Modern Python Standard Library Cookbook

Dictionary with fallback

When working with configuration values, it's common to look them up in multiple places—maybe we load them from a configuration file—but we can override them with an environment variable or a command-line option, and in case the option is not provided, we can have a default value.

This can easily lead to long chains of if statements like these:

value = command_line_options.get('optname')
if value is None:
    value = os.environ.get('optname')
if value is None:
    value = config_file_options.get('optname')
if value is None:
    value = 'default-value'

This is annoying, and while for a single value it might be just annoying, it will tend to grow into a huge, confusing list of conditions as more options get added.

Command-line options ...