_classes)


class ConfigInvalidModuleError(ConfigError):
    def format(self):
        return "no class found"

remove unreachable code